•  Amino acids are made from intermediates of the citric acid cycle and other major pathways

Glutamate dehydrogenase catalyzes the reductive amination of α-ketoglutarate to glutamate. A transamination reaction takes place in the synthesis of most amino acids. At this step, the chirality of the amino acid is established. Alanine and aspartate are synthesized by the transamination of pyruvate and oxaloacetate, respectively. Glutamine is synthesized from NH⁴⁺ and glutamate, and asparagine is synthesized similarly.

Proline and arginine are derived from glutamate. Serine, formed from 3-phosphoglycerate, is the precursor of glycine and cysteine. Tyrosine is synthesized by the hydroxylation of phenylalanine, an essential amino acid. The pathways for the biosynthesis of essential amino acids are much more complex than those for the nonessential ones. Activated Tetrahydrofolate, a carrier of one-carbon units, plays an important role in the metabolism of amino acids and nucleotides. This coenzyme carries one-carbon units at three oxidation states, which are interconvertible: most reduced—methyl; intermediate—methylene; and most oxidized—formyl, formimino, and methenyl.

Fatty Acid/ Lipid Biosynthesis

Fatty acid biosynthesis occurs in following phases;

1. 1.
2. Synthesis of malonyl-CoA via Acetyl-CoA Carboxylase

2. Fatty Acid Synthase

3. Fatty acid elongation and desaturation

Site: Synthesis of fatty acids takes place in the cytoplasm and involves initiation of synthesis by the formation of acetoacetyl-ACP and then an elongation cycle where 2 carbon units are successively added to the growing chain.

Acyl carrier protein (ACP) serves as a chaperone for the synthesis of fatty acids. The growing fatty acid chain is covalently bound to ACP during the entire synthesis of the fatty acid and only leaves the protein when it is attached to the glycerol backbone of the forming lipid. ACP is one of the most abundant proteins in the bacterial cell (60,000 molecules per E. coli cell) which makes sense given the amount of lipid that must be synthesized to make an entire cell membrane. The formation of acetoacetyl-ACP can be catalyzed by a number of enzymes, but in all cases the starting substrate is acetyl-CoA. Once formed, acetoacetyl-ACP enters the elongation cycle for fatty acid synthesis. This cycle is the reverse of the β-oxidation of fatty acids discussed earlier.

The first step in the elongation cycle is condensation of malonyl-CoA with a growing acetoacetyl-ACP chain. This adds two carbons to the chain. The next three reactions use 2 NADPH to reduce the β-ketone and generate an acyl-ACP molecule two carbons longer than the original substrate.

The acyl-ACP molecule continues through the cycle until the appropriate chain length is reached. In E. coli fatty acid chains in lipids are 12-20 carbons long. The length of the fatty acid chains and the number of double bonds (unsaturation) is dependent upon the temperature the bacteria are growing at. The membrane must remain fluid. Using short chain fatty acids with higher degrees of unsaturation increases the fluidity of the membrane. As the temperature increases, longer fatty acid chains with fewer double bonds will be more prevalent in the membrane.