Biotin

Question 1

Structure of biotin

Answer 1

Biotin is an imidazole derivative . It consists
of a tetrahydrothiophene ring bound to an imidazole ring and a valeric acid side chain.

Question 2

Sources of biotin

Answer 2

• It is widely distributed in foods.
• Liver, kidneys, vegetables and egg yolk are the
  important sources of biotin.
• Biotin is also synthesized by intestinal bacteria.

Question 3

Active Form of Biotin

Answer 3

Enzyme-bound biotin, biocytin is an active form of biotin. Biotin is covalently bound to ε-amino group of lysine of an enzyme to form biocytin.

Question 4

Functions of Biotin

Answer 4

Biotin is a coenzyme of carboxylase reactions, where
it is a carrier of CO2. Some of the carboxylation reactions
requiring biotin are given below.
* Conversion of acetyl-CoA into malonyl-CoA
catalyzed by acetyl-CoA carboxylase in fatty acid synthesis.
* Conversion of pyruvate into oxaloacetate, catalyzed by pyruvate carboxylase in gluconeogenesis.
* Conversion of propionyl-CoA to D-methyl malonylCoA catalyzed by propionyl-CoA carboxylase in the pathway of conversion of propionate to succinate.
* It is also involved in the catabolism of branched chain amino acid catalyzed by β-methyl-crotonylCoA carboxylase.

Question 5

Biotin Independent Carboxylation Reaction

Answer 5

There are few carboxylation reactions which do not require biotin. For example:
* Formation of carbamoyl phosphate by carbamoyl
phosphate synthetase in urea cycle.
* Addition of CO2 to form C6 in purine ring.
* Conversion of pyruvate to malate by malic enzyme.

Question 6

Nutritional Requirements of biotin

Answer 6

A daily intake of about 150–300 μg is recommended for adults. Biotin is synthesized by intestinal microorganisms in such a large quantities that a dietary source
is probably not necessary.

Question 7

Deficiency Manifestation of biotin

Answer 7

• Since biotin is widely distributed in plant and animal
  foods and intestinal bacterial flora supply adequate
  amounts of biotin, the natural deficiency of biotin is
  not well characterized in humans.
• The experimentally induced symptoms of biotin
  deficiency are nausea, anorexia, glossitis, dermatitis,
  alopecia (loss of hair), depression and muscular pain.
• Deficiency of biotin occurs in:
  – The people with the unusual dietary habit of
  consuming large amounts of uncooked eggs. Egg
  white contains the glycoprotein avidin, which
  binds the imidazole group of biotin and prevents
  biotin absorption.
  – Use of antibiotics, that inhibit the growth of
  intestinal bacteria, eliminates this source of biotin
  and leads to deficiency of biotin.