Types of Biochemical Reactions

Each of the processes shown in Figure 2.8 can be described by a Michaelis-Menten type of biochemical reaction, a standard generalized mathematical equation describing the interaction of a substrate with an enzyme. Michaelis and Men ten realized in 1913 that the kinetics of enzyme reactions differed from the kinetics of conventional... 

Table 10-1 Types of Biochemical Reactions with Ionic Mechanisms... 

The removal of the a-amino group from amino acids involves two types of biochemical reactions transamination and oxidative deamination. Both reactions have been described (Section 14.2). (Recall that transamination reactions occupy important positions in nonessential amino acid synthesis.) Because these reactions are reversible, amino groups are easily shifted from abundant amino acids and used to synthesize those that are scarce. Amino groups become available for urea synthesis when amino acids are in excess. Urea is synthesized in especially large amounts when the diet is high in protein or when there is massive breakdown of protein, for example, during starvation. 

The degradation of most amino acids begins with removal of the a-amino group. Two types of biochemical reactions are involved in amino group removal transamination and oxidative deamination. 

Enzymes are proteins that catalyze chemical reactions that take place in living systems. A catalyst is a molecule that speeds up a reaction but is not consumed by that reaction. Enzymes are particularly interesting because they often are quite specific, capable of catalyzing only one type of biochemical reaction. Learning about the structure of the active site is often a... 

The decarboxylation of pyruvic acid is an example of a more general type of biochemical reaction the decarboxylation of a-keto acids. The reaction is complex and occurs in several consecutive steps. The intermediates have been identified, but little is known of the enzymes involved. The reaction starts with the complexion of pyruvic acid with one molecule of enzyme-bound thiamine pyrophosphate. This is followed by decarboxylation of pyruvic acid and the formation of an intermediate, 2-acetylthiamine pyrophosphate, in which the aldehyde carbon of the acetyl is bound to the carbon 2 of the thiozole ring of the thiamine pyrophosphate. In the second step, the aldehyde is oxidized, the disulfide bond of enzyme-bound lipoic acid is reduced, and the free enzyme-bound thiamine pyrophosphate is restored. The tWrd step of the reaction involves the transacylation from reduced lipoic acid to CoA. Finally, lipoic acid is reoxidized by the catalytic activity of an NAD-dependent flavoprotein, lipoic dehydrogenase (see Fig. 1-14). 

One of these is by heterotrophic CO2 fixation, first discovered in propionic acid bacteria by Wood and Workman (1936, 1938). The CO2 fixation is especially high when the bacteria are grown on glycerol. Both the fixation of carbon dioxide and formation of succinate by propionibacteria are inhibited by NaF (Wood and Workman, 1940). Another pathway of the condensation of C3- and Ci-compounds was discovered by Swick and Wood (1960), who showed that lliese bacteria contain a transcarboxylase that has a role in producing propionate firom methylmalonyl-CoA. This was preceded by an observation (Wood and Leaver, 1953) fiiat propionic acid bacteria have two mechanisms for CO2 fixation and only one of these is inhibited by NaF. The enzyme, discovered by Swick and Wood, catalyzed a new type of biochemical reactions— transcarboxylation between a carboxyl donor and an acceptor ... 

Among biochemical compounds, S-adenosylmethionine owns a unique versatility its chemical structure, with three substituents attached to the sulfonium pole, permits its participation in several different types of biochemical reactions involving both low and high molecular weight compounds as acceptors of alkyl groups. 

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