Cell Structure Part A

Isoprene?

 5-carbon compound generated in the mevalonate pathway

Isoprene purpose

Is used by Archaea (and some bacteria) in the synthesis process of membrane lipid ethers; used by Eukarya to make cholesterol

Cell wall purpose

gives the cell shape and allows it to withstand turgor pressure

Turgor pressure?

osmotic pressure caused by differences in concentrations across the membrane

Peptidoglycan/murein composition?

glycan chains that are linked across by peptide cross-bridges

Glycan composition?

Glycan is made up of long polymer chains of two amino sugars (N-acetylglucosamine & N-acetylmuramic, NAG & NAM) connected glycosidic/sugar bonds

What amino acids make up the peptide cross-bridges?

 L-Alanine, D-Glutamic acid, m-Diaminopimelic acid, D-Alanine

Between which two amino acids does the cross-link form?

 m-DAP and D-alanine.

Which sugar of the glycan is the peptide cross-bridge attached to?

 N-acetylmuramic (NAM)

How does penicillin/cephalosporin affect cell wall/peptidoglycan formation?

They blocks the release of D-Alanine. Specifically, the enzyme transpeptidase (during Transpeptidation) so no cross-links form

How does vancomycin affect cell wall/peptidoglycan formation?

Vancomycin blocks croos-bridge formation (blocks Transglycosylation) by binding D-Ala—D-Ala instead of D-Ala—DAP in gm- & D-Ala—Lys in gm+

Peptidoglycan differences in Gm(+) vs. Gm(-)

Gm- : direction connection of DAP—D-Ala; Thin peptidoglycan layer; moderate strength

Gm+: connection via interbridge (5 Gly) connecting D-Ala—L-Lys; thick multi-layered peptidoglycan + extra coss-links = very strong

Where does synthesis of peptidoglycan occur?

Occurs in the cytoplasm and the extracytoplasmic (periplasmic) space

Describe the process of synthesizing PG (8 steps)

1.) Amino acids are added to UDP-NAM

2.) D-Alnine peptide attaches

3.) NAM pentapeptide picked up by bactoprenol, UMP leaves, molecule is moved (this is known as Lipid I)

4.) UDP-NAG linked up to the NAM and UDP leaves (this is now Lipid II)

5.) Flippase flips NAM-NAG outside the membrane

6.) Transglycosylation: NAM-NAG disaccharide added to existing PG chain (sugar-sugar backbone)

7.) Transpeptidation: A pentaglycine forms the cross-link sidechains (L-Lys—D-Ala + pentaglycine bridge for Gm(+) and D-Ala—m-DAP for Gm(-))

8.) One phosphate removed, and lipid returns to the cytoplasmic side of the membrane. Process starts again

What are Transglycosylase and Transpeptidase?

PBPs, Penicillin-binding proteins

What does bacitracin do in PG formation?

Inhibit incorporation of disaccharide units during transpeptidation

Phylum?

a group of organisms that share a common ancestor that diverged early from other bacteria, based on rRNA sequence

Gram-positive bacteria example?

Bacillus anthracis— anthrax, thick cell wall

Gram-negative bacteria example?

Escherichia coli, thin cell wall + outer membrane

Teichoic acids? What’s special about them? Where are they present?

Long polymers made out of glycerol or ribitol units that are linked by phosphates.
Have - charge due to them
In Gm(+) cell wall

Purpose of teichoic acids? (4 things)

Bind divalent metal ions (ex: Mg2+ & Ca2+), aid in ion transport, help wall stability, act like antigens

Lipoteichoic acids? Where are they present?

Teichoic acids bound to membrane lipids (they long)
In Gm(+) cell walls

Sortase enzymes? Where are they mainly found?

convalently link proteins to the amino acids of the PG cross-links via peptide bonds. 

Gm(+) cells

Glycosyl chains in Gm(+) cell walls?

Chains of NAM-NAG sugars

Not always present, but sometimes

Help maintain shape and resist osmotic lysis

What does Gm(-) bacteria envelope look like?

plasma/inner membrane + periplasmic space + outer membrane

What is the outer membrane? Where is present?

The outer membrane is a bilayer with inner and outer facing surfaces

Present in Gm(-) bacteria

What do Bruan’s/Murein lipoproteins do?

Are attached to the inner surface of the outer membrane via 3 fatty acids; are also attached to the peptidoglycan layer, thus connecting the two via covalent bonds

Porin complex purpose?

Allow small molecules and ions to enter, but not large ones

Periplasmic space?

Space between the outer membrane and the cytoplasmic membrane/inner membrane in Gm(-)

Lipopolysaccharide (LPS)?

Lipid molecule on the outside of the outer membrane that responds to infection in the host 

What happens is a lipopolysaccharide (LPS) is free floating?

It is an endotoxin and will stimulates defenses in the host, can lead to endotoxic shock

Core polysaccharide?

Part of LPS that links together Lipid A and the O antigen. 

O-antigens purpose?

Prevent phagocytosis (where cell eats bacteria, debris, dead cells, etc.) by the host cells

Lipopolysaccharides (LPS) structure?

O side chain (O antigen), core polysaccharide, lipid A

Lipid A has sugars with branched lipids and short fatty acids

What proteins does Gm(-) outer membrane have?

Has porin and transporter proteins (make it more permeable than inner membrane)

What makes Mycobacterium tuberculosis & M. Leprae cell envelopes special?

Mycolic acids & Arabinogalactans (Note: structure goes cell membrane—> PG—> Galactan—> Arabinan—> Mycolic acid—> capsule)

Mycolic acids? Purpose?

Very long chains of fatty acid

They form a thick waxy/hydrophobic outer layer that traps the dye carbol fuchsin during acid–fast staining

Purpose of outer membrane?

Permeability barrier to large molecules and harmful chemicals. 

Arabinogalactans? Purpose?

an unusual polysaccharide that acts like a linker between the PG layer and the mycolic acid layer

Results of Mycobacterial special features?

Typically harder to stain, harder for antibiotics to enter, and harder for the host immune system to kill (why tuberculosis is so difficult to treat) 

S-layer?

A crystalline layer of thick protein or glycoprotein subunits, which unit contains large pores

S-layer function?

Contributes to cell shape, protects from osmotic stress, helps in biofilm formation, and in pathogenicity (causing disease)

S-layer differences in Gm(+) vs. Gm(-)?

Gm(-): adheres to inside of outer membrane

Gm(+): adheres to the PG surface

Who has s-layer?

Found in archaea and in bacteria

Structure of archaeal envelopes and cell walls?

They do not have PG, and no OM

Typically have s-layer and cytoplasmic membrane

SOMETIMES have cytoplasmic membrane + pseudopeptidoglycan/pseudomurein + s-layer

What is pseudopeptidoglycan/pseudomurein? Where is it found?

A polysaccharide composed of N-acetylglucosamine and N-acetylalosaminuronic acid

In archaeal cells between cytoplasmic membrane & S-layer

Mycoplasma? Features?

No cell wall

Design and synthesis of minimal bacterial genome (lowkey just a membrane). Takes nutrients from host cell

Thermoplasma? Features?

Archaea, growth at low pH and high temperature

No S-layer, but some have glycocalyx (polysaccharide)