Introduction to Prokaryotes Flashcards
Prokaryote Definition
Cellular organisms with no true nucleus
Prokaryotic Shape
- Cocci (spherical):
- Streptococci: Chains
- Staphylococci: Grape like clusters
- Bacili (rods):
- Coccobacili: Very short rods
- Vibrios: Resemble rods, comma shaped
- Spirilla: Rigid helices
- Spirochetes: Flexible helices
- Filamentous
Common features in all prokaryotes
- Cell envelope = plasma membrane plus all layers outside of it (cell wall etc. )
- Cytoplasm (no organelles, no ER, no GA)
- External structures (fimbriae, flagella, some bacteria have a capsule)
Structure of Polar membrane lipids
- Polar & hydrophilic head (phosphate group and glycerol mol)
- Long, non polar, hydrophobic fatty acid chain tail
- Lipid composition of membrane phospholipids changes with environmental temp to maintain membrane fluidity.
Bacterial membrane proteins
Integral Proteins:
- 70-80% of total membrane proteins - Amphipathic, embedded in membrane - carry out important functions
Peripheral Proteins:
- 20-30% of total membrane proteins - Loosely connected to membrane, easy to remove
Hopanoids
Sterol-like molecules found only in bacterial membranes
Peptidoglycan
- Also called Murein
- Rigid structure that lies just outside the bacterial cell membrane (Makes up the cell wall)
Gram-Positive Bacteria
- Stain purple
- thick layer of peptidoglycan (20-80nm)
Gram-Negative Bacteria
- Stains pink or red
- Thin peptidoglycan layer (2-7nm) plus an outer membrane
Functions of the bacterial cell wall
- Maintains shape of the bacterium
- Help protect cell from osmotic lysis
- Helps protect from toxic molecules
- May contribute to pathogenicity in pathogenic bacteria
Peptidoglycan Structure
- Mesh-like polymer of identical subunits forming long strands
o Forms a helical spiral with some peptides facing vertically and some facing horizontally. - Each subunit consists of:
o 2 sugars: N-acetylglucosamine (NAG)
& N-acetylmuramic acid (NAM)
o Alternating D- and L- amino acids - Side-chain of peptide always ONLY added to NAM unit and not NAG
- Strands have a helical shape and short peptides extend from glycan backbone at right angles to each other.
o Means that each peptidoglycan strands can be cross-linked to peptidoglycan strands above it, below it and the either side of it. - Chains are cross-linked by peptides for strength:
o Direct crosslinking – amino of 1 to amino of another
o Interbridges may form (bridge made of short peptide chain between the short peptidoglycan peptides)
o Various structures occur (determined by the amino acids used in the cross-linking peptides)
Transpeptidation
o Bond formed between adjacent peptide chains, catalysed by enzyme transpeptidase.
- Enzyme removes C5 (terminal C) during reaction. - Antibiotics like penicillin interfere with transpeptidation (inactivate transpeptidase), which makes the peptidoglycan mesh around a bacterial cell weaker so it is more susceptible to death by lysis.
Peptidoglycan as a drug target
o Penicillin’s and cephalosporins and glycopeptides only kill growing cells (because all of these antibiotics target peptidoglycan synthesis).
o Peptidoglycan is an important target for beta-lactam antibiotics and glycopeptide antibiotics.
Peptidoglycan as an enzyme target
o Lysozyme hydrolyses the B(1-4) glycosidic bonds in glycan chains of peptidoglycan.
o This hydrolysis makes bacteria susceptible to lysis by osmotic effects.
o Lysozyme is an antimicrobial enzyme produced by humans and animals, present in human secretions like tears, saliva etc.
o Lysozyme is most effective against Gram-positive pathogens like Streptococcus.
Gram-Negative Cell Walls Structure
- Thin layer of peptidoglycan surrounded by an outer membrane
- More complex than gram-positive cell walls
- Outer membrane composed of lipids, lipoproteins and lipopolysaccharides
- No techoic acids
- Harder to treat gram-negative infections because of double membrane and periplasmic space (space between outer Membrane and peptidoglycan)
- Many enzymes present in the periplasmic space. Space constitutes 20-40% of cell volume
Gram-Negative Outer Membrane Permeability
- More permeable than plasma membrane due to porin proteins and transporter proteins
- Porin proteins form channels through which small mols can pass
Gram- Positive Cell Wall Structure
- Composed primarily of thick layer of peptidoglycan
- May contain large amounts of techoic acids (Neg charge):
- Helps maintain structure of cell envelope
- Protects from environmental substances (like antibiotics and host defence molecules)
- May help pathogens bind to host tissues in initiating infection
- Some gram+ bacteria have layer of proteins on surface of peptidoglycan
- Periplasmic space between plasma membrane and cell wall, very few proteins, smaller than in Gram-.
Techoic Acid Structure
- Polymers of glycerol or ribitol linked by phosphate groups
- ONLY in Gram+ bacteria
- Amino Acids or sugars attached to glycerol/ribitol units
- Techoic acids contribute to negative charge of Gram+ cells
Lipopolysaccharides (LPs)
- In the Gram negative bacterial cell wall
- Consist of 3 parts:
- Lipid A - anchors polysaccharide in outer membrane
- Core polysaccharide
- O-side chain - O antigen; varies between bacterial strains
- Lipid A embedded in outer membrane; Core polysaccharide and O-side chain extend out from the cell
Importance of Lipopolysaccharides
- Contribute to negative charge on cell surface
- Help stabilise outer membrane structure
- May contribute to attachment to surfaces and biofilm formation
- Creates a permeability barrier
- Protection from host defences
- Lipid A part of LPS can act as an endotoxin
Exoenzymes
Enzymes secreted by Gram+ bacteria
Aid in degradation of polymeric nutrients
S Layers
- Regularly structured layers of protein or glycoprotein that self-assembles in bacterial cell walls.
- In Gram- bacteria the S layer adheres to the outer membrane
- In Gram+ bacteria it is associated with the peptidoglycan surface
S Layer Function
- Protects from ion & pH fluctuations, osmotic stress, enzymes and predation
- Maintains shape and rigidity
- Promotes adhesion to surfaces
- Protects from host defences
- Potential use in nanotechnology (because of its spontaneous association)
Main groups of Archaea
- Methanogens (Produce methane)
- Halophiles (salt loving)
- Thermophiles (Heat loving)
Archaeal Cell envelope differences to Prokaryotes
- S-layer may be only component outside plasma membrane
- Some archaea lack a cell wall
- Capsules and slime layers are rare
Archaeal Cell Walls
- Lack peptidoglycan
- Most common cell wall is an S-layer
- May have protein sheath external to S-layer
- S-layer may be outside the membrane and separated by a psuedomurein
Pseudomurein subunit composition
- Peptidoglycan-like molecule
- L-amino acids rather than D-amino acids
- NAG & NAT (N-acetyltalosaminouronic) not NAM
- B(1-3)glycosidic bonds not B(1-4)glycosidic bonds
Archaeal Membranes
- Have distinctive membrane lipids (different from BOTH bacteria and eukaryotes)
o Branched-chain hydrocarbons bonded to glycerol by ETHER bonds, not fatty acids linked by ESTER bonds. - Contain diglycerol tetraether
- May also have polar lipids: phospholipids, sulpholipids and glycolipids
- Can mix diethers, tetraethers and other lipids to give membranes of differing rigidity and thickness.
- Some have monolayer structure instead of a bilayer structure (unique to archaea)
What makes the archaeal membrane different to that of bacteria and eukaryote?
- Some have monolayer structure instead of a bilayer structure
- Contains diglycerol tetraether
- Has distinctive membrane lipids (different from BOTH bacteria and eukaryotes)
o Branched-chain hydrocarbons bonded to glycerol by ETHER bonds, not fatty acids linked by ESTER bonds.
What are 6 similarities between Archaeal and Bacterial Molecular Biology?
- Pyrococcus exhibits bidirectional DNA replication from a single origin (like bacteria)
- Archaeal mRNA similar to bacterial mRNA
o Polycistronic mRNAs (RNA from operon) are produced and NO evidence for mRNA splicing – very bacterial like. - Archaeal promoter similar to those in bacteria
- Ribosomes are 70S (Like bacteria) BUT shape is variable and can be different from bacteria and eukaryote.
- Archaeal small-subunit rRNA is 16S like bacteria
- Both have single circular chromosome per cell
What are 5 similarities between Archaeal and Eukaryote Molecular Biology?
- EF-2 (elongation factor) sensitive to Diptheria toxin (inhibits protein synthesis)
- Archaeal DNA replication proteins similar to those in eukaryotes
- Archaeal RNA polymerase also similar to that in eukaryotes: INSENSITIVE to rifampicin and streptolydigin (which inhibit bacterial transcription)
- Protein synthesis is sensitive to anisomycin, resistant to kanamycin and chloramphenicol.
- Most aspects of archaeal DNA, RNA and protein synthesis resemble those in eukaryotes
Common Properties to All Archaea
- Cell Walls
- Membrane Lipids
- Transcription & Translation machinery
- Coenzymes
- Mechanism of autotrophic CO2 fixation
Are Archaea more closely related to Eukaryotes or Bacteria?
Eukaryotes
What are 8 Cellular Structures common to all Prokaryotes? (Bacteria & Archaea)
- Protoplast
- Cytoplasm
- Cytoskeleton
- Nucleoid
- Plasmids
- Pili
- Fimbriae
- Flagella
