J. Biol. Chem., Vol. 259, Issue 4, 2252-2256, Feb, 1984
The interaction between Escherichia coli aspartokinase-homoserine dehydrogenase and 3-acetylpyridine-adenine dinucleotide phosphate (reduced), an analog of NADPH
K Muller and JR Garel
The interaction of 3-acetylpyridine-adenine dinucleotide phosphate, a
structural analog of NADPH, with aspartokinase-homoserine dehydrogenase has
been studied by fluorescence and activity measurements. This analog binds
to the same site and with the same affinity as does the natural coenzyme.
Also, the binding of homoserine to the dehydrogenase site or that of
threonine to the regulatory site is the same whether NADPH or its analog is
bound to the enzyme. So NADPH and its analog appear as equivalent in the
formation of various stable enzyme-ligand(s) complexes. The analog
resembles NADPH enough so that it is a substrate that the enzyme can use to
reduce aspartate semialdehyde; the maximum velocity of this dehydrogenase
reaction is however reduced by 90% as compared to that with NADPH. It seems
as if one of the catalytic steps is affected by the replacement of a--CONH2
group by--COCH3. Another difference between the two coenzymes is that the
reaction with the analog is insensitive to threonine, whereas that with
NADPH is inhibited. The lack of inhibition is not due to a lack of binding,
but rather to a difference in the ternary complexes composed of enzyme,
coenzyme, and substrate. A possible relationship between the inhibition by
threonine and the mechanism of the dehydrogenase reaction is thus suggested
by this comparison between NADPH and its analog.
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