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The amino acid sequences of the a subunits of tryptophan synthase from ten different microorganisms were aligned by standard procedures. The alpha helices, beta strands and turns of each sequence were predicted separately by two standard prediction algorithms and averaged at homologous sequence positions. Additional evidence for conserved secondary structure was derived from profiles of average hydropathy and chain flexibility values, leading to a joint prediction. There is good agreement between (1) predicted beta strands, maximal hydropathy and minimal flexibility, and (2) predicted loops, great chain flexibility, and protein segments that accept insertions of various lengths in individual sequences. The a subunit is predicted to have eight repeated beta-loop-alpha-loop motifs with an extra N-terminal alpha helix and an intercalated segment of highly conserved residues. This pattern suggests that the territory structure of the a subunit is an eightfold alpha/beta barrel. The distribution of conserved amino acid residues and published data on limited proteolysis, chemical modification, and mutagenesis are consistent with the alpha/beta barrel structure. Both the active site of the a subunit and the combining site for the beta 2 subunit are at the end of the barrel formed by the carboxyl-termini of the beta strands.

L-Malate dehydrogenase from the extremely thermophilic mathanogen Methanothermus fervidus was isolated and its phenotypic properties were characterized. The primary structure of the protein was deduced from the coding gene.The enzyme is a homomeric dimer with a molecular mass of 70 kDa, possesses low specifity for NAD' or NADP' and catalyzes preferentially the reduction of oxalacetate. The temperature dependence of the activity as depicted in the Arrhenius and van't Hoff plots shows discontinuities near 52"C, as was found for glyceraldehyde-3-phosphate dehydrogenase from the same organism.With respect to the primary structure, the archaebacterial L-malate dehydrogenase deviates strikingly from the eubacterial and eukaryotic enzymes. The sequence similarity is even lower than that between the L-malate dehydrogenases and L-lactate dehydrogenases of eubacteria and eukaryotes.The phylogenetic meaning of this relationship is discussed.Archaebacteria differ from eubacteria and eukaryotes in many respects, encouraging studies on the physiological and biochemical bases of their phenotype as well as on their phylogenetic position within the living world.One of the most striking features of many archaebacteria is represented by the ability to grow under extreme conditions, i.e. at high salinity, low pH, or at extremely high temperatures. In order to understand the biochemical background of thermoadaptation, we have been investigating the structure and function of enzymes from extremely thermophilic archaebacteria.Sequence comparisons of glyceraldehyde-3-phosphate dehydrogenases [phosphorylating ~-glyceraldehyde-3-phosphate : NAD(P)+ oxidoreductase (E.C. 1.2.1 . -)I from closely related meso-and thermophilic methanogenic bacteria [l] provided preliminary evidence for the structural prerequisites of thermoadaptation. To analyze the structure/function relationship of thermophilic enzymes from archaebacteria on a broader basis, we included L-malate dehydrogenase in our studies.Like glyceraldehyde-3-phosphate dehydrogenase, L-malate dehydrogenase, occurs in all three urkingdoms. Detailed structural analyzes, however, have only been done with eubacterial and eukaryotic enzymes [2 -101. Thus, structural work on the archaebacterial L-malate dehydrogenase should provide new information, not only for the biochemical Enzymes. L-Malate dehydrogenase (NAD+) (EC 1.1.1.37); L-malate dehydrogenase (NADP') (EC 1.1.1.82); ~-glyceraldehyde-3-phosphate dehydrogenase (NAD') (EC 1.2.1.12); D-glyceraldehyde-3-phosphate dehydrogenase (NAD+/NADP') (EC 1.2.1.-); L-lactate dehydrogenase (EC 1.1.1.27).Note. The novel nucleotide sequence data published here have been deposited with the EMBL sequence data bank and are available under the accession number X51714. The novel amino acid sequence data have been deposited with the EMBL sequence data bank.thermoadaptation, but also for the evolutionary differentiation of the enzyme throughout all three urkingdoms.Here we describe some phenotypic properties of L-malate dehydrogenase from the extremely thermop...

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