When I visited Thailand years ago, I had the pleasure of tasting about 20 different types of exotic, tropical fruits like pineapple and mango, some of which were first in my life. Each had a unique color, shape, smell, and taste. It occurred to me then that there were plants and fruits in places that we would never visit in our lifetimes and we would never have the chance to taste or smell them, but our tongue was still capable of tasting, smelling, and getting pleasure out of every one of them.

Take the orange for instance. Since it is more easily available and not as exotic as other fruits, we may incline to underestimate it; but in fact it is one of the countless bounties we are blessed with. There are currently over 600 known different types of oranges and they serve as a rich source of vitamin C that helps enhance our body’s resistance against illnesses such as the common cold. Therefore, our need for this vitamin increases particularly in the winter. With a beautiful color and smell, oranges contain a plethora of beneficial minerals such as thiamine, and vitamins such as vitamin A [1]. Vitamin C, another vitamin richly found in oranges, is an excellent antioxidant substance that fights carcinogenic free radicals. It is also essential for the protection of skin health. Oranges are also good sources of fibers and potassium that are crucial for heart health. Potassium lowers the risk of high blood pressure. Fibrous content of oranges helps to prevent diabetes development [2].

This fruit is only one of so many blessings that we are unable to count. For us to be able to appreciate these blessings we need to see, taste, feel, smell, and digest them, each of which is another reason to be grateful.

Seeing

We have to see and like the appearance of a food first before deciding to eat it, don’t we? We do not want to eat it if we feel disgusted from the way it looks. Our eyes are windows unto life so that we can see and observe the great book of the universe which is filled with miraculous works of art and get to know the divine names and attributes that manifest on them.

“Indeed, the Compassionate Provider, in order to give to them provision in more generous measure has created each of man’s subtle capacities – eye and ear, heart, imagination, and intellect – in the form of a key to His treasury of mercy. For example, the eye is a key to the treasury containing such precious jewels as the fairness and beauty to be seen on the face of the universe, and the same holds true of all the others mentioned; they all benefit through faith.” [3]

Everything – from fruits and vegetables to the eggs served by the chicken and the honey put together by bees – is created subtly and packaged in a way to appeal to our eyes in unique and protective enclosures.

Appetite

We have to have appetite so that we develop a desire to eat food. This desire is a capacity encoded into our being so we can enjoy good food and feel aversion for certain things.

“Appetite and desire for sustenance are a sort of innate or instinctive thanks.” [4]

Consuming sustenance with appetite is a way of expressing our thankfulness to the One who provides them for us, for we have appetite for the things we are appreciative of. If we do not have any appetite then we would have no desire to eat even the most delicious food. This is sometimes the case when we are ill and refuse to eat even our most favorite dishes. But think of our commonplace orange again. Like other fruits and vegetables, the orange is created with a shape or allure that will whet our appetite. God has placed sustenance at the very center of His workings in the world of living beings and guides us to this sustenance through the urges of appetite. His servants, on the other hand, are supposed to respond to these blessings with remembrance, reflection, and thankfulness.

Touching

For us to be able to taste and eat an orange, we first have to hold it in our hands. For an action as apparently simple as touching or holding anything, what “we” have to do is only to exhibit willingness to do so. Most of the processes in our bodies occur beyond our control. The joints in our fingers, the size of our hands, and the design of our arms meet our needs in the best way.

It may be an ordinary act for us to move our hands and finger joints with the help of the muscles wrapped around our bonds and triggered into action with electric signals coming through nerve cells. However, when we contemplate on all of these processes we come to the conclusion that they are not casual at all:

“Yes, we see for instance that the members and bodily systems of a fly or human being, and even the cells of the body and red and white corpuscles in the blood, are placed with so sensitive a balance and fine a measure, and they are so fitting and suitable for each other, and their mutual proportion with the other members of the body is so orderly, and they are in such harmony with them, that it is in no way possible that one lacking infinite knowledge could have given them those situations.” [5]

Smelling

The Prophet Muhammad (pbuh) likens believers who read the divine revelation to an orange “whose fragrance is sweet and whose taste is sweet” and those believers who don’t recite are like a date fruit, which tastes sweet, but with no fragrance [6].

Bediuzzaman Said Nursi draws attention to the manner of benefiting from the bounties of God as follows:

“Know, O Friend, that the gifts God has ordained that humanity attain or make use of come with conditions. Some of these conditions are established by God, while others pertain to human beings themselves. For example, light, air, food, and speech are God’s gifts, and how much we benefit from them depends upon our respective organs’ soundness and health. All senses and organs have been created by God Almighty; our role is to keep them sound and healthy.” [7]

The role of our willpower is limited to the functioning of these organs which are required for obtaining and consuming the sustenance needed for our survival.

When we look at an orange closely its shiny skin with thousands of tiny holes catches our attention. After the fruit is picked from a tree it remains fresh for a long time thanks to air coming through the holes in its peel.

It sends out a fragrant smell as it is sliced down. The inner parts of the peel are covered with a white, thick, and soft layer as if it is plastered. There are round, tiny buds with voids between them immediately beneath the outer peel. Like cushions, they add flexibility and strength to the peel. Inside we find segments that look like each other and are placed in an aesthetically pleasing manner. The segments are covered with a strong membrane and protected with white, fibrous walls. The segments contain hundreds of shiny and swollen tiny sacs, each of which are arranged regularly like miniaturized grapes and are also covered with membranes. These juice sacs are protected in this manner because the juice in these sacs contains vitamin C which quickly degrades upon contact with air.

“Let us imagine an army which consists of four hundred thousand nations, and each nation requires different provisions, uses different weapons, wears different uniforms, undergoes different drill, and is discharged from its duties differently. If this army and camp has a miracle-working commander who on his own provides all those different nations with all their different provisions, weapons, uniforms, and equipment without forgetting or confusing any of them, then surely the army and camp point to the commander and make him loved appreciatively.” [8]

Digesting

“Especially the members, faculties, and senses of a single of the innumerable members of those species; they are related to each other with so fine a balance and equilibrium that their balance and mutual proportion point to an All-Wise and Just Maker so clearly as to be self-evident.” [9]

The arrangement of teeth, the shape of the tongue, and the structure of the mouth function in perfect harmony with the alimentary canal, the stomach, and other digestive organs. Teeth are responsible for biting and grinding food, but if the structure and arrangement order of our teeth had differed, i.e., if our grinders had been replaced with incisors, the acts of biting and chewing would be much harder.

If our tongue did not assist us in the acts of turning and grinding food in our mouth, or if it had been larger or smaller, this would have complicated food consumption for us. This also applies to many other wise purposes related to the mobility of our chin or functions of saliva.

Oranges, or whichever blessing you may take as an example, are a sign for us to contemplate and be grateful to the One who generously provides them for us.

“He it is Who sends down water from the sky, and therewith We bring forth vegetation of every kind (from their seeds under the soil), and then from it We bring forth a lively shoot, from which We bring forth close-packed and compounded ears of grain, and from the palm-tree – from the spathe of it – dates thick-clustered hanging (ready to the hand), and gardens of vines, and the olive tree, and the pomegranate: alike (in the fundamentals of life and growth) and diverse (in structure, look, taste, and smell). Look at their fruit, when they begin to fruit and as they ripen. Surely in that there are signs for people who will believe and who will deepen in faith (as they see new signs)” (An-An’am, 6/99).

References

1. “What to know about oranges”, www.medicalnewstoday.com/articles/272782.
2. Bediuzzaman Said Nursi, Şualar (Rays), Istanbul: Şahdamar Yayınları, 2010, p. 162.
3. Bediuzzaman Said Nursi, Mektubat (Letters), Istanbul: Şahdamar Yayınları, 2010, p. 412.
4. Bediuzzaman Said Nursi, Şualar (Rays), Istanbul: Şahdamar Yayınları, 2010, p. 635.
5. Bukhari, At’imah, 30.
6. Bediuzzaman Said Nursi, Mesnevî-i Nȗriye (Epitomes of Light: Mathnawi al-Nuriya), Istanbul: Şahdamar Yayınları, 2010, p. 84.
7. Bediuzzaman Said Nursi, Asâ-yı Musa (Staff of Moses), Istanbul: Şahdamar Yayınları, 2010, p. 20.
8. Bediuzzaman Said Nursi, Şualar (Flashes), Istanbul: Şahdamar Yayınları, 2010, p. 384.

Among tropical fruits, bananas are probably the most popular, and with good reason. They are delicious and nutritious, and can be consumed by everyone, from babies to seniors.

The banana plant belongs to the Musa genus. The varieties preferred for cultivation are two hybrid species, Musa accuminata and Musa balbisiana. These types are mostly produced in Southeast Asia, Africa, and South and North America.

There are many varieties of banana. Short Cavendish, mid-sized Grand Nines, and longer Chiquita are some of those. The main harvesting occurs in September and October, but bananas can be produced year round in greenhouses.

Bananas are not reproduced by seeds. They reproduce through tissue culture – pieces of tubers or underground shoots. Underground perennial tubers spread horizontally and grow roots. Once the leaves and sheaths inside the annual aerial pseudostem reach a certain number, a flower bud is developed. The flower stalk, rising among the leaf bundles that are in the center of the pseudostem, carries the purple colored flowers that will become fruit.

When these flowers bloom, sounds can be heard. These sounds occur during the tearing of stem when the flowers force away from the crust to form the banana clusters; this is also known as inflorescence. While the buds are quickly developing, the purple leaves open and flowers become visible. Once the flower inflorescence emerges completely, it bends towards the ground. Fruits then form. Since the banana is a parthenocarpic plant, its fruit is generated without pollination from female flowers, like seedless grapes. The approximate time required for flowers to give fruit and become ripe is three months (1).    

Bananas are rich in nutrients. There is 1.1 gr. of protein, only 0.2 gr. of fat, 22 gr. of carbohydrates (fructose, glucose, sucrose and starch, cellulose, pectin), along with minerals like potassium, calcium, iron, phosphate, copper, zinc, and magnesium in 100 grams of a banana. It also contains fruit acids, along with vitamins A, B1, B2, B6, B9, C, D, E and P vitamins. Before bananas ripen, when they are still green, they contain approximately 1% sugar and 20% starch. As they ripen, the sugar content rises to 20% and the starch level drops to 1%.  

Bananas are harvested green and unripe and are matured in a closed environment. Unripe bananas can be stored for up to 15 days at 5-10 degree Celsius and 80-90% relative humidity. Maturation is enabled via ethylene gas, in storage rooms or during shipment. The shell becomes completely yellow, with brown spots, when the banana ripens. These spots indicate the sufficient conversion of starches into sugars. It is recommended to consume the ripe fruit as soon as possible.

In the past, the transportation of mature fruits to remote regions was a major problem, but this is not the case today. It is because methylcyclopropene (MCP) is used to delay the ripening of the fruit. When MCP is used, it binds to ethylene receptors, and slows down maturation (2). The proper storage temperature is 13-15 degrees (Celsius), so they get darker in the fridge faster. Therefore, it is advised to store them in a suitable place in a paper bag (3).

Even though they are usually eaten raw, as a fruit, some banana types are consumed after cooking. They can also be utilized as chips, baby food, puree, flour, or juice. In some places, banana flowers are used in salads and as decoration. Furthermore, bananas are employed in facial and skin care products. The leaves and stalks of the banana plant may also be used in the construction of roofs, and the fibers can be used as ropes, upholsteries, or even hats.  

According to the statistics of the Food and Agriculture Organization of the United Nations (FAO), the annual production was 55-60 million tons in 1995, whereas it is 90-100 million tons today. 25% of the production occurs in India, and the majority of the rest takes place in the Philippines, China, Brazil, and Ecuador. Even though they are grown in various parts of the world and traded widely, bananas are mostly imported by developed countries.

Benefits of the banana

Bananas can help prevent many diseases or provide complementary aid to different therapies. When a banana is blended with milk, it can serve as an ideal starter food for babies, and can also reduce wear and tear on the body, delaying aging.

Bananas have been shown to help after stomach or intestinal bleeding, and in patients with ulcers or various laryngeal problems. They can help prevent acid reflux. The sodium and potassium contained in bananas are effective for restoring heart beat rhythms and the body’s osmotic balance. Since bananas are rich in iron, they’re beneficial for anemia. Due to a high amount of serotonin, bananas can actually make people happier, improving our decision making and concentration. Because they can actually make us less depressed, bananas are a recommended snack (5).

Bananas help support the development of children’s skeletal structures, and can ease the pain of menstrual cramps. Aside from being an energy source, bananas help the nervous system function properly, maintaining the acid-base balance of bodily fluids and strengthening the immune system. Because of high magnesium and potassium, they help us sleep better – thus making a banana the perfect midnight snack!

Carbohydrates can cause instant blood sugar spikes, but they are also the most important energy source of the brain, central nervous system, and muscles. When blood sugar rises, it is stored as sugar or fat with the help of insulin secreted by the pancreas. This can cause obesity, or even diabetes, if a person ingests too many carbohydrates. Therefore, one must consume high sugar foods like bananas at the right time, and in the right amount.

Beta-carotene, one of the precursors of vitamin A, happens to be abundant in bananas. Beta-carotene helps with the neutralization of free radicals, supports the immune system, prevents cardio-vascular disease, and is protective against cancer. It’s more beneficial when taken together with vitamin E and C, which are also found in bananas.

The good news keeps coming! Due to a high amount of fiber, bananas are great for dieting and can also prevent colon and bowel cancers. Bananas can also help to regulate bowel functions.

Genetically modified banana plants are also employed for the synthesis of vaccine proteins used against Hepatitis, rabies, dysentery, cholera and other intestinal infections (7).

As you can see, the banana has been imbued with an extraordinary amount of positive properties. It’s not difficult to look at bananas as an amazing blessing bestowed upon humanity.

The heart is composed of four compartments: two atriums and two ventricles. The blood that reaches the heart first accumulates in the atriums. From here, it is sent to the ventricles. Afterwards, it is redistributed to the body by the contractions of the ventricles. The harmony of this process depends on the electrical currents in our hearts.

How is the electrical current formed?

There is a particular region in the heart called the sinus node. The sinus node is strip of a muscle that is 15 mm in length, 3 mm in width, and 1 mm in thickness, and is located in the right atrium of the heart. The cells of this strip are responsible for producing electrical currents, and are created in a different fashion from the rest of the cells that are responsible for producing contractions. This is where the electrical currents in our hearts are periodically produced. Every cell in the body contains elements such as sodium, calcium, potassium and chlorine that are electrically charged. The elements which are electrically charged are called ions. These ions also exist in the extracellular environment. The intra cellular and extra cellular concentrations of these ions are different from each other. This situation causes a difference in the electrical potential between the interior and exterior of a cell. This difference is called a membrane potential. Periodically, the membrane potentials of the sinus cells show sudden jumps – meaning they suddenly increase and then suddenly decrease. Since the cells are in close contact with each other, such a jump in the membrane potential of one cell triggers a jump in the membrane potential of another cell. The electrical currency that enables the contraction of the heart is produced by this continuous triggering of cells. On average, 70 electrical currents per minute are produced in the sinus node. These currents start being produced while a person is in the womb of their mother and continues their whole lifetime. The heart of an embryo starts beating while it is only 22 days old. However, the height of the embryo at this point has not even reached 1 cm. Isn’t it an amazing force that creates the beating heart of such a small embryo and keeps it going a lifetime?

How is the electrical current distributed?

Another node called the atrio ventricular node was created in between the atriums and ventricles in our heart. While the current coming from the sinus node is spread to the whole of the atrium, it is by this node that the current is sent to particular fibers. The task of this node is to hang on to the current coming from the sinus node for a while. Why does the current need to be held on to? Because blood can only enter the ventricles while it is resting and by holding on to it, the contraction of the ventricles is disabled while the contraction of the atriums is taking place. By this process, the blood coming from the atrium can enter the ventricle. Therefore, the blood fills in the ventricles and can be distributed throughout the body. The blood circulation is enabled in a flawless manner by allowing the atriums to do their duty while the ventricles wait.

After passing through the atrio ventricular node, the electrical currency eventually goes through the purkinje fibers. These fibers surround the ventricles like a web and are composed of cells that can conduct electrical current in a very fast manner. Compared to the atrio ventricular node, the electrical current can be conducted 150 times faster in the purkinje fibers. Therefore, the current reaches every point of the ventricles in a very short period of time. Every muscle in the ventricles contracts in a time shorter than one tenth of a second.

The muscles in the ventricles rapidly contract, one by one, depending on when the current reaches them. The contraction starts at the end of the ventricles and carries on towards the main veins exiting the heart. By this orderly and harmonious contraction, the blood is pumped from the end of the heart towards the main veins exiting the heart to be distributed among the body. Because all the ventricle muscles are stimulated very fast, the contraction also happens very fast, resulting in a strong pumping effect. The design of this system is incredibly wise, right down to its most minute detail.

Movement in heart muscle potential

As all cells in our body, the cells in the heart also have a membrane potential. We had stated before that this membrane potential is the result of the difference in intra and extra cellular ion concentrations. The charges of these ions are different from each other. For example, sodium and potassium have plus one (+1) charges, calcium has a plus two (+2) charge, and chlorine has a negative one (-1) charge. The resting potential of a cell is negative. This means that there are more negative ions within the cell when compared to its environment. Sodium, calcium, and potassium ions are mobile through the membrane. While sodium and calcium have a higher concentration outside the cell, potassium has a higher intra cellular concentration compared to its environment. There are channels created on the cell membrane that allow ions to pass through the membrane. The sudden increase in the membrane potential that was explained before causes a sudden rush of sodium ions inside the cell. This is such a rapid movement that it is concluded in a tenth of a second. Right after the entrance of the sodium ions, calcium ions also enter. Because these ions are positively charged, the membrane potential becomes positive.

With the entering of calcium ions into the cell, calcium ions are also released from the storages within the cell. By triggering the protein necessary for these contractions, the calcium ions become a means for the contraction of the heart muscles. Meanwhile, the potassium channels open and these ions within the cell pass to the extra cellular environment. The loss of positive ions results in the membrane potential being negative again. Therefore, the sudden jump in membrane potential that is the basis for the electrical current is created.

However, at this point there are extra amounts sodium and calcium within the cell and extra amounts of potassium outside the cell. The concentrations need to be returned to their original values for the next jump in the membrane to be possible. This task is given to a protein called the sodium-potassium pump that pumps out sodium from the cell and pumps in potassium. If this pump had not been created, the ion balance in any of the cells within the body would be impossible to re-establish. As a result, the life of the cells would come to an end. However, because of the remarkable intricacy of our cells, life is made possible for us.

Afterwards, some amount of the calcium ions are pumped out of the cell with a similar pump, while the rest are stored within the cell. The decrease in the concentration of calcium relaxes the muscle. Now the heart muscle has gone into relaxation and therefore is ready for the next contraction.

If the movement of the ions becomes unbalanced, the rhythm of our heart is disturbed. The unbalance in the ion movements or blockage in heart veins can be reasons for heart rhythm disorders. Even small heredity-based defects in the ions pumps affect the movement of these ions and can cause heart rhythm disorders. This situation shows that nothing is created by coincidence.

Movement in the sinus node

The jump in the membrane potential of a heart cell depends on the membrane potential jump of the previous cell. Through the gaps in between the cells that are in contact with each other, the positive ions that exit a cell reach the membrane of the cell next to it and trigger the opening of its ion pumps. As a result, the membrane potential of that cell starts changing. At this point, you may have this question: how does the electrical current start in one end of the sinus node that is not previously triggered by any cell?

This concept is explained by the ion transfer mechanism of the node cells being different than the muscle cells. Before explaining this, it should be noted that even while resting, a mechanism for allowing an ion exchange of the cell with its surrounding has been created. In the node cells, this exchange while at rest has been created in a way that the sodium and calcium exchange is larger and the potassium exchange is lower compared to the muscle cells during resting conditions. Therefore, the membrane potential of the node cells is less negative and slowly increases with time. As a result of this slow but steady increase, after a while it reaches a threshold. When it reaches it, the calcium channels in the membrane suddenly open and there is a rush of calcium ions into the cell. Thus, the jump in the membrane potential is created independently from another cell.

As it can be observed, even a single contraction of our heart depends on a very detailed, delicate, and complex system. Moreover, this system is repeated over a hundred thousand times within one day. After reflecting on this, how can we claim this system runs by coincidence or chance?

Open heart surgery is performed after putting the patient to sleep under a general anesthetic. The chest is then opened by the surgeon, and the heart is temporarily bypassed or deactivated for the duration of surgery (although in some new techniques like beating heart surgery or minimal invasion heart surgery, the operation is possible without deactivation of the heart) During this period the functions of the heart are performed by an artificial lung mechanism called the heart-lung machine (cardiopulmonary bypass machine). Performing surgery on a working heart cases where there is no facility for beating heart surgery would be like trying to repair the engine of a car while it is in motion. This is why it is necessary to temporarily prevent the functions of the heart during the operation, which requires great care and accuracy.

Stopping the heart

During this procedure the patient is connected to the machine, thin pipes called cannulae are inserted into the main veins which lead to the heart, and thus the blood which goes to the heart is directed into the heart-lung pump, fed with oxygen, and then redirected into the body. Preventing the function of the heart is not a very difficult process. When the heart-lung machine is activated and the blood is cooled and redirected into the blood vessels, the body temperature is reduced to below 30°C, and this lowers the heart rate and assists the heart to stop functioning. The actual stopping of the heart is performed by feeding a serum containing a concentrated solution of potassium ions into the coronary artery, which feeds the heart muscle. Potassium ions are normally found in the human body but in a fixed proportion; potassium is an electrolyte which, if increased, causes a defect in the heart’s rhythm and can lead to ceasing of the heart function. Feeding the coronary artery rapidly with a rich potassium solution causes the heart to stop within a few seconds and allows the surgeon to perform the operation on a non-functioning, motionless heart.

The heart should not be stopped from functioning for a long period, even if the heart-lung pump is performing the function of the heart successfully. Under normal conditions the pump cannot perform the whole duty of the actual heart and lungs. When the body temperature is reduced, there is a reduction of functioning in many organs of the body to such an extent that they almost stop working, especially the brain. This means that every organ freezes and if the patient’s pulse were taken within this period, they would be assessed as dead.

Restarting the heart

To restart the heart following the operation a reversal of the procedure performed at the beginning of surgery is necessary; the temperature of the body is increased to 36.5–37°C again with the help of the heart-lung pump, and at the same time the amount of potassium in the blood is reduced to a normal level. This is usually executed by ensuring the normal function of the kidneys which discard the potassium from the body. This is when the function of the heart is monitored closely because there is a reversal in the process of inducing low body temperature and the excess of potassium which caused the heart to stop. In other words, the barrier which stopped the flowing river is removed; therefore, according to the laws of physics, the trapped fluid should flow again at great speed, and although following surgery the majority of hearts do begin to function again when these procedures are performed, there is unfortunately no actual guarantee. There may be certain complications or even causes which we have not yet discovered, in which case an electric shock of 10–20 joules is delivered directly to the heart muscle to encourage it to function normally. If this is unsuccessful, medication such as adrenalin, which induces the functioning of the heart, is given to the patient. If, following these repeated procedures, there is no effect, and, regardless of all the effort, the heart does not function, then everything is performed again from the beginning, including the operation. But there is always the possibility that the desired result may not be achieved. Human beings always face the prospect of death in daily life, and although there is a very slim chance of death, with such a big operation there is always the possibility.

The result

We are normally totally unaware of the rhythmic incidents, a combination of great harmony, occurring within our bodies. Even breathing, a necessity for every living creature to stay alive, is not an action which we activate and continue of our own will. Sight, hearing, hunger and senses are acts of nature over which we have little direct will or power. Nevertheless, they are all events which we can only describe as divine miracles and what a great blessing it is that none of these complex functions of our bodies have been left to us humans.

In May 2003, Roderick Mackinnon’s research group at Rockefeller University has published the three-dimensional atomic structure of a voltage-gated potassium channel. MacKinnon also received the Nobel Prize in 2003 for the first potassium channel structure

How electrical signals are conducted through nerve cells has been understood with some detail in the last fifty years, after the seminal work of Hodgkin and Huxley, which earned them a Nobel Prize in 1963. After 40 years, another study is appreciated in the scientific community as the end of one era and the beginning of another one.

Cells, body’s building blocks, are bound by membranes that are impermeable to ions, atoms that carry positive or negative charge. Nerve cells are specialized for having very long arms that can carry electric potentials on their membranes for very long distances. The electric potential (voltage difference) is the result of unequal number of charges across the inside and outside of the cell membrane. Hodgkin and Huxley explained the movement of the electric potential on the membranes by the selective opening of ion channels, proteins embedded in membranes to allow the passage of certain ions. As channels are opened in one part of the nerve cell to allow an electrical potential change, the neighboring stretches somehow sense the change in voltage and open their channels as well. The propagation of the electric potential depends on the fast sensing, opening and closing of these channels. The molecular details of these voltage-gated channels have been somewhat obscure; however important properties have been determined using methods pioneered by Hodgkin and Huxley, generally dubbed electrophysiology, the study of biological material using electrical methods. Scientists

have imagined channel proteins as many helices running through the membrane, some of which could be pulled to the inside or outside of the cell depending on the voltage across the

membrane. Movement of these helices across the membrane could open channels for potassium or sodium. Finally in May 2003, Roderick Mackinnon’s research group at Rockefeller University has published the three-dimensional atomic structure of a voltage-gated potassium channel. MacKinnon also received the Nobel Prize in 2003 for the first potassium channel structure, which explained the selectivity of a potassium channel for allowing only potassium ions to pass, but nothing else.

MacKinnon is a mixed structural biologist and electrophysiologist. His group has isolated potassium channel molecules from membranes, and tested them for strict functionality as it has been observed that protein molecules isolated out of their natural environments may lose their activities and chemical properties. The next step of their study was to crystallize the channel molecules-much like forming salt crystals-and diffract x-rays through the aligned molecules in the lattice of their crystals. These diffraction patterns are mathematically solved to give atomic positions of electrons, hence atoms, in the protein crystal. Several research groups failed at this step, since purified potassium channels never formed crystals, as flexible protein molecules are unlikely to form crystals. Here, MacKinnon cleverly used antibodies they developed specifically against the channel. These antibodies, used radically differently than they are in nature, overcame the flexibility of the voltage-sensing regions of the channel, by binding to the channel and fixing it to a specific conformation. The ordered channel crystallized and gave the structure of the potassium channel and the antibody bound to it.

The structure of the channel consists mostly of helices, which in the center surround a potassium-conduction pore, and “voltage-sensing paddles” in the periphery. When the gate is closed, the helical paddles lay parallel at the cellular side of the membrane. Gate opening only happens when the paddles are pulled through the membrane to the outer side as voltage pulls the positive charge of the paddles to the outside (see figure). Now, it looks like 50 years of data collected by electrophysiologists and biochemists can be explained coherently (such as the observed 14 positive charges residing on the paddles moving through the membrane). As is usual with such discoveries, many new questions are asked, such as how the charged paddles can go through the oily, highly uncharged membranes. There are also valid scientific objections, such as the antibody distorting the potassium channel structure. However, this is one of those bright moments in any field of research, when researchers go over the threshold of collection of enough observations to discovery and understanding.

References

• Jiang, Y., Lee, A., Chen, J., Ruta, V., Cadene, M., Chait, B.T., MacKinnon, R., Nature 423, 33-41 (2003).
• Jiang, Y., Ruta, V., Chen, J., Lee, A., MacKinnon, R., Nature 423, 42-48 (2003).
• Sigworth, F.J., Nature 423, 21-22 (2003).