Organic Chemistry

Section 25.5 Biotin: Vitamin H 1053

3-D Molecule:

Biotin

Biotin is the coenzyme required by enzymes that catalyze carboxylation of a carbon
adjacent to a carbonyl group. For example, pyruvate carboxylase converts pyruvate—the
end product of carbohydrate metabolism—to oxaloacetate, a citric acid cycle intermedi-
ate (Figure 25.2). Acetyl-CoA carboxylase converts acetyl-CoA into malonyl-CoA, one
of the reactions in the anabolic pathway that converts acetyl-CoA into fatty acids
(Section 19.21). Biotin-requiring enzymes use bicarbonate for the source of
the carboxyl group that becomes attached to the substrate.

Mechanism for Biotin
In addition to requiring bicarbonate, biotin-requiring enzymes require and
ATP. The function of is to decrease the overall negative charge on ATP by
complexing with two of its negatively charged oxygens (Section 27.5). Unless its
negative charge is reduced, ATP cannot be approached by a nucleophile. The func-
tion of ATP is to increase the reactivity of bicarbonate by converting it to “activated
bicarbonate,”a compound with a good leaving group (Section 27.2). Notice that
“activated bicarbonate”is a mixed anhydride of carbonic acid and phosphoric acid
(Section 17.1).

Nucleophilic attack by biotin on activated bicarbonate forms carboxybiotin. Because
the nitrogen of an amide is not nucleophilic, it is likely that the active form of biotin
has an enolate-like structure. Nucleophilic attack by the substrate (in this case, the
enolate form of acetyl-CoA) on carboxybiotin transfers the carboxyl group from biotin
to the substrate.

+ + ADP

activated bicarbonate

O−

Mg^2 +

O

O−

P

O− −OO

O

C

HO

O

C

bicarbonate

O

O−

P

−OO adenosine

O

O−

P

O

O

O−

P

Mg^2 +

Mg^2 +

Mg^2 +

(HCO 3 - )

Biotin is required by enzymes that

catalyze the carboxylation of a carbon

adjacent to a carbonyl group.

acetyl-CoA

++HCO 3 − ATP ++ADP

O

CH 3 C SCoA

malonyl-CoA

O O

−OCCH

2 C SCoA

acetyl-CoA

carboxylase

Mg^2 +

biotin

−O OH

O

−O

P

pyruvate

++HCO 3 − ATP ++ADP

O O

CH 3 C C
oxaloacetate

O O O

−OCCH

2 C C O

O− −

pyruvate

carboxylase

Mg^2 +

biotin

−O OH

O

−O

P

O −

N

S

Mg NH

2 +

(CH 2 ) 4 CNH(CH 2 ) 4 E 2 −

O

−O +

O

O

C

O

O−

P

O−

O −

O

−O C N

S

NH

PO 3 (CH^2 )^4 CNH(CH^2 )^4 E

O

O

“enolate-like” structure of

enzyme-bound biotin Mg

2 +