Biochemistry

Some enzymes act on substrates having only a certain spatial configuration, i.e., they exhibit spatial, optical, stereoisomeric specificity. For example, yeast cell enzymes catalyze fermentation - the breakdown of D-glucose to form ethanol and carbon monoxide, but do not affect L-glucose. The stereoisomeric specificity is determined not only by D and L, but also by other types of isomerism. An enzyme acting on a substrate with a cis configuration, as a rule, does not attack the transisomer of this compound, and vice versa. Of the other properties of enzymes should indicate the reversibility of their action. Depending on the concentration of the initial and final reaction products, enzymes can catalyze both direct and reverse reactions, i.e., cause both decomposition and synthesis processes. For the first time, the outstanding Russian chemist A.Ya.Danilevsky (1886) drew attention to this feature. He showed that the gastric juice enzyme pepsin, which breaks down proteins into peptones and albumoses (fragments of a protein molecule), under certain conditions can catalyze the synthesis of proteins from these intermediate products of protein metabolism. However, it was later proved that the reversibility of the action is not characteristic of all enzymes. Many processes of decay and synthesis occur not only under the influence of various enzymes, but also according to a different mechanism. In a living organism, cleavage and synthesis are carried out in most cases by different enzymes, even when this enzyme is able to catalyze the reaction in different directions. The synthesis involves enzymes that use the energy of the ATP hydrolyzate and some other compounds. So, during hydrolysis of glycogen in the liver with the help of amylase and maltase, glucose is formed. Glycogen is synthesized from glucose in the same place in the liver, but its formation is not a consequence of the reversible hydrolysis reaction, but occurs in a more complex way. Enzymes are characterized by sensitivity to temperature changes (thermolability), especially to its increase. High temperature, denaturing proteins, inactivates enzymes (Fig. 3.3). Figure 3.3 - Thermolability of enzymes 85