Branched Ketoacid Dehydrogenase.

Thiamine pyrophosphate is the cofactor bound to the E1 subunit, and it is the chemistry that lets the branched-chain ketoacid lose its carboxyl group. Without adequate thiamine the committed decarboxylation step slows and the ketoacids back up. This is textbook cofactor biochemistry, not an inferred pairing.

The E2 subunit carries a lipoamide arm that shuttles the acyl group from E1 to coenzyme A, and lipoic acid is that arm's chemistry. It is covalently attached to a lysine residue in the protein rather than floating free. The relationship is structural to the complex.

The E3 subunit, dihydrolipoamide dehydrogenase, is a flavoprotein holding FAD derived from riboflavin, and it is what re-oxidises the lipoamide arm so the complex can turn over again. Riboflavin supply therefore sits on the same cycle. Cofactor biochemistry.

NAD+ is the terminal electron acceptor for the E3 step, so the reaction's direction depends on the mitochondrial NAD+ to NADH ratio. Niacin is the precursor of that nucleotide. A cofactor relationship rather than a measured combination.

Coenzyme A accepts the acyl group at the E2 step, producing the branched acyl-CoA that continues down the pathway, and pantothenic acid is the backbone of coenzyme A. Every acyl transfer in this pathway consumes CoA availability. Established biochemistry.

Thiamine pyrophosphate binds its enzyme site as a magnesium complex, so magnesium is required for the E1 cofactor to sit correctly. This is the same magnesium dependence seen across thiamine-dependent decarboxylases. Structural, not additive.

Branched-chain aminotransferase performs the step immediately upstream, converting leucine, isoleucine and valine into the ketoacids this complex then handles, and it uses pyridoxal 5-phosphate. Without that step there is no substrate for the dehydrogenase. Sequential pathway biochemistry.

Leucine is transaminated to alpha-ketoisocaproate, the substrate this complex disposes of, and that ketoacid also inhibits the kinase that switches the complex off. So a leucine load both supplies substrate and indirectly turns up the machinery that clears it. Both halves of that loop are established.

Valine is transaminated to alpha-ketoisovalerate, one of the three ketoacids this single complex handles. The three branched-chain amino acids share one dehydrogenase rather than each having their own. That shared bottleneck is why they are discussed as a group.

Whey is unusually rich in branched-chain amino acids, so a whey load raises the flux this complex has to process. Circulating branched-chain amino acids and their ketoacids rise and then fall as that disposal happens. The flux relationship is well characterised; nothing here is a claim about an outcome.

Two carboxylases further down the branched-chain route are biotin enzymes: 3-methylcrotonyl-CoA carboxylase on the leucine branch and propionyl-CoA carboxylase on the valine and isoleucine branches. Biotin supply therefore governs whether the products of this complex keep moving. Downstream cofactor biochemistry.

Methylmalonyl-CoA mutase, which sits downstream on the valine and isoleucine branches, requires adenosylcobalamin. That step is what feeds those carbons into the citric acid cycle as succinyl-CoA. Established pathway biochemistry, no combination trial needed.

Excess isovaleryl-CoA on the leucine branch is conjugated with glycine and excreted in urine as isovalerylglycine, which is a recognised disposal route when the acyl-CoA pool backs up. Glycine availability therefore affects that overflow channel. This describes a metabolic route, not a benefit.

Carnitine conjugates accumulating acyl-CoA species into acylcarnitines, which frees coenzyme A and gives the mitochondrion a way to export excess acyl groups. That buffering is why acylcarnitine profiles are used to read branched-chain flux. Established as a mechanism; the practical size of the effect from supplemental carnitine is less settled.

HMB arises from alpha-ketoisocaproate through a minor cytosolic branch that bypasses this complex entirely. Supplying it feeds the leucine pathway from the side rather than through the mitochondrial dehydrogenase step. The pathway relationship is clear; what that means practically is not established.