Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 23 Vitamins
Cyanocobalamin
H 2 O
NH 2
CH 2 O
H 2 N C
H 2 C H 2 C
CH 3
CH 3
O
H 2 N
C
CH 2
CH 2 C
NH 2
O H 3 C
CN CH 2 CH 2 C N
N
H 3 C
O
Co
H
O
N
N
CH 3
H 2 N
C
CH 2
CH 3
O
HO
O
H
CO
CH 2 CH 2
CH 3 CH CH 2 CH 2 C NH 2
N
3
NH CH 2
2
H CH 3 C C H N
3
N
N
O
O
-
P
N
O
CH 3
O HO
H 2 O
N
N
Ribose
CH 3
N 5 -CH 3 -THFA
ATP
CH 2
CH 2
CH 2
CH 2 CH 2 CH 3 CH 3
CH 2
H 3 C
N N
CH 2
CH 2
H 3 C
H
Co
CH 2
N N
CH 3
CH 3
(Substrate)
C
X
Ado
C
H
CH 2
C C
X
Ado CH 3
CH 2 CH 2 CH 3 CH 3
CH 2
H 3 C
N N
CH 2
CH 2
H 3 C
H
Co
CH 2
N N
CH 3
CH 3
C
X
Ado
C
H
CH 2
N
N
CO(II)
N
N
N
N
CO(II)
N
N
C C
X
Ado CH 3
N
N
N
N
CO(II)
CO(II)
N
N
C
C
C
C
N
N
C
C
H
Ado
X
CH 2
Ado
X
CH 3
H X
(Product)
N
CO(II)
N
N
CO(II)
N
N
N
N
N
COCoA
H 3 C
C
H
COOH
Methylmalonyl CoA
COCoA
Vitamin B-12
H 2 C
C
H 2
COOH
Succinyl CoA
FIGURE 23-22 Cyanocobalamin. Commercial preparations of vitamin B 12 usually have a cyano group coordinated with the cobalt (associated with
the corrin ring of vitamin B 12 ). In an aqueous environment, the cyano group can be displaced by water for the eventual transfer of a methyl group
(donated by N 5 -methyl-THFA) or an adenosyl moiety (from ATP). Methylated vitamin B 12 serves as a cofactor for methionine synthetase (see Fig. 23-7 ).
Adenosylated vitamin B 12 serves as a cofactor for unusual isomerase reactions, such as the conversion of methylmalonyl CoA to succinyl CoA. A mechanism
for this process is shown, which involves the redox of cobalt in the corrin ring of cobalamin (Co 3 ↔ Co 2 ).
(and more recently as small-molecular-weight haptocorrin)
and intrinsic factor (IF). Vitamin B 12 first binds to R
proteins and is apparently released in the intestinal lumen
by the action of pancreatic and intestinal proteinases and
peptidases. Next, vitamin B 12 associates with an intrinsic
factor, a binding protein made in the stomach. The vitamin
B 12 -intrinsic factor complex then interacts with receptors
on the intestinal brush border localized in the midgut