Large and small molecules

Looked at from a traditional chemical view­point, biochemistry can be divided roughly into two parts. One is concerned with large molecules, the other with small ones. In or­ganic chemistry, most of the familiar polymers (large molecules) are made up of hundreds of repeating units of one or two different small molecules. In living systems, three main types of polymers occur: polysaccharides, proteins, and nucleic acids. The polysaccharides tend to be like the industrially important polymers in that they are generally made up of only one or two different monomers (small molecules). Proteins, however, are made up from a much greater number of monomers. Although the basic core of the monomer is the same, there are differences in side chains. This leads to the immense variety of proteins found in living or­ganisms.

The structures of proteins are stored in coded form by the nucleic acid polymers ribo­nucleic acid (RNA) and deoxyribonucleic acid (DNAi. Although the nucleic acids also have a simple, repeating backbone, they use only a few different side chains. These are arranged in an extremely precise way. By contrast, most commercial polymers that incorporate more than one monomer have only an average structure. Thus, two molecules of monomer A may be incorporated for each three molecules of monomer B in a batch of a polymer. How­ever, a short length of polymer chain might still have a structure in which this ratio is re­versed. Only the overall composition of the batch, which contains a large number of indi­vidual molecules, has the correct ratio of A andB.

The control of living systems needs to be much more exact than a statistical average. Consequently, the natural synthesis of nucleic acid polymers is controlled very carefully. For example, the DNA strand that makes up a given gene in an organism always has the same structure. This is due to the individual way in which it is made. This involves the cop­ying of an existing molecule one monomer at a time. Where the gene ceases to be a precise

Biochemistry 105

copy, the result may be fatal for the organism concerned. The role of genetic mutation—as such change is called—in the development of cancer cells is an example.

Control mechanisms

The maintenance of life frequently depends on a complex series of controls. Many of the small molecules important in biochemistry carry out control functions. Examples of such biochemical controllers are hormones.

Small molecules are also needed for other functions. The ability of certain metals to cata­lyze (bring about or speed up) chemical reac­tions is apparent from a study of both inor­ganic and organic chemistry. Life would be impossible without catalysis. For a plant to capture the energy of sunlight, magnesium is essential. The oxygen we need to survive is captured for us with the help of iron atoms. In each case, however, the metal is held in a complex molecule, which is itself associated with an even more complex biochemical sys­tem.

More than 600 years ago, it was discovered that certain diseases could be cured by changes in the diet. It is only during the twen­tieth century that the relatively small mole­cules responsible—the vitamins—have been analyzed chemically. In many cases, vitamins have been shown to be low-molecular weight substances that can combine with specific pro­teins to form powerful catalytic agents called enzymes. Many enzymes, however, are com­posed of protein alone, with no vitamin-de­rived "coenzyme."

Interactions

Catalysts, whether biological or not, work by providing a more favorable environment for a reaction to take place. Complex biological catalysts—enzymes—also require a favorable environment to work effectively. Much of bio­chemistry is concerned with the environment in which molecules in living systems are found and with the ways this affects their behavior.

Chemistry provides a more detailed ac­count of the composition of materials than does classical physics, which is concerned mainly with their overall physical properties. In a similar way, biochemistry involves one more factor than chemistry. Inorganic and, de­spite its name, organic chemistry both deal with inanimate materials. Biochemistry studies reactions in living organisms.

Thus, a study of proteins and lipids (fats) in isolation can provide considerable informa­tion about them. But it requires a biochemical study of their interactions to help explain the properties of membranes in living cells. These membranes are made up of an intimate and organized physical combination of proteins and lipids.

Biochemistryis the chem­istry of life. Some of its most significant features are illus­trated in the photograph above. The starling (Sturnus vulgaris) is feeding. The food will have to go through the biochemical process of digestion. Flight requires energy, which comes from the biochemi­cal processes of metabo­lism. Green plants are the means by which solar en­ergy is biochemically con­verted into an edible energy source. The pigments that produce plant and animal colors are also created bio­chemically.

Two porphyrinsare the

plant pigment chlorophyll (top left) and the blood pig­ment heme (bottom left). These porphyrins play vital roles in the biochemical processes of photosynthesis and respiration respectively. Porphyrins are organic pig­ments that form complexes with metal radicals. The rad­icals account for the differ­ent properties of otherwise similar complex molecules.

Date: 2015-12-11; view: 1286