Lesson Twelve

Lesson Twelve

Enzymes, the catalysts of biological systems, are remarkable molecular devices that determine the patterns of chemical transformations. They also mediate the transformation of one form of energy into another. The most striking characteristics of enzymes are their catalytic power and specificity. Catalysis takes place at a particular site on the enzymes called active site. Nearly all known enzymes are proteins. However, proteins do not have an absolute monopoly on catalysis. The discovery of catalytically active RNA molecules provides compelling evidence that RNA was an early biocatalyst.

Proteins as a class of macromolecules are highly effective catalysts for an enormous diversity of chemical reactions because of their capacity to specifically bind a very wide range of molecules. By utilizing the full repertoire of intermolecular forces, enzymes bring substrates together in an optimum orientation, the prelude to making and breaking chemical bonds. They catalyze reactions by stabilizing transition states, the highest energy species in reaction pathways. By selectively stabilizing a transition states, an enzyme determine which one of several potential chemical reactions actually takes place.

Enzymes accelerate reactions by factors of as much as a million or more. Indeed, most reactions in biological systems do not take place at perceptible rates in the absence of enzymes. Even a reaction as simple as the hydration of carbon dioxide is catalyzed by an enzyme, called carbonic anhydrase. The transfer of CO2 from the tissues into blood and then to the alveolar air would be less complete in the absence of this enzyme. In fact, carbonic anhydrase is one of the fastest enzymes known. Each enzyme molecule can hydrate 106 molecules of CO2 per second. This catalyzed reaction is 107 times as fast as the uncatalyzed one. We will consider the mechanism of carbonic anhydrase in Chapter 9. Enzymes are highly specific both in the reactions that they catalyze and in their choice of reactants, which are called substrates. An enzyme usually catalyzes a single reaction or a set of closely related reactions. Side reactions leading to the wasteful formation of by—products are rare in enzymes catalyzed reactions, in contrast with the uncatalyzed ones.

Proteolytic enzymes differ markedly in their degree of substrate specificity. Subtilisin, which is found in certain bacteria, is quite undiscriminating: it will cleave any peptide bond with little regard to the identity of the adjacent side chains. Trypsin, a digestive enzyme, is quite specific and catalyzes the splitting of peptide bonds only on the carboxyl side of lysine and arginine residues. Thrombin, an enzyme that participates in blood clotting, is even more specific than trypsin. It catalyzes the hydrolysis of Arg—Gly bonds in particular peptide sequence only.

DNA polymerase I, a template—directed enzyme, is another highly specific catalyst. It adds nucleotide to a DNA strand that being synthesized, in a sequence determined by the sequence of nucleotide in another DNA strand the serve as a template. DNA polymerase I is remarkably precise in carrying out the instructions given by the template. It inserts the wrong nucleotide into a new strand less than one in a million times.

The specificity of an enzyme is due to the precise interaction of the substrate with the enzyme. This precision is a result of the intricate three—dimensional structure of the enzyme protein.

The catalytic activity of many enzymes depends on the presence of small molecules termed cofactors, although the precise role varies with the cofactor and the enzyme. Such an enzyme without its cofactor is referred to as an apoenzyme; the complete, catalytically active enzyme is called a holoenzyme. Cofactors can be subdivided into two groups: metal and small organic molecules. The enzyme carbonic anhydrase, for example, required Zn+2 for its activity. Glycogen phosphorylases, which mobilize glycogen for energy, require the small organic molecule pyridoxal phosphate (PLP).

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