Consider the following questions about glutamate dehydrogenase.
(a) The reaction as shown on page 569 has NH3 as a reactant, instead of NH4+, which is far more abundant at physiological pH. Why is NH3 preferred?
(b) Glutamate dehydrogenase has a KM for ammonia (NH3) of ∼ 1 mM. However, at physiological pH the dominant ionic species is ammonium ion, NH4 + (pKa = 9.2). Calculate the velocity (as a fraction of Vmax) that would be achieved by glutamate dehydrogenase if the total intracellular ammonia concentration (NH3 + NH4+) is 100 µM (approximate physiological concentration). Assume a mitochondrial matrix pH of 8.0.
(c) The thermodynamic equilibrium for the reaction greatly favors a-ketoglutarate reduction, yet in mitochondria the enzyme acts primarily to oxidize glutamate to a-ketoglutarate. Explain.
(d) Propose a reasonable mechanism for this reaction.