tricarboxylic acid cycle. For amino acid metabolism, methylmalonyl-CoA mutase works in the degradation pathways of isoleucine, threonine, valine, and methionine. These amino acids are degraded into propanoyl-CoA which is then further degraded into (S)-methylmalonyl-CoA. This substrate must be further metabolized by a very similar enzyme, methylmalonyl-CoA epimerase, which converts the (S) form of methylmalonyl-CoA into the (R) form. This is finally transformed using methylmalonyl-CoA mutase. L-methionine is also metabolized through a longer superpathway (see Figure 2). After transformation to L-homocystein, it is combined with L-serine to make L-cystathione, which is hydrolyzed by cystathione gamma lyase to create 2-oxobutanoate. This substrate is transformed to propionyl-CoA and undergoes the same metabolism previously described for propionyl-CoA. The cholesterol superpathway follows the degradation of cholesterol down to various substrates, however only a couple of these biotransformed molecules see propionyl-CoA as a byproduct. The conversion of 3,24-dioxocholest-4-en-26-oyl-CoA to 2-oxochol-4-en-24-oyl-CoA sees the release of a propionyl-CoA molecule. Additionally, the conversion of 3-oxo-23,24-bisnorchol-4-en-17-ol-22-oyl-CoA to androst-4-ene-3,17-dione release of a propionyl-CoA molecule. Finally, the degradation of (S)-4-hydroxy-2-oxohexanoate to pyruvate and propanal, in turn releases a propionyl-CoA substrate after the propanal is converted. All these propionyl-CoA substrates are converted to succinyl-CoA following the methylmalonyl pathway For amino acid metabolism, methylmalonyl-CoA mutase works in the degradation pathways of isoleucine, threonine, valine, and methionine. These amino acids are degraded into propanoyl-CoA which is then further degraded into (S)-methylmalonyl-CoA. This substrate must be further metabolized by a very similar enzyme, methylmalonyl-CoA epimerase, which converts the (S) form of methylmalonyl-CoA into the (R) form. This is finally transformed using methylmalonyl-CoA mutase. L-methionine is also metabolized through a longer superpathway (see Figure 2). After transformation to L-homocystein, it is combined with L-serine to make L-cystathione, which is hydrolyzed by cystathione gamma lyase to create 2-oxobutanoate. This substrate is transformed to propanoyl-CoA and undergoes the same metabolism previously described for propanoyl-CoA. Odd chain fatty acids are also metabolized through the methylmalonyl pathway. The degradation of odd chain fatty acids releases Acetyl-CoA and propionyl-CoA. Propionyl-CoA is then converted to succinyl-CoA, and both succinyl-CoA and propionyl-CoA are interjected into the tricarboxylic acid cycle for continued production of reductant. Metabolic pathology The final product of methylmalonyl-CoA mutase activity is succinyl-CoA which is a tricarboxylic acid cycle substrate. A side effect of excess methylmalonyl-CoA is an interruption of the enzymes responsible for other transformations earlier in the metabolism of propionyl-CoA, leading to propanoic acidemia as well. Excess methylmalonyl-CoA leads to