Biotin B7

other name

vitamin H

structure

ureido ring joined to a thiophene ring, with a valeric acid side chain

enzyme type

carboxylases, which add a carboxyl group

pyruvate carboxylase

pyruvate → oxaloacetate (needs ATP and Mg²⁺). activated by acetyle CoA

why pyruvate carboxylase matters

acetyl CoA is plentiful but oxaloacetate is low, so the enzyme makes oxaloacetate and lets acetyl CoA enter the krebs cycle

acetyl CoA carboxylase

acetyl CoA + CO₂ + ATP → malonyl CoA. first step and rate-limiting enzyme of fatty acid synthesis

propionyl CoA carboxylase

incorporates bicarbonate into propionyl CoA → methylmalonyl CoA → succinyl CoA. needs ATP and Mg²⁺

sources of proprionyl CoA

Isoleucine, threonine, methionine, and odd-chain fatty acids

odd chain vs even chain fatty acids

even chains end in acetyl CoA. odd chains end in propionyl CoA (3 carbons) in the last beta-oxidation round. odd chain fatty acids are made by intestinal bacteria

beta-methylcrotonyl CoA carboxylase

Catabolism of leucine (a branches-chain amino acid) → acetoacetate or acetyl CoA