Prokaryotic cytoplasmic membrane consists of
A phospholipid bilayer composed of phospholipids and proteins
Fluid mosaic model
The membrane is fluid because there is extensive lateral mobility of bulk proteins and phospholipids
What can cross the selectively permeable cytoplasmic membrane?
Water, gases, and small hydrophobic molecules
What cannot cross the selectively permeable cytoplasmic membrane?
Most polar compounds such as amino acids, organic acids, and inorganic salts.
What is a phospholipid made of?
2 fatty acid chains attached to the 2 carbon atoms of glycerol by ester bonds
A phosphate is attached to the third carbon atom of glycerol
Small organic groups linked to the phosphate group give additional variety of phospholipids
What fatty acid feature makes membranes more fluid?
Unsaturated fatty acids (i.e. double bonds within the fatty acid chain) and branched fatty acids make membranes more fluid.
Palmitic Acid (C16)
The predominant saturated fatty acid in bacteria
4 fatty acids present in bacteria
Palmitic acid (C16, most common), myristic (C14), stearic (C18), and lauric (C12)
Main unsaturated fatty acids in bacteria
Oleic acids (C18 monounsaturated)
(T/F) Polyunsaturated fats are not found in bacteria
True
Fatty acid features common in bacteria
Branched, hydroxylated, methylated, or cyclopropane ring-containing
What catalyzes fatty acid biosynthesis and what substrates are needed?
Fatty acid synthetase uses acetyl-CoA and malonyl-CoA as substrates
How is malonyl-CoA made?
From the carboxylation of acetyl-CoA using ATP as the energy source
Acyl carrier protein (ACP)
Serves as the carrier for the growing fatty acid chain
What is required as the reductant in fatty acid biosynthesis?
NADPH
Why do fatty acids always have an even number of carbons?
Because fatty acid synthetase adds two carbons at a timeto the carboxy end of the growing fatty acid chain
What happens to the inital carbon added to form malonyl-CoA at the end of fatty acid biosynthesis?
It is lost as CO2.
Pentose phosphate pathway
the source of reduced NADP (NADPH) used in fatty acid biosynthesis
What are the steps of phospholipid biosynthesis?
- FA CHAINS ADDED: ACP-fatty acid chains are added onto glycerol-3-phosphate, forming phosphatidic acid.
- COUPLING TO CDP: Phosphatidic acid is coupled to CDP -> CDP-diglyceride (CDP is the carrier)
- Head groups are added to CDP-diglyceride
- Serine is added to make phosphatidylethanolamine
- Glycerol-3-phosphate is added, making phosphatidylglycerol
The two most common phospholipids found in the bacterial membrane:
Phosphatidyl ethanolamine and phosphatidyl glycerol
Functions carried out by membrane proteins (7)
- Solute transport (e.g. ABC transporter for nutrient uptake)
- Electron transport (e.g. cytochromes)
- ATP synthesis (ATPase_
- Protein secretion (e.g. Sec translocase)
- Motility (flagella motor)
- Sensing environmental signals (sensor histidine kinase of a two component regulatory system)
- Biosynthesis of cell wall polymers and lipids
Function/ components of the Sec system
Involved in translocation of membrane-bound proteins
- Leader peptide (on membrane protein)
- Chaperon protein (SecB)
- Membrane bound SecYEG complex
- Peripheral ATPase (SecA)
3 ways that membrane proteins can be retained on the cytoplasmic membrane
- Leader peptide is not recognized and therefore not cleaved by the signal peptidase
- An internal hydrophobic region of amino acids called the “stop transfer sequence”
- Both the leader peptide and stop-transfer sequence
Once proteins are at the membrane, they will fold into a _________, which requires:
Conformation that is most stable thermodynamically.
- Most of the AA side chains must be non-polar (Ala, Val, Leu, Ile, Phe) to allow interactions with phospholipids
- Polar part of the protein backbone of transmembrane segments must hydrogen bond, which is accomplished via alpha-helices or beta-barrels
How is H bonding achieved in membrane proteins?
- When all peptide bons are H-bonded internally, an alpha-helix forms
- Beta sheets can also achieve H-bonding, provided that the beta-strands form closed beta-barrels
Bacteriorhodopsin
A proton pump produced by some archae to capture light energy for pumping out protons (to generate a proton motor force)
Lipids in archaeal cell membranes consist of either:
- C20 isopranoid alcohols ether-linked to a glycerol to form monoglycerol diethers
- C40 isopranoid alcohols ether-linked to 2 glycerols to form diglycerol tetraethers
Monoglycerol diethers form a
Lipid bilayer
Diglycerol tetraethers form a
Monolayer, because they are long enough to span the entire membrane
(T/F): Archaeal cell membranes can only contain either diethers or tetraethers
False. The ratio of diethers and tetraethers varies depending upon the bacterium
Two key differences between archaeal and bacterial membrane lipids
- Bacterial lipids are ester bonded ad are made of fatty acids
- Archaeal lipids are ether bonded and are made of isoprene (isopranoid alcohols)
Mycoplasma
Bacteria that lack a cell wall (just have a membrane, no PG).
The membrane contains sterols that hold the cell shape in addition to phospholipids
Sterols (where are they found and how are they acquired?)
In mycoplasma cytoplasmic membrane. Hold the cell shape.
Sterols are acquired from the environment, usually as cholesterol from the host.
Why are beta-lactam antibiotics ineffective against mycoplasma?
Beta-lactam antibiotics cannot attack mycoplasma because they dont have a cell wall
Mycoplasma human pathogens
M. pneumoniae and M. gentalium
Polymyxin
An antibiotic that affects membrane function
- Cationic antimicrobial peptide (cyclic peptide, long FA tail)
- Inserts into and disrupts membranes rich in phosphatidyl ethanolamine
- Can kill bacteria that is not growing
- Topical use only (e.g. Polysporin)