(Choice A) Alanine transaminase catalyzes the transfer of an amino group from alanine to a-ketoglutarate, generating pyruvate that can be used for gluconeogenesis.
(Choice 8) Aspartate is a nonessential amino acid; it can be converted into oxaloacetate for use in the TCA cycle by aspartate transaminase.
(Choice C) Glutamate is deaminated by glutamate dehydrogenase to form the TCA cycle intermediate aketoglutarate.
(Choice D) Lysine and leucine are essential amino acids that are strictly ketogenic. They are metabolized into acetyl-CoA, which is a precursor for ketone bodies.
(Choice E) Phenylalanine is converted to tyrosine by the enzyme phenylalanine hydroxylase. Tyrosine is further converted into fumarate (TCA cycle intermediate) and acetoacetate (ketone body).

(Choice A) 3-Hydroxy-3-methylglutaryl-CoA (HMG CoA) lyase is a mitochondrial enzyme necessary for ketogenesis. It is also responsible for metabolism of the ketogenic amino acid, leucine.
(Choice B) Ornithine transcarbamylase catalyzes the combination of ornithine and carbamoyl phosphate to form citrulline in the urea cycle. This reaction occurs within the mitochondria.
(Choice C) Pyruvate carboxylase catalyzes the initial step in gluconeogenesis by converting pyruvate to oxaloacetate. This enzyme requires biotin as a cofactor, and functions within the mitochondria.
(Choice D) Succinate dehydrogenase is a TCA cycle enzyme that converts succinate to fumarate. It is an inner mitochondrial membrane protein and functions as part of the electron transport chain.

The majority of ATP used for cellular processes is generated by the oxidation of acetate in the tricarboxylic acid (TCA) cycle. The enzymes of the TCA cycle are located in the mitochondria and generate reduced nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH2) (Choices C, D , F, and H). These molecules drive the process of oxidative phosphorylation, which converts their reducing potential into high-energy ATP via the electron transport chain.
ATP can also be generated by substrate-level phosphorylation, which involves the direct transfer of a phosphate group from a reactive intermediate to a nucleotide diphosphate (eg, ADP, GDP). Succinyl-CoA synthetase converts succinyl-CoA to succinate and uses the high-energy thioester present in succinyl-CoA to drive GTP synthesis. This GTP can then be used to transphosphorylate ADP to ATP, or rt may be utilized by specific GTPhydrolyzing enzymes, such as phosphoenolpyruvate c-arboxykinase (converts oxaloacetate to phosphoenolpyruvate during gluconeogenesis).