Fig. 2: Biochemical reactions in amino acid metabolism.

a Reverse-transsulfuration pathway: Cysteine can be produced from methionine through the reverse-transsulfuration pathway. This pathway is a combination of the methionine cycle and transsulfuration pathway. Homocysteine, the intermediate of the first step in the transsulfuration pathway, is generated from the methionine cycle. Serine condenses with homocysteine, producing cystathionine. Cystathionine is then converted to cysteine and alpha-ketobutyrate by CGL. Key enzymes are in red circle. THF tetrahydrofolate, CBS cystathionine β-synthase, SAM S-adenosylmethionine, CGL cystathionine γ-lyase. b Polyamine synthesis: Polyamines (putrescine, spermine, and spermidine) are synthesized from the amino acid arginine, and are converted from one to another (in the order of putrescine to spermidine to spermine). SAM, as the precursor of dcSAM, is the major donor for constructing polyamine structures. Key enzymes are in red circle. ODC ornithine decarboxylase, AMD S-adenosylmethionine decarboxylase, SAM S-adenosylmethionine, dcSAM decarboxylated S-adenosylmethionine. c Nitrogen and carbon source for nucleic acids: Aspartate, glycine, and glutamine provide nitrogen, and glycine and one-carbon units from the folate cycle (as a form of formate) provide carbon for purines. Glycine is formate’s indirect precursor through one-carbon metabolism, providing formate for biochemical reactions in purine biosynthesis. Aspartate and glutamine are the main amino acids involved in pyrimidine synthesis. Carbon (C) is in yellow, and nitrogen (N) is in green. d GSH and NADPH as antioxidants: Reactive oxygen species (ROS) bind and damage cellular macromolecules. The oxidation of NADPH and GSH allows ROS to be reduced to an inactive state. GSH reduces hydrogen peroxide to water and becomes oxidized to GSSG by GPX. Oxidized glutathione (GSSG) is then reduced back to GSH by GR in the presence of NADPH. Enzymes are shown in red circles. GPX glutathione peroxidase, GR glutathione reductase, GSH reduced glutathione, GSSG oxidized glutathione, NADPH reduced nicotinamide adenine dinucleotide phosphate, NADP+ oxidized nicotinamide adenine dinucleotide phosphate. e Amidation reaction for asparagine synthesis: Asparagine is synthesized by an amidotransferase reaction, catalyzed by asparagine synthetase (ASNS). The conserved amide group nitrogen is in a red box, while the enzyme is in a red circle.