where do branched chain amino acids come from
Branched-chain amino acid Five enzymes play a major role in the parallel synthesis pathways for isoleucine, valine, and leucine: threonine dehydrogenase, acetohydroxyacid synthase, ketoacid reductoisomerase, dihydroxyacid dehygrogenase and aminotransferase.[3] Threonine dehydrogenase catalyzes the deamination and dehydration of threonine to 2-ketobutyrate and ammonia. Isoleucine forms a negative feedback loop with threonine dehydrogenase. Acetohydroxyacid synthase is the first enzyme for the parallel pathway performing condensation reaction in both steps – condensation of pyruvate to acetoacetate in the valine pathway and condensation of pyruvate and 2-ketobutyrate to form acetohydroxybtylrate in the isoleucine pathway. Next ketoacid reductisomerase reduces the acetohydroxy acids from the previous step to yield dihydroxyacids in both the valine and isoleucine pathways. Dihydroxyacid dehygrogenase converts the dihyroxyacids in the next step. The final step in the parallel pathway is conducted by amino transferase, which yields the final products of valine and isoleucine.[3] A series of four more enzymes – isopropylmalate synthase, isopropylmalate isomerase, isopropylmalate dehydrogenase, and aminotransferase – are necessary for the formation of leucine from 2-oxolsovalerate.[3]
Transfer RNA The covalent attachment to the tRNA 3’ end is catalyzed by enzymes called aminoacyl tRNA synthetases. During protein synthesis, tRNAs with attached amino acids are delivered to the ribosome by proteins called elongation factors, which aid in association of the tRNA with the ribosome, synthesis of the new polypeptide and translocation (movement) of the ribosome along the mRNA. If the tRNA's anticodon matches the mRNA, another tRNA already bound to the ribosome transfers the growing polypeptide chain from its 3’ end to the amino acid attached to the 3’ end of the newly delivered tRNA, a reaction catalyzed by the ribosome.