Basso LA, Engel PC, Walmsley AR. Kinetic studies on the binding of 1,N6-etheno-NAD+ to glutamate dehydrogenase from Clostridium symbiosum.
BIOCHIMICA ET BIOPHYSICA ACTA 1997;
1340:63-71. [PMID:
9217015 DOI:
10.1016/s0167-4838(97)00032-0]
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Abstract
The mechanism of the binding of reduced coenzyme (NAD+) to clostridial glutamate dehydrogenase (GDH) was determined by transient kinetics. The fluorescent 1,N6-ethenoadenine analogue of NAD+ (epsilonNAD+) was used as a probe of nucleotide binary and ternary complex formation because the binding of NAD+ is optically silent. The kinetics of epsilonNAD+ binding were consistent with a 3-step binding process. The enzyme was found to oscillate between two conformational forms, termed E1 and E2, in the presence and absence of L-glutamate. However, L-glutamate shifted the equilibrium from 96.8% to 99% of the enzyme in the E1 form. The rapid-equilibrium binding of epsilonNAD+ to the E2 form was rate limited by a slow isomerisation of the ternary complex as the binary complex became saturated with epsilonNAD+. The L-glutamate binary complex had a greater affinity for the coenzyme (Kd = 11 microM) than the free enzyme (Km = 39 microM), indicative of a positive interaction of the substrate and coenzyme binding sites. Steady-state studies were also indicative of a positive interaction in the formation of the catalytic complex, with this complex having a Kd for epsilonNAD+ of 6.8 microM. Consequently, there is stabilization of successive complexes on the reaction pathway.
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