Lactate dehydrogenase forms

Electrons are transferred from NADH to pyruvate by lactate dehydrogenase, forming NAD and lactate (Figure 6-1). 

Figure 8.10 The quaternary structure of proteins. The enzyme lactate dehydrogenase (EC 1.1.1.27) has a relative molecular mass of approximately 140 000 and occurs as a tetramer produced by the association of two different globular proteins (A and B), a characteristic that results in five different hybrid forms of the active enzyme. The A and B peptides are enzymically inactive and are often indicated by M (muscle) and H (heart). The A4 tetramer predominates in skeletal muscle while the B4 form predominates in heart muscle but all tissues show most types in varying amounts.

L-Amino acid oxidase has been used to measure L-phenylalanine and involves the addition of a sodium arsenate-borate buffer, which promotes the conversion of the oxidation product, phenylpyruvic acid, to its enol form, which then forms a borate complex having an absorption maximum at 308 nm. Tyrosine and tryptophan react similarly but their enol-borate complexes have different absorption maxima at 330 and 350 nm respectively. Thus by taking absorbance readings at these wavelengths the specificity of the assay is improved. The assay for L-alanine may also be made almost completely specific by converting the L-pyruvate formed in the oxidation reaction to L-lactate by the addition of lactate dehydrogenase (EC 1.1.1.27) and monitoring the oxidation of NADH at 340 nm. 

The isoenzymes within a particular family will operate under slightly different circumstances or may respond differently to metabolite feedback regulation. In this case there is some degree of structural similarity between the different isoenzymes. The usual example used to illustrate this point is lactate dehydrogenase (LD), which has five isoenzymes, each composed of four sub-units. The subunits are of two types, H or M, so the five forms arise as follows ... 

The remaining three antiparallel /3 structures form a miscellaneous category (see Fig. 84). Lactate dehydrogenase d2 and gene 5 protein each has several two-stranded antiparallel j8 ribbons, but they do not coalesce into any readily described overall pattern. The N-terminal domain of tomato bushy stunt virus protein has a unique /3 structure in which equivalent pieces of chain from three different subunits wrap around a 3-fold axis to form what has been called a /3 annulus (Harrison et ah, 1978). Each of the three chains contributes a short strand segment to each of three three-stranded, interlocking /3 sheets. This domain provides one of the subunit contacts that hold the virus... 

Figure 6.1 Pathways involved in glucose oxidation by plant cells (a) glycolysis, (b) Krebs cycle, (c) mitochondrial cytochrome chain. Under anoxic conditions. Reactions 1, 2 and 3 of glycolysis are catalysed by lactate dehydrogenase, pyruvate decarboxylase and alcohol dehydrogenase, respectively. ATP and ADP, adenosine tri- and diphosphate NAD and NADHa, oxidized and reduced forms of nicotinamide adenine dinucleotide PGA, phosphoglyceraldehyde PEP, phosphoenolpyruvate Acetyl-CoA, acetyl coenzyme A FP, flavoprotein cyt, cytochrome e, electron. (Modified from Fitter and Hay, 2002). Reprinted with permission from Elsevier...

The active form of lactate dehydrogenase (mass 144 kDa) is a tetramer consisting of four subunits (1). Each monomer is formed by a peptide chain of 334 amino acids (36 kDa). In the tetramer, the subunits occupy equivalent positions (1) each monomer has an active center. Depending on metabolic conditions, LDH catalyzes NADH-de-pendent reduction of pyruvate to lactate, or NAD -dependent oxidation of lactate to pyruvate (see p. 18). 

The active center of an LDH subunit is shown schematically in Fig. 2. The peptide backbone is shown as a light blue tube. Also shown are the substrate lactate (red), the coenzyme NAD (yellow), and three amino acid side chains (Arg-109, Arg-171, and His-195 green), which are directly involved in the catalysis. A peptide loop (pink) formed by amino acid residues 98-111 is also shown. In the absence of substrate and coenzyme, this partial structure is open and allows access to the substrate binding site (not shown). In the enzyme lactate NAD"" complex shown, the peptide loop closes the active center. The catalytic cycle of lactate dehydrogenase is discussed on the next page. 

Different abortives may be formed with alternative products or substrates. Such procedures can be useful in helping to distinguish Theorell-Chance mechanisms from ordered systems with abortive complexes . In the case of lactate dehydrogenase, the E-pyruvate-NAD+ and E-lactate-NADH abortive complexes may play a regulatory roles in aerobic versus anaerobic metabolism. 

A sequential enzyme-catalyzed reaction mechanism in which two substrates react to form two products and in which there is a preferred order in the binding of substrates and release of products. Several enzymes have been reported to have this type of binding mechanism, including alcohol dehydrogenase , carbamate kinase , lactate dehydrogenase , and ribitol dehydrogenase. ... 

RGURE 7 An oxidation-reduction reaction. Shown here is the oxidation of lactate to pyruvate. In this dehydrogenation, two electrons and two hydrogen ions (the equivalent of two hydrogen atoms) are removed from C-2 of lactate, an alcohol, to form pyruvate, a ketone. In cells the reaction is catalyzed by lactate dehydrogenase and the electrons are transferred to a cofactor called nicotinamide adenine dinucleotide. This reaction is fully reversible pyruvate can be reduced by electrons from the cofactor. In Chapter 13 we discuss the factors that determine the direction of a reaction. 

When animal tissues cannot be supplied with sufficient oxygen to support aerobic oxidation of the pyruvate and NADH produced in glycolysis, NAD+ is regenerated from NADH by the reduction of pyruvate to lactate. As mentioned earlier, some tissues and cell types (such as erythrocytes, which have no mitochondria and thus cannot oxidize pyruvate to C02) produce lactate from glucose even under aerobic conditions. The reduction of pyruvate is catalyzed by lactate dehydrogenase, which forms the l isomer of lactate at pH 7 ... 

Lactate, formed by the action of lactate dehydrogenase, is the final... 

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