Micronutrients: Biotin

which vitamin is biotin

vitamin B7

how was biotin first discovered?

during experiments examining the cause of “egg white injury” (eating raw eggs led to hair loss, dermatitis, etc. which led to the discovery of avidin, a biotin-binding protein)

what are some sources of biotin

1. made by intestinal bacteria (not enough to meet the needs of humans)
2. widely found in foods bound to proteins

explain the absorption of biotin

must be removed from proteins prior to its absorption → proteolysis by pepsin in the stomach breaks down proteins and releases biotin

free biotin is absorbed near completion

what inhibits the absorption of biotin?

alcohol

what form of biotin is found in the blood?

in the free form (\~80%) with a bit bound to albumin

what is the RDA for biotin

there is no RDA or deficiency established because it’s so prevalent

explain biotin-mediated carboxylation

* 2 rings with N and S and a side chain with a carboxyl group
* the carboxyl group interacts with the NH2 group of a lysine side chain in an enzyme (e.g. pyruvate carboxylase)
* 2 step reaction
* carboxylate the N in the biotin ring structure
* transfer the carboxyl group to another molecule (pyruvate \[3C\] → oxaloacetate \[4C\])

what are 3 key reactions involving biotin?

1. pyruvate carboxylation (production of oxaloacetate, can only form is biotin is attached)
2. malonyl CoA formation (fatty acid synthesis)
3. conversion of propionate into glucose (important in ruminants, SCFAs)

what does pyruvate carboxylase require to be active

a biotin cofactor

explain how biotin being anchored to a lysine side chain allows it to function in carboxylation

it allows it to “reach” into the carboxylation reactive site (#1 where it is carboxylated) and then swing to the active site #2 which brings the carboxyl group into close proximity with the substrate (in this case, pyruvate)