An exploration of restricted lactate dehydrogenase subunit association

The enzyme lactate dehydrogenase (LDH) is important to organisms because of its role in the metabolic process of glycolysis and it provides organisms with a vital source of the coenzyme NAD under anaerobic conditions (Markert et al. 1975). Like many other proteins, this enzyme exists in multiple molecular forms, or isozymes, all of which have the same chemical function (Markert et al. 1975). As the result of two types of subunits combining to form tetramers, LDH exists in five isozymes, a system that characterizes most mammals, birds, fish, and amphibians (Sherr 1968, Markert et al. 1975). Usually, all five isozymes can all be found within an organism at varying concentrations in different tissues (Sherr 1968). However, some vertebrates, such as certain amphibians and fish, exhibit restricted subunit association (Markert et al. 1975). In particular, the Red-Spotted Newt, Notophthalmus viridescens, is restricted to the production of only the LDH homotetramers (Sherr 1968), and the Chinese Warty Newt, Paramesotriton chinensis, is restricted to the production of the homotetramers and the A2B2 tetramer. After sequencing the LDH genes that encode for the two types of subunits in both of these species of newt, the primary structures were analyzed in order to determine if there is a mutation at one, or both, of the loci that is interfering with random subunit association. Secondary and tertiary structures were modeled based on these amino acid sequences and the human LDH homologs due to the fact that human LDH has been crystallized and humans form all five isozymes. Detailed analyses predict particular amino acid residues as well as secondary structures that may be responsible for restricted subunit association in these two species of newt