Advances in Enzymology and Related Areas of Molecular

  • Encompasses the most recent advances within the box.
  • New sequence editor, Daniel Purich, is a well known biochemist and enzymologist.


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They proposed that binding of Kþ was linked to formation of the NAD binding site, a concept that has become a consistent theme over the decades. The authors presented a cogent discussion of influence of the substrate concentrations employed on the apparent kinetic mechanism because of differences in binding rates. 1 M concentration of Kþ needed for saturation the secondorder binding rate of Kþ is much faster than the binding rates for IMP or NAD at their typical $10À4 M concentrations. During steady-state turnover a significant proportion of the enzyme is in complexes with the XMP product since XMP release is partially rate limiting.

This is in accord with the Mþ-independent rate of hydrolysis of 2-Cl-IMP. The structure of the ribavirin monophosphate complex of the T. foetus enzyme (1me8) showed a density attributed to a Naþ ion at a similar location, coordinated to the carbonyls of Gly314, Gly316, and Cys319 from one subunit and Glu485*, Gly486*, and Gly487* from another subunit (19). Prosise et al. (19) proposed that in the presence of IMP the formation of the covalent intermediate is required for “recruitment” of the cation, but the smaller ring system in ribavirin monophosphate allows the cation to bind in the absence of a covalent linkage.

The Asp338Ala mutant (corresponding to human Asp364; INOSINE 50 -MONOPHOSPHATE DEHYDROGENASE 23 see Figure 10A) affected kcat more than 600-fold and increased the Km for IMP, consistent with the crystallographically observed binding of the homologous residue to the ribose hydroxyls of the substrate in other IMPDHs and suggesting a role in catalysis. Figure 10. Structures of the active site. (A) The substrate site as seen in the hamster E-XMP*ÁMPA structure (28). (B) SAD bound to the NAD site as seen in the complex with the human type 2 6-Cl-IMP adduct (31).

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