Figure 1 Synthesis and degradation of amino acids are closely related to cell growth and FFA production. Six amino acids are grouped as acetyl-CoA module (lysine, tryptophan, leucine), succinyl-CoA module (methionine), and α-ketoglutarate module (histidine, arginine). The investigated amino acids are colored in red, and compounds marked in orange are intermediates of amino acids involved in the tricarboxylic acid (TCA) cycle.