SN7 SHORT NOTE 5M

Describe the functions and deficiency of biotin and pantothenic acid.

Biochemistry · 1st Professional — Pre-Clinical

Biotin

Biotin (vitamin B7) = a sulphur-containing vitamin that acts as the coenzyme for carboxylation reactions, bound covalently to the enzyme as biocytin.

Enzyme	Reaction	Pathway
Pyruvate carboxylase	Pyruvate → oxaloacetate	Gluconeogenesis; anaplerotic
Acetyl-CoA carboxylase	Acetyl-CoA → malonyl-CoA	Fatty acid synthesis
Propionyl-CoA carboxylase	Propionyl-CoA → methylmalonyl-CoA	Odd-chain fatty acid oxidation
β-Methylcrotonyl-CoA carboxylase	—	Leucine catabolism

All four require ATP and CO2 (as bicarbonate)
Sources — liver, egg yolk, nuts, pulses; also synthesised by intestinal bacteria
Deficiency is rare, and occurs with raw egg white (avidin), prolonged antibiotics, or total parenteral nutrition
Features — dermatitis, alopecia, glossitis, depression, hypotonia, and in infants a characteristic periorificial rash
Multiple carboxylase deficiency (holocarboxylase synthetase or biotinidase deficiency) responds dramatically to large doses of biotin — and is screened for in the newborn

Pantothenic Acid

Pantothenic acid (vitamin B5) = the precursor of coenzyme A and of the acyl carrier protein (ACP) of fatty acid synthase.

The name is from Greek pantothen, "from everywhere" — it is ubiquitous in food, so deficiency is very rare
Coenzyme A carries acyl groups in over 100 reactions — the TCA cycle, β-oxidation, ketogenesis, cholesterol and haem synthesis, acetylcholine formation, drug acetylation
The reactive part is the –SH group of the phosphopantetheine arm, which comes from cysteine
Deficiency, produced experimentally, causes "burning feet syndrome", fatigue, and impaired adrenal function

Comparison of the Two

Feature	Biotin	Pantothenic acid
Coenzyme	Biocytin	Coenzyme A, ACP
Reaction type	Carboxylation (adds CO2)	Acyl group transfer
Binding	Covalent (prosthetic group)	Freely diffusible
Antivitamin	Avidin	None known
Deficiency	Rare; raw egg white, inborn errors	Very rare; burning feet

Applied Aspects

Biotinidase deficiency is treatable and is included in newborn screening — untreated it causes deafness, optic atrophy and developmental delay; treated it is entirely preventable
High-dose biotin supplements interfere with immunoassays that use the biotin–streptavidin system — giving falsely abnormal thyroid function and falsely low troponin, which has led to missed myocardial infarction. Patients should stop biotin before blood tests
"Burning feet syndrome" was described in prisoners of war and is attributed to pantothenate deficiency, though other B vitamins were also lacking
Both vitamins are synthesised by gut flora, which is why prolonged broad-spectrum antibiotics can contribute to deficiency

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