A tentative primary structure of the NADP-specific glutamate dehydrogenase [L-glutamate: NADP oxidoreductase (deaminating), EC 1.4.1.41 from Neurospora crassa has been determined. The proposed sequence contains 452 amino-acid residues in each of the identical subunits of the hexameric enzyme. Comparison of the sequence with that of the bovine liver enzyme reveals considerable homology in the amino-terminal portion of the chain, including the vicinity of the reactive lysine, with only shorter stretches of homology within the carboxyl-terminal regions. The significance of this distribution of homologous regions is discussed.Investigation of structure-function relationships in enzymes has progressed to the state where assignment of a specific function to certain amino-acid residues in a protein is possible. Information on the function of individual residues in the sequence can be obtained from specific chemical modifications, from amino-acid replacements during the course of evolution, or from artificially induced mutations within a species. Investigations of the sequence of the NADP-specific glutamate dehydrogenase (GDH) from Neurospora crassa were undertaken with a view to attacking all phases of this problem. Thus, comparison of the sequence proposed herein with that obtained earlier for the vertebrate enzymes should afford some general information on the importance of groups of residues for catalysis, for regulation of activity, and for conformation. Moreover, determination of the sites of substitution in the mutants described by Fincham and coworkers (1, 2) should provide specific information on the role of individual residues. In this communication a summary of our findings will be presented; the laboratories involved will report separately upon the experimental details elsewhere.
RESULTS AND DISCUSSIONLike the vertebrate enzymes, GDH (NADP) of Neurospora is composed of six identical polypeptide chains each having a molecular weight of about 48,800 (3, 4), somewhat less than the subunit weight of 55,393 calculated from the sequence of the bovine enzyme (5). The proposed sequence of the Neurospora enzyme, shown in Fig. 1, was derived by studies in Leeds of the peptides obtained by hydrolysis of the intact Abbreviation: GDH, glutamate dehydrogenase.
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