What Is Difference Between D-Biotin and Biotin？

Biotin is a water-soluble B-complex vitamin involved in a variety of human metabolic processes. Eight possible stereoisomers of biotin exist. Stereoisomers are molecules that have the same molecular formula and sequence of bonded atoms, but differ in the three-dimensional orientation of their atoms in space. D-+-biotin is the only stereoisomer found in nature, and it is the only stereoisomer that is enzymatically active.
Structure of Biotin
Biotin has the chemical formula C10H16N2O3S1, so it contains 10 carbon atoms, 16 hydrogen atoms, two nitrogen atoms, three oxygen atoms and one sulfur atom for every one molecule of biotin.

These molecules are arranged in a bi-cyclic, or two-ring structure. One ring of biotin contains a ureido, or -N-CO-N- group, and the other ring contains a sulfur atom as part of the tetrahydrothiophene ring.

Biotin has three chiral carbons and can therefore exist in eight different stereoisomer forms; biotin, epibiotin in cis form, allobiotin, and epialloboiotin in trans form, in both D(+) and L(-) forms. Only the D-(+) biotin stereoisomer is physiologically active.

Alternative Names
Because D-biotin is the only naturally occurring isomer, it is often used synonymously with the simple term, biotin, which is also known as Vitamin B-7, Vitamin H or Coenzyme-R.

The formal chemical name of biotin is hexahydro-2-oxo-1H-thieno[3,4-d]imidazole-4-pentanoic acid, but it is also known as beta-Alanine, N-(2,4-dihydroxy-3,3-dimethyl-1-oxobutyl)-, monosodium salt, (R)-; Sodium D-Pantothenate; Vitamin H（58-85-5）; Hexahydro-2-oxo-, [3aS-(3a alpha,4beta, 6a alpha)]- 1H-Thieno[3,4-d]imidazole-4-pentanoic acid; Biopeiderm, as well as d-cis-tetrahydro-2-oxothieno [3,4]-imidazoline-4-valeric acid.

Physical Properties
D-Biotin exists as a white, or off-white, crystalline powder, and sometimes also as a colorless crystal. It has a molecular weight of 244.31. It is water soluble and insoluble in organic solvents. It has a melting temperature of 444 to 451 degrees Fahrenheit, at which point, it decomposes.

Function
Biotin acts as a co-enzyme in four different carboxylase enzymes of the body: acetyl-CoA carboxylase, propionyl-CoA carboxylase, b-methylcrotonyl-CoA carboxylase and pyruvate carboxylate. These enzyme complexes play a role in the metabolism of lipids, proteins and carbohydrates.
Biotin is also involved in making glucose and some amino acids and may play a role in DNA replication as it interacts with the DNA binding proteins known as histones.
Dietary Sources
Humans cannot make biotin, although there is some controversy as to whether enteric bacteria can make biotin for human use. Notwithstanding that possibility, biotin still must be taken in from dietary sources. Fortunately, biotin is widely available in a variety of food sources, and biotin deficiencies are quite rare. The richest source of biotin can be found in yeast, liver and kidneys, as well as egg yolks, soybeans, nuts and cereals.