Organic Chemistry

1054 CHAPTER 25 The Organic Mechanisms of the Coenzymes • Metabolism

All biotin-requiring enzymes follow the same three steps: activation of bicarbonate

by ATP, reaction of activated bicarbonate with biotin to form carboxybiotin, and transfer

of the carboxyl group from carboxybiotin to the substrate.

25.6 Pyridoxal Phosphate: Vitamin

Pyridoxal phosphate (PLP)is the coenzyme required by enzymes that catalyze

certain transformations of amino acids. The coenzyme is derived from pyridoxine, the

vitamin known as vitamin The name “pyridoxal”indicates that the coenzyme is a

pyridine aldehyde. A deficiency in vitamin causes anemia; severe deficiencies can

cause seizures and death.

Several different kinds of amino acid transformations are catalyzed by PLP-requiring

enzymes. The most common are decarboxylation, transamination, racemization (the in-

terconversion of L- and D-amino acids), bond cleavage, and

RCH 2 NH 3 +

+

+

O

RCH

NH 3

C O− CO 2

E

PLP

decarboxylation

Ca¬Cb a,b-elimination.

pyridoxine

vitamin B 6

N

H

CH 2 OH

OH

+ CH 3

E

HOCH 2

pyridoxal phosphate

PLP

N

H

HC

OH

+ CH 3

2 −O

3 POCH 2

O

the coenzyme is bound to the

enzyme by means of an imine

linkage with a lysine residue

N

H

HC H

O

CH 3

(CH 2 ) 4

+

2 −O

3 POCH 2

N hydrogen

bond

B 6

B 6.

B 6

3-D Molecule:

Pyridoxal phosphate (PLP)

Pyridoxal phosphate (PLP) is required
by enzymes that catalyze certain
transformations of amino acids.

+

+ PO 4 −^3 Mg^2 +

CH 2

O–

SCoA

C

CH 2

OH

SCoA

C

−O

O

C

O −

CH 2

C N

C

SCoA

−O

O S

NH

(CH 2 ) 4 CNH(CH 2 ) 4 E

O

O

N

S

NH

(CH 2 ) 4 CNH(CH 2 ) 4 E

O

O

−O CH 2

O

SCoA

C N

S

NH

(CH 2 ) 4 CNH(CH 2 ) 4 E

O

O−

O

C

carboxybiotin

malonyl-CoA

CH 3

O

SCoA

C

acetyl-CoA

+ H+