III

III. Cell wall, peptidoglycan structure & synthesis

A. Functions of the Cell Wall

confers a shape to the bacteria

directly in contact with the environment. Its interaction w/the outside world may determine the successful survival of the cell

involved in many pathogenic properties of the bacteria

acts as a barrier to some molecules

Inside Cell Outside Cell

high solute conc. low solute conc.

great pressure on membrane

Q. What does this mean?

a cell wall protects cells from osmotic lysis

B. General Properties of the Cell Wall

1. 2 basic types of bacterial cell wall structures

a. Gram positive (G+)

b. Gram negative (G-)

2. Peptidoglycan = murein

a thick rigid layer that is found in both G+ and G- cells

glycan strands - 2 sugars crosslinked by peptide chains

2 sugars are N-acetyl glucosamine (NAG) & N-acetyl muramic acid (NAM)

attached to NAM is a peptide side chain generally of 4 amino acids

L-Ala

G-Glu

Diamino-pimelic acid (DAP) or L-Lys

D-Ala

1 NAM, NAG, & aa side chain form a single peptidoglycan unit that can link w/ other units via covalent bonds to form a repeating polymer

the polymer is further strengthened by cross links between aa 3 (DPA or L-Lys) of one unit and aa 4 (D-Ala) of the next glycan tetrapeptide

C. G+ Cell Wall

20-80nm

A thick peptidoglycan layer constitutes most of the G+ cell wall (> 50%)

As a result, the G+ cell wall is very sensitive to the action of lysozyme and penicillin

Teichoic acid - alcohol phosphates. Covalently linked to muramic acid, links various layers of the peptidoglycan mesh together

D. G- Cell Wall

1. General characteristics

10nm

a more complicated structure

2 separate areas w/ an additional membrane besides the cellular membrane - outer membrane (OM)

a thin layer of peptidoglycan (10-20%)

an open area - the periplasmic space

a. Peptidoglycan in G-

less cross-linking with no peptide interbridge

covalently bound to the peptidoglycan is Braun's lipoprotein, which has a hydrophobic anchor in the OM that strongly binds the peptidoglycan to the OM

b. Outer Membrane

role in the virulence of G- bacteria

a lipid bilayer like the cell membrane, containing lipids & proteins, but it also contains lipopolysaccharides (LPS)

has distinctive sides, w/ the side that faces the outside containing the LPS

1) LPS

composed of 2 parts, Lipid A & a polysaccharide chain that reaches out into the environment

Lipid A is a derivative of 2 NAG units w/ up to 7 fa connected to it that anchor the LPS in the membrane

attached to Lipid A is a conserved core polysaccharide that contains, KDO, heptose, glucose & glucosamine sugars

rest of the polysaccharide consists of repeating sugar units called the O-antigen

O-antigen gets its name from the fact that it is exposed to the outer environment

G- protect themselves from hosts defenses by varying the make-up of the O-antigen

LPS is biologically very impt because it has activities in humans - LPS is why G- septicemia kills

2) Outer membrane proteins (OMP)

Ex. porins - proteins that form pores in the OM wide enough to allow passage of most small hydrophilic molecules

3) Functions of the outer membrane

negative charge to cell

pores for entrance of hydrophilic molecules

phage receptors

pathogenic properties

keeps enzymes in periplasm

4) Periplasm

a hydrated gel in between PG and OM, contains many different proteins

proteins function to detect the environ. & transport needed nutrients into the cell

Examples:

1.hydrolytic enzymatic enz

phosphatases - degrade phosphate containing compounds

proteases - degrade proteins & peptides

endonucleases - degrade nucleic acids

2.binding proteins - recognize specific solutes & transport across membrane:

sugars

inorganic ions

vitamins

3.chemoreceptors - help cells interpret chem. composition of their environ.

4.detoxifying enz - alter harmful agents before they get into cell, ex. beta-lactamase

5.osmotic protection. - to protect themselves, bacteria synth small molecules to balance the osmotic stress - compatible solutes

A comparison of G- and G+ cell wall types

Property	Gram +	Gram -
Thickness	20-80nm	10nm
Number of layers	1	2
Peptidoglycan content	> 50%	10-20%
Teichoic acid	+	-
LPS	-	+
Sensitive to penicillin	+	Not as much
Digested by lysozyme	+	Not as much

E. Synthesis of Peptidoglycan

multi-step process

inside & outside the cell

1. Stage 1 - Assembly of structural units - cytoplasm

synth. of precursors into NAM-UDP & NAG-UDP
addition of aa to form pentapeptides

L-Ala

G-Glu

DPA or L-Lys

D-Ala

peptide bonds are synth. by enz. & not on ribosomes

requires ATP

Q. How will cell get sugars through cytoplasmic membrane to growing cell wall?

2. Stage 2 - transport across cytoplasmic membrane

NAM-pentapeptide is transferred to a carrier molecule called C₅₅ carrier lipid or bactoprenol on inner cytoplasmic membrane

NAG is transferred to the bactoprenol-NAM to form a bactoprenol-NAG-NAM (2 glycan repeat)

this molecule moves across cytoplamic membrane to outside of the cell

transferred from bactoprenol to NAG end of a growing chain called nascent peptidoglycan

3. Stage 3 - cross-linking - cell wall

nascent peptidoglycan is covalently linked to existing cell wall by formation of cross-bridges between nascent peptidoglycan & existing cell wall peptidoglycan - by enz transamidase - called transpeptidation