Vitamin B7- Biotin

What foods are good sources of biotin?

Liver and eggs

Vitamin B7

Biotin

What form can biotin be found as?

Can be found free or protein bounded, usually through a lysine residue

Why can eating excessive amounts of raw egg white prevent biotin absorption?

In raw eggs whites, avidin irrevesibly binds to biotin in a very strong noncovalent bond

How can you prevent compromise to biotin absorption?

Cooking the egg whites, since avidin is heat liable (destroyed by heat)

Digestion prior to absorption process

Protein-bound biotin requires digestion by enzymes prior to absorption

Protein bound biotin 1-> biocytin 2-> free biotin

1. Proteolysis (breakdown) by pepsin and intestinal proteases
2. Hydrolyzed by biotinidase

Biotinidase deficiency

autosomal recessive inborn error of metabolism

How does biotinidase deficiency affect digestion?

-Insufficient intestinal biotinidase impairs digestion of lysine-bound biotin and limits bioavailability of absorption
-Insufficient extraintestinal biotinidase impairs biotin release and thus recycling of biotin in tissues

Free biotin absorption occurs by

-passive at pharmacological doses
-carrier-mediated transport at physiological doses

Where is free biotin absorbed primarily?

in proximal small intestine

What carrier is involved in absorption of biotin?

SMVT

SMVT transcription ___ as biotin concentration ___

decreases; increases

coenzyme carrier for the transfer of

activate bicarbonate to substrates

Non-coenzyme roles of biotin

regulating gene expression

Biotin in glucose metabolism

pyruvate -> oxaloacetate, with pyruvate dehydrogenase

oxaloacetate used

surplus of ATP- gluconeogenesis
deficiency of ATP- condensed with acetyl-CoA and enters Krebs cycle

Biotin in fatty acid metabolism

acetyl CoA -> malonyl CoA, with either Acetyl-CoA carboxylase 1 or 2

Malonyl CoA used in ACC1

as rate limiting substrate in fatty acid synthesis

Malonyl CoA used in ACC2

inhibits fatty acid uptake and thus b-oxidation

Biotin in both amino acid catabolism and B-oxidation

propionyl CoA -> D-malonylmalonyl CoA, with Propionyl CoA carboxylase

Propionyl CoA

-created from catabolism of isoleucine, valine, threonine, and methionine
-arises from catabolism of odd chain fatty acids

biotin in just amino acid catabolism

B-methylcrotonyl CoA -> B-methylglutaconyl CoA, with B-methylcrotonyl CoA carboxylase

B-methylcrotonyl CoA -> B-methylglutaconyl CoA importance

in the degradation of the essential branched chain amino acids

non coenzyme roles in gene expression

biotin function biotinylation of histone proteins

biotinylation of histone

causes the histones to uncoil and create pores through which transcription factors can enter to reach DNA and activate gene promoter sequences

Mediated by holocarboxylase synthetase

Biotinylation of histone

biotin recycling

1. biotin -> holocarboxylases
2. biocytin -> free biotin
3. free biotin can then be reused as new apocarboxylase

biocytin -> free biotin hydrolyzed by

biotindase

Biotin deficiency symptoms

-Lethargy
-Paresthesia in extremities
-Hypotonia (reduced muscle tones)
-Depression and hallucinations
-Dermatitis
-Anorexia, nausea
-Alopecia
-Brittle nails
-Muscle pains
-Hair thinning/hair loss

Populations at risk

-Inborn error in genes for biotinidase and holocarboxylase syntheases
-GI disorders
-Alcoholism
-Individuals on anticonvulsant drug therapies