Mitochondria glutamate dehydrogenase

Energy-linked transhydrogenase, a protein in the inner mitochondrial membrane, couples the passage of protons down the electrochemical gradient from outside to inside the mitochondrion with the transfer of H from intramitochondrial NADH to NADPH for intramitochondrial enzymes such as glutamate dehydrogenase and hydroxylases involved in steroid synthesis. 

The other part of the urea cycle that has occurred is the conversion of the carbons of aspartate to fumarate. The fumarate is recycled back to oxaloacetate through TCA cycle reactions in the mitochondrion. Transamination with glutamate regenerates aspartate. The glutamate comes from the glutamate dehydrogenase reaction. 

Reactions of the urea cycle occur in both the mitochondria and cytosol of liver cells. Glutamate dehydrogenase, the citric acid cycle enzymes, carbamoyl phosphate synthetase I, and ornithine transcarbamoylase are localized in the mitochondrion, whereas the rest of the cycle occurs in the cytosol. This means that ornithine must be transported into mitochondria, and citrulline must be exported to the cytosol, in order for the cycle to proceed. 

Figure 16.S illustrates the reactions and the com-partmentalization of the enzymes of the urea cycle. The first reaction in urea biosynthesis is the mitochondrial formation of carbamoyl phosphate, the substrate of the urea cycle. The reaction utilizes an ammonium (NH4 ) ion, delivered into the mitochondrion as glutamate by the action of both the glutamate-aspartate (Section 11.3) and the glutamate-hydroxyl ion antiport carriers. Oxidative deamination of glutamate by glutamate dehydrogenase releases an NH4 ion.

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