PMID: 6113246Jun 25, 1981Paper

Evidence for a change in the structure of glutamate dehydrogenase during its catalytic cycle under cryogenic conditions.

The Journal of Biological Chemistry
R E JohnsonH F Fisher

Abstract

Glutamate dehydrogenase binds alpha-ketoglutarate and NADPH to form a ternary complex whose ultraviolet difference spectrum exhibits a blue-shifted coenzyme absorption band and a distinctive aromatic amino acid perturbation. When ammonia is added to this complex at -42 degrees C in 50% methanol, initiating the enzymatic reaction, these two spectral features disappear at different rates. The kinetic independence of these two features is especially evident in the presence of excess L-glutamate. We propose that, under cryogenic conditions at least, there are two forms of the enzyme. The blue shift of the coenzyme absorption band reflects only the physical presence of an alpha-ketoglutarate molecule at the active site, while the distinctive aromatic amino acid perturbation reflects a change in enzyme structure caused by alpha-ketoglutarate binding which may persist in the absence of any bound alpha-ketoglutarate molecule. Simple red and blue shifts of model tyrosine and tryptophan compounds cannot be used to simulate the observed aromatic amino acid perturbation.

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