Organic Chemistry

Section 19.21 Reactions at the -Carbon in Biological Systemsa 827

Before collagen molecules can cross-link, the primary amino groups of the lysine
residues of collagen must be converted to aldehyde groups. The enzyme that catalyzes
this reaction is called lysyl oxidase. An aldol condensation between two aldehyde
residues results in a cross-linked protein.

A Biological Claisen Condensation
Fatty acids are long-chain, unbranched carboxylic acids (Sections 17.13 and 26.1). Most
naturally occurring fatty acids contain an even number of carbons because they are syn-
thesized from acetate, which has two carbon atoms.
In Section 17.20, you saw that carboxylic acids can be activated in biological systems
by being converted to thioesters of coenzyme A.

One of the necessary reactants for fatty acid synthesis is malonyl-CoA, which is
obtained by carboxylation of acetyl-CoA. The mechanism for this reaction is discussed
in Section 25.5.

Before fatty acid synthesis can occur, however, the acyl groups of acetyl-CoA and
malonyl-CoA are transferred to other thiols by means of a transesterification reaction.

O

CH 3 CRSHSCoA +

SCoA

O

CH 3 C SR + CoASH

+ RSH + CoASH

OO

−OCH

C 2 C SR

OO

−OCH

C 2 C

O

CH 3 C SCoA + HCO 3 −

OO

−OCH

C 2 CSCoA

acetyl-CoA malonyl-CoA

lysyl

oxidase

H 3 NCH 2 CH 2 (CH 2 ) 2 CH

NH

CO

+ +

CH(CH 2 ) 2 CH 2 CH 2 NH 3

NH

O C

aldol

condensation

HCCH 2 (CH 2 ) 2 CH

NH

CO

CH(CH 2 ) 2 CH 2 CH

NH

O C OO

C(CH 2 ) 2 CH

NH

CO

HC O

CH(CH 2 ) 2 CH 2 CH

NH

O C

lysine

residues

cross-linked collagen

O

CH 3 C O− + CoASH + ATP

O

CH 3 C

O

−O O−O O−

P

O

O−

SCoA ++AMP P

acetate coenzyme A acetyl-CoA

pyrophosphate