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Monday, January 30, 2006

N92/2
Discuss the importance of hydrogen bonding in biology
Hydrogen bonding plays an integral role in the structure of various biological molecules, all of which are essential to life in one form or another. Since biology is basically the study of living organisms, hydrogen bonding is thus of great importance to the study of biology.

One of the main categories of biological molecules in which hydrogen bonding plays an important role in is protein. Proteins form a component of all living organisms, and they are important for the growth, reproduction, repair and reproduction of an organism- which basically constitute the study of biology. Hydrogen bonding is critically important to the formation and maintenance of the structure of one the basic building blocks of life, protein; it plays a role in maintaining the secondary, tertiary and quaternary structures a protein, thus allowing the protein molecule to maintain its specific shape and configuration, which in turn allows the protein to perform its function more effectively. Although hydrogen bonds are not as strong as covalent bonds (e.g. disulphide bonds), the mere frequency with which they occur in the secondary structure proteins gives them great importance in maintaining the structure and stability of the polypeptide chain (and so the protein). Examples of different forms of secondary structures of proteins which depend mainly on intramolecular hydrogen bonding are the alpha-helix and the beta-pleated sheet and the triple helix. In the alpha-helix, which takes the form of an extended spiral spring, hydrogen bonds which occur between the –C=O and –NH groups within the polypeptide chain stabilize the molecule. In the beta-pleated sheet, hydrogen bonds between the –C=O and –NH groups of one part of the backbone and the –C=O and –NH groups of an adjacent part of the backbone hold the structure together. The secondary structure of a protein is considered to be the most important in fibrous proteins- hydrogen bonding is important in maintaining the structure of proteins, particularly fibrous ones, which mostly have supportive functions, for examples, collagen and keratin. In the tertiary and quaternary structures of protein, hydrogen bonding plays a part in holding the various subunits together, and maintaining the optimal surface configuration of the protein molecule, which allows the protein to perform its function better. The maintenance of a specific surface configuration is crucial to the function of proteins (which is essential to the functioning of a living organism). Some proteins may function as enzymes, hormone receptors, transport proteins, et cetera. For enzymes, the rate of reaction between the enzyme molecule and the substrate molecule is the fastest when the surface configuration of the enzyme (especially that of the active sites) is in optimal condition- the enzyme is most suited to bonding with its substrate molecule when all the bonds that help to maintain the secondary, tertiary and quaternary structure of the protein are intact (and this includes hydrogen bonds).Enzymes are necessary in living organisms to catalyze biochemical reactions that occur within the cell, which are essential to life. For example, isocitrate dehydrogenase is involved in the Krebs cycle in the respiratory pathway, which produces ATP. Hydrolysis of ATP releases energy which the cell can use for various biochemical activities. Thus, as can be seen, hydrogen bonds are very important to the structure and function of proteins. With regard to enzymes, this importance can be illustrated using a reverse example. When exposed to high temperatures and/or extreme pH, a protein molecule is denatured, i.e. the bonds that help to maintain the secondary, tertiary and are disrupted (this includes hydrogen bonds too) and the protein molecule unfolds, losing its secondary, tertiary and quaternary structure. In the case of enzymes, the specific configurations of the active sites are lost, and the enzyme is thus unable to carry out its function. To sum up, hydrogen bonding has an integral role to play in maintaining the structure of proteins, which is crucial to the function of one of the basic building blocks of life.

Another important class of biological molecules in which hydrogen bonding also plays a part in the maintenance of structure are the nucleic acids. Nucleic acids can be divided into two broad categories, ribonucleic acids (RNA) and deoxyribonucleic acids (DNA). Nucleic acids, in general, are the means by which genetic material is transferred form one generation to the next- with the help of suitable enzymes, they are able to self-replicate. Nucleic acids could thus be said to be the basic building blocks of life, even more so than proteins. Hydrogen bonding plays a crucial role in the maintenance of the stability of the double helix of DNA found in most eukaryotic cells, which functions as the main heriditory material. Hydrogen bonding between the A-T and C-G base pairs of two polynucleotide chains maintains the stability and integrity of the double helix- this is crucial as DNA functions as heriditory material. This function of DNA would not be well carried out if the DNA molecule were unstable- which would be the case if not for the hydrogen bonds between complementary base pairs. With regard to the DNA molecule, it can be established that hydrogen bonding plays a crucial role in its structure and function. Hydrogen bonding also occurs in other nucleic acids other than DNA- it is also important in the formation of DNA-RNA duplexes, which are formed during transcription. Hydrogen bonds between complementary base pairs hold the two nucleic acids together as the DNA strand is being read and the mRNA is being formed. Transcription is one of the two main steps that consist protein synthesis in the cell- the other being translation. Earlier, the function of proteins (in which hydrogen bonds play an important part in the maintenance of structure) has already been mentioned. Hydrogen bonds are involved in the translation step of protein synthesis- hydrogen bonds between the complementary base pairs on the mRNA codon and the tRNA anticodon. Since hydrogen bonding are involved in both steps of protein synthesis, and relating this to the importance of proteins in living organisms discussed earlier, it can be seen that hydrogen bonding is crucial to life, and thus to the study of biology. Besides the above, hydrogen bonding is also involved in the intramolecular bonds within nucleic acids- such as the folding of a tRNA molecule into a clover shape and the formation of hairpins in DNA molecules.

Next, I will move on to discuss the importance of hydrogen bonding between water molecules. All life on earth is dependant on this polar molecule, which are all held together by hydrogen bonds. Although individual hydrogen bonds are relatively weak, collectively they form important forces which hold water molecules together. Water is widely considered to be the universal solvent, and this property of water is particularly important to living organisms. For example, water forms a large part of blood plasma, which is used to dissolve and transport a large variety of substances around the body, such as glucose, a respiratory substrate, and waste products such as urea for disposal. Water is able to act as a universal solvent mainly because hydrogen bonding occurs between the polar groups in the solute molecule and the polar water molecules. Thus, hydrogen bonding is key to water’s property of being a universal solvent, which is important to all living organisms. The high specific heat of water, which is also important to life, is also attributed to hydrogen bonding, this time between water molecules. This property of water is important to living organisms as it helps in the regulation of body temperature (a component of homeostasis). This ensures that the internal environment of an organism is kept relatively constant (with respect to temperature) in comparison to fluctuations in the temperature of the external environment- water, as mentioned above, forms a large part of blood plasma which circulates around the body and is able to act as a temperature buffer, due to the hydrogen bonds that exist between water molecules. This property of water is important to living organisms because enzymatic reactions can only occur over a narrow temperature range (which is around body temperature)- enzymes are involved in many biochemical reactions in the body that help sustain life. Furthermore, enzymes are denatured or inactivated at extreme temperatures and then are unable to carry out their function, which would be severely detrimental to life. Hence the property of water which allows it to function as a temperature buffer in the body (which is due to the intermolecular hydrogen bonds) is essential to life. Furthermore, most organisms are unable to tolerate wide variations in temperature. Another thermal property of water which is important to life, and also due to the intermolecular hydrogen bonds between water molecules, is the high heat of vaporization. Basically, this means that a lot of heat can be lost with minimal loss of water from the body of organism; this is because many intermolecular hydrogen bonds must be broken for water to evaporate. Once again, this property of water is important for the regulation of body temperature in living organisms. Water’s special freezing properties and the density of ice in comparison to liquid water are also essential to life- and these, once again, are due to the intermolecular hydrogen bonds between water molecules. Ice is less dense than water at zero degrees Celsius because the hydrogen bonds between the molecules make crystal spacious, and this enables water to remain at 4 degrees Celsius during winter, as the layer of ice insulates the water below, preventing the complete solidification of the water body- this allows organisms to survive beneath the ice during winter. If hydrogen bonding were not present between water molecules, leading to ice being denser that liquid water, water would freeze from the bottom upwards, killing aquatic organisms. As can be seen, hydrogen bonding between water molecules is of great importance to life. Another property of water which again depends on the intermolecular hydrogen bonds is the cohesion and high surface tension of water. The constant formation and reformation of these intermolecular hydrogen bonds give liquid water high cohesion- which allows water to move in a continuous stream against gravity (important for water transportation in plants, transpiration stream in the xylem). Because of the large cohesive forces in water, water molecules lie close to one another, making water a useful means of support. This property of water, due to the intermolecular hydrogen bonds, is important to life- turgidity in plants is maintained by the osmotic influx of water into plant cells, and animals such as earthworms depend on their hydrostatic skeleton for support. The high cohesion and surface tension of water due to hydrogen bonding also accounts for the high tensile strength of a water column, for example the transpiration stream in the xylem vessel- transpiration prevents the plant from overheating and allows for the uptake of minerals and ions that are required for photosynthesis. Thus, hydrogen bonding is essential to life in the following ways, with regard to water, and so is important to biology.

Hydrogen bonding is also plays an important role in maintaining the structure of cellulose, an important plant carbohydrate. Adjacent chains of beta-glucose molecules are linked by hydrogen bonds between the hydroxyl groups of carbon atom two- cross-linking between chains is thus established, accounting for the high tensile strength of cellulose. This high tensile strength of cellulose attributed to the hydrogen bonds that exist between chains is important to life as it prevents plant cells from bursting when water enters by osmosis- call walls of plant cells are made of cellulose. The hydrogen bonds between adjacent chains of beta-glucose molecules also ensures that there is a lot of space between glucose residues- cellulose is thus fully-permeable to water and solutes. The cellulose cell wall of plant cells is thus fully-permeable and this is an important property in the functioning of plant cells. Thus, hydrogen bonding plays an important role in the structure of cellulose, which in turn plays an important role in plants.

From all the above, it can be established that hydrogen bonding is critically important to life in all its forms in one way or another. Hydrogen bonding is found in some of the most basic building blocks of life and so is exceedingly important to the study of biology.

References: Campbell and Reece (6th Edition)