Like glutamate, aspartate is synthesized by a simple one-step transamination reaction catalyzed by aspartate aminotransferase, AST (formerly referred to as serum glutamate-oxalate transaminase, SGOT). Humans express two different AST enzymes, both of which function as homodimeric enzymes. One AST enzyme is a cytosolic enzyme and the other is a mitochondrial enzyme. The cytosolic AST enzyme is synthesized by the GOT1 gene (glutamate-oxalate transaminase 1) that is located on chromosome – and is composed of 9 exons that encode a 413 amino acid protein. The mitochondrial AST enzyme is synthesized from the GOT2 gene that is located on chromosome 16q21 and is composed of 10 exons that generate two alternatively spliced mRNAs that encode two different isoforms: isoform 1 (430 amino acids) and isoform 2 (387 amino acids).
Biotin is mainly required as a coenzyme for carboxylation reactions and the main examples are carboxylation of-i) pyruvate to oxaloacetate (first step of gluconeogenesis); ii) Acetyl co A to Malonyl co A (first step of fatty acid synthesis) and iii) Propionyl co A to D-Methyl malonyl co A (in the conversion of propionyl co A to Succinyl co A to gain entry to TCA cycle). In biotin deficiency, out of the given options, defective fatty acid synthesis is the most suited option because of the impaired conversion of acetyl co A to malonyl co A.