Throughout biochemistry there are many bonds without which life as it is on earth today would not be possible. One of the most important bonds of these is the hydrogen bond, a weak chemical bond that is present in essential biological molecules such as water and polypeptides. A hydrogen bond is defined by Campbell and Reece as occurring when a hydrogen atom is covalently bonded to an electronegative atom but attracted to another electronegative atom. In water molecules, there are several key reasons why hydrogen bonds can be formed and explaining them in water a good way to show the chemistry. Firstly, the presence of covalent bond between the hydrogen and the oxygen means that the electrons in the outer shells of both atoms are shared- 1 electron from hydrogen and 1 electron from oxygen. Since the 2 electrons are shared, they are free to move within the covalent bond to the atom that is the most electronegative. In the case of water, this is oxygen. As a result of the electrons moving to the oxygen side of the bond, the hydrogen becomes less electron-dense and becomes a slight positive charge known as a delta-positive charge. It is this positive charge that has the ability to attract other negatively charged objects, since opposite electrostatic charged atoms attract each other. On the oxygen atom of each water molecule there is a lone pair of electrons that are negatively charged, which makes oxygen delta-negative. This means that between water molecules, the delta-positive hydrogen of one molecule is able to attract a lone pair of electrons from the delta-negative oxygen atom of another water molecule (Fig. 1).
Fig. 1 Hydrogen bonding in water
A hydrogen bond, however, is comparatively weak to covalent or ionic bond, as much as 22 times time weaker [Libes 2009], so in order to explain why hydrogen bonds are so necessary in life it is perhaps not significant that hydrogen bonds are weak on their own, since the majority of their use within strong structures is facilitated by their strength as a large number of hydrogen bonds. For example, the fundamental strength of tendons and skin lies within the many hydrogen bonds in the collagen protein.
For formation of collagen, the strength of hydrogen bonds is required to firstly join two amino acid chains (polypeptides) together into a helix. Three helices are then bound into a triple helix by yet more hydrogen bonds. The result is a fibrous quaternary protein structure with a high tensile strength that the mammalian skeletal muscles could not function without. Tendons attach skeletal muscles to their respective bones and we would simply not be able to move without them. Other uses of hydrogen bonds in proteins include contributing to the specific conformational shape of globular proteins, called protein folding. A precise 3D shape is required in most enzymes so that the shape of binding site (active site) is complementary to the chemical reacting with the enzyme (substrate). Hydrogen bonds are essential, along with ionic bonds, covalent bonds, disulphide bonds and hydrophobic interactions, for making secondary structures (i.e. alpha-helices and beta-pleated sheets) coil into a tertiary structure. A tertiary structure, or a quaternary structure after further protein folding, can then be utilized as a specific enzyme within organisms to carry out specific metabolic reactions.
It is the hydrogen bonding found in water, in fact, that makes the metabolic reactions in the human body so efficient. The slight increase of strength between water molecules caused by hydrogen bonds means that in comparison to other fluids without hydrogen bonds, water requires a lot of energy to raise the temperature of it. This is called high heat specific capacity and may be defined as the amount of energy required to change the temperature of 1g of a substance by 1C, an attribute that is especially useful when the body is actively...