Enzymes, branching

Transaminase enzymes (also called aminotransferases) specifically use 2-oxoglutarate as the amino group acceptor to generate glutamate but some have a wide specificity with respect to the amino donor. For example, the three branched-chain amino acids leucine, isoleucine and valine, all serve as substrates for the same enzyme, branched-chain amino acid transaminase, BCAAT ... 

Transamination Removal of the amino groups of all three amino acids is catalyzed by a single enzyme, branched-chain a-amino acid aminotransferase. 

R-Enzyme was first isolated from broad beans and potatoes by Peat and coworkers. Purification was difficult in view of the relatively small amounts present and the large number of contaminating enzymes, particularly a maltodextrinyl transferase (D-enzyme), branching (Q-) enzyme, and alpha-amylase. However, a procedure was developed that gave preparations suflBciently pure to permit routine use of this enzyme during some ten years for debranching of amylaceous polysaccharides. ... 

Fig. 1. The revised pathway of 22 6n-3 biosynthesis from 18 3n-3. 18 3n-3 is converted to 24 6n-3 in microsomes then 24 6n-3 enters peroxisomes for one cycle of fatty acid (3-oxidation, via straight-chain acyl-CoA oxidase, D-bifunctional protein, and 3-oxo-acyl-CoA thiolase or sterol carrier protein X to produce 22 6n-3. The peroxisomal fatty acid (3-oxidation enzymes, branched-chain acyl-CoA oxidase, and L-bifunctional protein are not thought to participate in 22 6n-3 synthesis.

This enzyme has two forms. One is a homodimer made from the nnion of two identical amino acid chains, both encoded by the phenylalanine hydroxylase gene [6]. The other is a homotetra-mer made from the union of four identical amino acid chains, all encoded by the phenylalanine hydroxylase gene [6]. In contrast maple syrnp urine disease (MSUD) is caused by a deficiency of the enzyme branched-chain keto acid dehydro-... 

Endogenous production of HMB occurs in muscle and liver (Figure 12.1) and possibly other tissues. The first step in HMB formation is the transamination of leucine to KIC, which occurs in both the cytosol and mitochondria of muscle cells. In the mitochondria, KIC is irreversibly oxidized to isovaleryl-CoA by the enzyme branched-chain a-keto acid dehydrogenase. Isovaleryl-CoA then undergoes further metabolic steps within the mitochondria (Figure 12.1), yielding P-hydroxy- -methylglutaryl-CoA (HMG-CoA). Further metabolism by the enzyme HMG-CoA lyase results in the end products acetoacetate and acetyl-CoA. Approximately 90% of KIC is oxidized to isovaleryl CoA in liver mitochondria and ultimately to acetoacetate and acetyl-CoA. 

The glucose molecules are added on to the existing chain in such a way that they are joined in the 1 4a configuration and repetition of the process results in the production of the linear polymer amylose (Fig. 5.11). The synthesis of branched molecules of amylopection is achieved by the operation of another enzyme branching enzyme) which joins linear polymers in the 1 6a configuration (see p. 108 for information on the phosphorylase reaction which may also be involved in a minor way in the synthesis and degradation of 1 4a-polyglucans). 

Cyanohydrin Synthesis. Another synthetically useful enzyme that catalyzes carbon—carbon bond formation is oxynitnlase (EC 4.1.2.10). This enzyme catalyzes the addition of cyanides to various aldehydes that may come either in the form of hydrogen cyanide or acetone cyanohydrin (152—158) (Fig. 7). The reaction constitutes a convenient route for the preparation of a-hydroxy acids and P-amino alcohols. Acetone cyanohydrin [75-86-5] can also be used as the cyanide carrier, and is considered to be superior since it does not involve hazardous gaseous HCN and also virtually eliminates the spontaneous nonenzymatic reaction. (R)-oxynitrilase accepts aromatic (97a,b), straight- (97c,e), and branched-chain aUphatic aldehydes, converting them to (R)-cyanohydrins in very good yields and high enantiomeric purity (Table 10). 

Hundreds of metabohc reac tions take place simultaneously in cells. There are branched and parallel pathways, and a single biochemical may participate in sever distinct reactions. Through mass action, concentration changes caused by one reac tion may effect the kinetics and equilibrium concentrations of another. In order to prevent accumulation of too much of a biochemical, the product or an intermediate in the pathway may slow the production of an enzyme or may inhibit the ac tivation of enzymes regulating the pathway. This is termed feedback control and is shown in Fig. 24-1. More complicated examples are known where two biochemicals ac t in concert to inhibit an enzyme. As accumulation of excessive amounts of a certain biochemical may be the key to economic success, creating mutant cultures with defective metabolic controls has great value to the produc tion of a given produc t. 

Thousands of reactions mediated by an equal number of enzymes are occurring at any given instant within the cell. Metabolism has many branch points, cycles, and interconnections, as a glance at a metabolic pathway map reveals... 

Kinetics is the branch of science concerned with the rates of chemical reactions. The study of enzyme kinetics addresses the biological roles of enzymatic catalysts and how they accomplish their remarkable feats. In enzyme kinetics, we seek to determine the maximum reaction velocity that the enzyme can attain and its binding affinities for substrates and inhibitors. Coupled with studies on the structure and chemistry of the enzyme, analysis of the enzymatic rate under different reaction conditions yields insights regarding the enzyme s mechanism of catalytic action. Such information is essential to an overall understanding of metabolism. 

Pantothenic acid, sometimes called vitamin B3, is a vitamin that makes up one part of a complex coenzyme called coenzyme A (CoA) (Figure 18.23). Pantothenic acid is also a constituent of acyl carrier proteins. Coenzyme A consists of 3, 5 -adenosine bisphosphate joined to 4-phosphopantetheine in a phosphoric anhydride linkage. Phosphopantetheine in turn consists of three parts /3-mercaptoethylamine linked to /3-alanine, which makes an amide bond with a branched-chain dihydroxy acid. As was the case for the nicotinamide and flavin coenzymes, the adenine nucleotide moiety of CoA acts as a recognition site, increasing the affinity and specificity of CoA binding to its enzymes. 

FIGURE 23.15 The reactions of glycogen debranching enzyme. Transfer of a group of three o -(l 4)-linked glucose residues from a limit branch to another branch is followed by cleavage of the o -(l 6) bond of the residue... 

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