section 16.2

Biosynthesis of Peptidoglycans

325

U TP + N-AcetylgIucosamine-1-phosphate

p

UDP-GIcNAc

C K O H

O — UDP

NADPH

+ H+

NADP+

NH— C — CH,

CH,OH

CO O H

HO \

CO O H

L -A la , d -G Iu , L-Lys

UDP-MurNAc

2(L-Ala)

\

A TP

2 (D -A I^ ^ A D P + F)

D-Ala-o-Ala

\

UDP-MurNAc-(L-Ala)-( D-y-Glu)-( L-Lys)

A T P — - V

ADP + F> -—

A

CH,OH

h

O — UDP

O — UDP

NH— C — C H a

UDP-N-Acetyl muramyl

pentapeptide

O

NH— C — CH 3

О

C H 3

О

Il

I

II

СН,— C H — c — NH— C H — C — (d -Y -Glu)— ( L-Lys)— ( D-Ala)

FIGURE 16-18

Formation of UDP-N-acetylmuramic acid pentapeptide: stage 1 of peptidoglycan biosynthesis. The steps in this

pathway are discussed in the text. MurNAc = N-acetylmuramic acid, PEP = phosphoenolpyruvate.

and peptide. N-Acetylglucosamine is enzymatically trans-

ferred to MurNAc from UDP-GIcNAc, forming the re-

peating disaccharide unit found in peptidoglycan. This

bond is susceptible to hydrolysis by lysozyme, an en-

zyme found in many bodily secretions, including tears

(Chapter 11). In most bacterial species, the peptide is

modified by ATP-dependent amidation of glutamic acid

residues and by sequential addition of five glycine residues

linked to the a-amino group of lysine. The resulting lipid-

linked disaccharide-peptide units are then polymerized

to form linear peptidoglycan molecules (Figure 16-19).

The high-energy phosphate ester linkage provides the

energy for the transfer. Undecaprenyl pyrophosphate is

liberated and hydrolyzed to undecaprenyl phosphate for