Organic Chemistry

Section 25.7 Coenzyme B 12 :Vitamin B 12 1061

PROBLEM 15

Which of the following compounds is more easily decarboxylated?

PROBLEM 16

Explain why the ability of PLP to catalyze an amino acid transformation is greatly reduced

if a PLP-requiring enzymatic reaction is carried out at a pH at which the pyridine nitrogen

is not protonated.

PROBLEM 17

Explain why the ability of PLP to catalyze an amino acid transformation is greatly reduced

if the OH substituent of pyridoxal phosphate is replaced by an

25.7 Coenzyme Vitamin

Enzymes that catalyze certain rearrangement reactions require coenzyme a coen-
zyme derived from vitamin The structure of vitamin was determined by
Dorothy Crowfoot Hodgkin, using X-ray crystallography. The vitamin has a cyano
group (or or ) coordinated with cobalt (Section 21.11). In coenzyme
the cyano group is replaced by a 5 ¿-deoxyadenosylgroup.

HO- H 2 O B 12 ,

B 12. B 12

B 12 ,

B 12 : B 12

OCH 3.

O

CH 2 CH 2

or

C O−

O

CH 2 C O−

+N

H

+N

H

imine between PLP and the amino acid:

before breaking the -C—H bond

before breaking the

• C—CO 2 bond

before breaking the

• C—R bond

after breaking the

• C—CO 2 bond

after breaking the

• C—R bond

electrons left behind after breaking the -C—H bond

have been delocalized into the conjugated system

N C

R

COO−

N

H

N C

H

R N C

COO−

H

C

R

COO−

N

C

C

H

R

O−

O

N

R

C

COO−

H

+N

N

• H –CO 2 – R