In humans, amino acids are classified into two categories: essential and non-essential amino acids. Essential amino acids cannot be synthesized by the human body and must be obtained through the diet, while non-essential amino acids can be synthesized by the body. The metabolic pathways involved in the biosynthesis of non-essential amino acids are as follows:1. Alanine: Alanine is synthesized from pyruvate through the transamination reaction, where the amino group is transferred from glutamate to pyruvate, forming alanine and -ketoglutarate.2. Asparagine: Asparagine is synthesized from aspartate by the enzyme asparagine synthetase, which transfers an amide group from glutamine to aspartate.3. Aspartate: Aspartate is synthesized from oxaloacetate through the transamination reaction, where the amino group is transferred from glutamate to oxaloacetate, forming aspartate and -ketoglutarate.4. Cysteine: Cysteine is synthesized from serine and homocysteine. Serine is first converted to cystathionine by the enzyme cystathionine -synthase, and then cystathionine is converted to cysteine by the enzyme cystathionine -lyase.5. Glutamate: Glutamate is synthesized from -ketoglutarate through the transamination reaction, where the amino group is transferred from another amino acid, usually alanine or aspartate, to -ketoglutarate, forming glutamate and the corresponding -keto acid.6. Glutamine: Glutamine is synthesized from glutamate by the enzyme glutamine synthetase, which adds an amide group to the -carboxyl group of glutamate.7. Glycine: Glycine is synthesized from serine by the enzyme serine hydroxymethyltransferase, which transfers a hydroxymethyl group from serine to tetrahydrofolate, forming glycine and 5,10-methylene tetrahydrofolate.8. Proline: Proline is synthesized from glutamate by a series of reactions involving the enzymes glutamate-5-kinase, glutamate-5-semialdehyde dehydrogenase, and pyrroline-5-carboxylate reductase.9. Serine: Serine is synthesized from 3-phosphoglycerate by a series of reactions involving the enzymes 3-phosphoglycerate dehydrogenase, 3-phosphoserine aminotransferase, and phosphoserine phosphatase.10. Tyrosine: Tyrosine is synthesized from phenylalanine by the enzyme phenylalanine hydroxylase, which hydroxylates phenylalanine to form tyrosine.The biosynthesis of essential amino acids differs from that of non-essential amino acids in that essential amino acids cannot be synthesized by humans and must be obtained through the diet. The metabolic pathways for essential amino acids are primarily found in plants, bacteria, and fungi. Some of the key pathways for essential amino acid biosynthesis include the shikimate pathway for aromatic amino acids phenylalanine, tryptophan, and tyrosine , the aspartate pathway for lysine, methionine, and threonine, and the pyruvate pathway for leucine, isoleucine, and valine.