The Initial Reactions Are Common To All Three Branchedchain Amino Acids

Reactions 1-3 of Figure 30-19 are analogous to those of fatty acid catabolism. Following transamination, all three a-keto acids undergo oxidative decarboxylation catalyzed by mitochondrial branched-chain a-keto acid dehydrogenase. This multimeric enzyme complex of a decarboxylase, a transacylase, and a dihydrolipoyl dehydrogenase closely resembles pyruvate dehydroge-nase (see Figure 17-5). Its regulation also parallels that of pyruvate dehydrogenase, being inactivated by phos-phorylation and reactivated by dephosphorylation (see Figure 17-6).

Reaction 3 is analogous to the dehydrogenation of fatty acyl-CoA thioesters (see Figure 22-3). In isova-leric acidemia, ingestion of protein-rich foods elevates isovalerate, the deacylation product of isovaleryl-CoA. Figures 30-20, 30-21, and 30-22 illustrate the subsequent reactions unique to each amino acid skeleton.

L-Cysteine

Cystathionine

nh4+

NAD+

a-Ketobutyrate

CoASH

NAD+

H3C.

S~CoA

Propionyl-CoA

Figure 30-18. Conversion of methionine to propi-onyl-CoA.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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