What is it about?
Under certain conditions clostridial GDH, a hexameric enzyme, shows a Hill coefficient close to the theoretical maximum of 6 for total cooperativity in glutamate binding.
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Why is it important?
At high pH and 40mM glutamate, clostridial GDH is inactive. We had attributed this to a pH-dependent, reversible conformational change. However, in this work we discovered that, going to still higher [Glu] there was a steep sigmoid increase to full activity at 200mM. The extraordinary Hill coefficient of ~5.8 is close to the theoretical maximum of 6 for a hexamer, implying total cooperativity - i.e. all subunits inactive OR all subunits active. A world record? It implies virtually no hybrid conformations - i.e. either all 6 active or all 6 inactive.
Perspectives
The story of GDH was dominated for decades by bovine GDH, commercially available and with complex regualtory behaviour. This story was hampered, however, by the failure to get a crystallographic structure for the enzyme through hte 1970s. 80s and 90s. We deliberately switched around 1980 to a bacterial GDH in the hope of crystals and also in the hope of a cloned gene, DNA sequence and mutagenesis (mammalian genes at that stage being seenas more challenging). This led in collaboration with Davidd Rice's group to the first high-resolution structure for a GDH and accordingly much insight into substrate specificity, catalysis, conezyme specificity etc. Studies of this enzyme also showed that it is allosteric and this is in itself revealing as the bacterial enzyme lacks about 50 amino acids thought to be responsible for allosteric regulation by ADP, GTP etc. in bovine GDH. More recently the structure has finally been solved for bovine GDH showing the 50 residues in question form an antenna region that binds the mononucleotide regulators. The subsequent discussion has seemed to ignore the fact that the hexameric core minus any antenna is already highly allosteric.
Professor Paul C Engel
University College Dublin
Read the Original
This page is a summary of: Positive Cooperativity with Hill Coefficients of up to 6 in the Glutamate Concentration Dependence of Steady-State Reaction Rates Measured with Clostridial Glutamate Dehydrogenase and the Mutant A163G at High pH, Biochemistry, September 1995, American Chemical Society (ACS),
DOI: 10.1021/bi00036a014.
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