Distinct conformation-mediated functions of an active site loop in the catalytic reactions of NAD-dependent D-lactate dehydrogenase and formate dehydrogenase

作者：Shinoda T; Arai K; Shigematsu Iida M; Ishikura Y; Tanaka S; Yamada T; Kimber MS; Pai EF; Fushinobu S; Taguchi H^*

来源：Journal of Biological Chemistry, 2005, 280(17): 17068-17075.

DOI：10.1074/jbc.M500970200

The three-dimensional structures of NAD-dependent D-lactate dehydrogenase (D-LDH) and formate dehydrogenase (FDH), which resemble each other, imply that the two enzymes commonly employ certain main chain atoms, which are located on corresponding loop structures in the active sites of the two enzymes, for their respective catalytic functions. These active site loops adopt different conformations in the two enzymes, a difference likely attributable to hydrogen bonds with Asn(97) and Glu(141), which are also located at equivalent positions in D-LDH and FDH, respectively. X-ray crystallography at 2.4-angstrom resolution revealed that replacement of Asn(97) with Asp did not markedly change the overall protein structure but markedly perturbed the conformation of the active site loop in Lactobacillus pentosus D-LDH. The Asn(97) --> Asp mutant D-LDH exhibited virtually the same k(cat), but about 70-fold higher K-M value for pyruvate than the wild-type enzyme. For Paracoccus sp. 12-A FDH, in contrast, replacement of Glu(141) with Gln and Asn induced only 5.5- and 4.3- fold increases in the K-M value, but 110 and 590-fold decreases in the k(cat) values for formate, respectively. Furthermore, these mutant FDHs, particularly the Glu(141) --> Asn enzyme, exhibited markedly enhanced catalytic activity for glyoxylate reduction, indicating that FDH is converted to a 2-hydroxyacid dehydrogenase on the replacement of Glu(141). These results indicate that the active site loops play different roles in the catalytic reactions of D-LDH and FDH, stabilization of substrate binding and promotion of hydrogen transfer, respectively, and that Asn(97) and Glu(141), which stabilize suitable loop conformations, are essential elements for proper loop functioning.

• 出版日期2005-4-29