Bacterial membranes, transport and secretion L5 Flashcards
what are the functions of the cell membrane
transport
energy production
interaction with environment
what is the transport function in cell membrane
controls what comes into and goes out of cell
what is the energy production function in cell membrane
bacteria have no intracellular organisms, no mitochondria, cell membrane involved in electron transport so all in cytoplasmic membrane
what is the interaction with environment functions of cell membrane
- acts as a receptor
- receives signals from both inside and outside the cell (e.g. MCP – chemotactic proteins)
- lots of receptor molecules in membrane – transfer information throughout cell
what is the amphipathic character
Common feature of molecules that spontaneously associate to form a lipid bilayer is an amphipathic character
how are lipids amphipathic
- hydrophilic region (polar or charged) that can interact with aqueous solvent
- hydrophobic region (two acyl groups or planar multi-ring systems) that can segregate from aqueous solvent
what contains sterols in their membrane
eukaryotes and Mycoplasma (cell wall free bacteria) do
what are the membrane proteins
Peripheral membrane proteins
Integral membrane proteins
what are the peripheral membrane proteins
- bound to bilayer surface
- can extend from either surface
- roles in anchoring, sensing, transport
what are the integral membrane proteins
- amphipathic, if was all hydrophilic would forced out of membrane, wouldn’t be able to stay – need a specialized structure, integral membrane proteins must be amphipathic
- roles in transport and synthesis
what is the name for all the proteins embedded in the membrane
fluid mosaic model
what is the cytoplasmic membrane barrier like
Gram-negative and Gram-positive bacteria:
Hydrophobic - good barrier to charged/hydrophilic molecules
Non-polar molecules pass through freely
Small uncharged polar molecules also pass though free
what generates NADH in electron transport
Energy from catabolism generates reduced NADH
where are electrons delivered to in electron transport
delivered to cytochromes
where are protons delivered to in electron transport
H+ pumped out of cell (into periplasm/pseudoperiplasm)
what is the effect of proton movement in electron transport
Generates a proton gradient across membrane since charge molecules cannot freely diffuse back into cell
what is catabolism
getting food molecules taken up by cell, fed into TCA cycle, energy generation from that, produce NADH
what happens when proton and electron fed into electron transport chain
H+ ions pumped out of cell, generates proton motive force as now have a gradient across membrane as charged molecules can’t get back in
some H+ ions can come back into cell through ATPsynthase, create ATP
where does electron transport occur in eukaryotes
mitochondria
where does electron transport occur in prokaryotes
e.g. bacterial cell this occurs in cytoplasm, in bacteria can be powered directly by H+ ions, don’t need ATP
what is the proton gradient used for
power other cellular processes
what is the gram-negative cells outer membrane barrier like compared to cytoplasmic membrane
unlike cytoplasmic membrane, outer membrane allows passage of some polar molecules to get to cytoplasm
what is the porins in the gram-negative outer membrane function
porins provide, water filled channels for the movement of small hydrophilic molecules
Other porins are highly specific with binding site for one or more molecules e.g. LamB binds maltose
what is a porin
porins are proteins, don’t allow passage of large molecules e.g. enzymes and other proteins
what is homotrimeric
type of porin structure
three copies of same protein, forming a complex, each with a hole in middle
what is barrel structure like
type of porin structure
side view, is hydrophobic – all around outside interacts with lipids, keeps it buried in membrane
what is the structure like when all the loop bits are hydrophilic
interact with molecules that are being taken up or the water/cytoplasm phase in general
what are the porin structures
Homotrimeric
Barrel structure
All loop bits are hydrophilic
how do large polar molecules and ions pass through cytoplasmic membrane barrier
don’t pass across cytoplasmic membrane freely; need specific transporters
how is the production of transport proteins regulated
in response to presence of substrates, can’t fill fluid membrane with every transport protein - only make transport proteins they need in response to the presence of those substrates
uniporter
one molecule moves in one direction (can also be outside to in) causes cell dissipation
symporter
two molecules move in one direction
antiporter
two molecules move in opposite direction
what is important in maintaining charge gradient across membrane
antiporter and symporter
how is maltose taken up in gram negative
what does this allow
Outer membrane porin protein
allows selective transport of maltose
what happens to maltose once taken up
In periplasm, need to get across inner membrane to be taken up by cell
Sugar picked up by specific transport protein – MalE can bind to maltose, will pick it up and transport it across cell to outer face of inner membrane
how does maltose affect ATP in uptake
During process ATP used to bring in sugars that will generate more energy
how does maltose uptake affect proteins
Complex of proteins will form a particular transport complex – waiting to react with MalE, delivers sugar and get uptake into cell, so maltose inside
what occurs in group translocation
Transported compound is chemically modified during the transport process
what happens in glucose transport system
During uptake sugar becomes phosphorylated, also first step required for glucose (to be fed into TCA cycle and for energy generation) metabolism, so uptake is energetically ‘neutral’
what is involved the phosphotransferase system
5 proteins some of which are cytoplasmic others are membrane protein
what happens in glucose translocation
glucose is targeted to PTS Enzyme 1 gets phosphate from PEP
PEP comes from TCA cycle
Phosphate is then transferred to Glucose via HPr and EII
HPr = Histidine Protein/Heat stable protein
Enzyme Complex 2 = 2 proteins (EIIA and EIIB)
what is the porin in the outer membrane like for group translocation
Porin in outer membrane to allow sugars through membrane
In this case is not a selective recognition system, is an open pore allow molecules through
what happens when glucose is moved into the periplasm
interact with complex in membrane
Glucose delivered in activate state, is energised from inside – enzyme complex 1, HPr, enzyme complex 2 (made of two parts A and B) and transport system in membrane – enzyme complex 2 part C
what does the PEP do, and what does it use
PEP transport phosphate group to enzyme complex 1, transported via HPr to enzyme complex 2
what happens when phosphate transported to enzyme complex 2
now providing phosphate on enzyme complex 2 B, glucose reacts with enzyme complex 2 C, phosphate transformed onto glucose = glucose-6-phosphate accumulates inside cell
how does glucose translocation affect the cell
energetics: if have glucose on outside and move it inside, chemical gradient changes – more glucose on inside, harder to get glucose into cell as up concentration gradient
BUT as soon as become glucose-6-phosphate no longer contributes to gradient as a different molecule – so more glucose on outside than in
why are protein transport systems important
transport of proteins outwards across cell envelope, important as relates to pathogens making protein toxins, will make toxin
what does the type I (ABC) transporter require
System requires three specific proteins, signal comes in causes a change in transport system, ATP binds causes conformational change of protein, creates channel, opens pore lets protein be translocated
ATP hydrolysis to energise all these functions
what does the ATP binding cassette protein do in the typeI (ABC) transporter
- provides energy for translocation
- recognises a C-terminal sequence, so cassette knows it’s going to transport out the right proteins
what does the inner membrane protein do in the type I (ABC) transporter
- membrane fusion protein (MFP), create a pore, let proteins cross inner and outer membrane in one movement
- provides substrate selection and “gate” for pore to control transport process, stops things leaking out of cell. Gate is triggered when the C-terminal signal is delivered into inner membrane proteins
what does the outer membrane protein do in the type I (ABC) transporter
allows protein to cross outer membrane and leave cell
what is an example of a type I system
Virulence factor secreted by a type I system - E. coli
what happens in the haemolysin type I system
Phospholipase enzyme – target the lipid in RBC, cause them to break open, release nutrients from inside RBC, so bacteria get nutrients
what is the test to see if something is haemolytic
lysis of RBC incorporated into agar, as make dish, let molten agar cool down mix with blood and add to petri dish then set, when do streaks, can see around single colonies are clear zones as phospholipase secreted by cell, get into agar, causes lysis zones
what are type II transporters
Most common and well-studied transport system, known as the General Secretory System
Made up of General Secretory Proteins (GSPs)
what do type II transporters require
two systems
what are the two steps that occur in type II transporters
- Inner membrane Sec transport system
2. Outer membrane transport system
what does the inner membrane system recognise
Inner membrane Sec complex recognises specific N-terminal sequences in proteins destined for secretion
= Leader or Signal sequence as N-terminal at start
what does the protein in the sec system do
Protein delivered to Sec complex by SecB, free in cytoplasm, first going to recognition event, pick proteins with N-terminal tag, deliver to inner membrane Sec complex, this transport is energised by ATP
what is the function of SecB and what does this lead to
SecB delivers protein that will be secreted to the complex, as it goes through N-terminal tag is snipped off by the protease – cleavage event, so protein that ends up in periplasm is now shortened, now picked up, must be delivered to specific outer membrane system
what interacts with the proteins in the sec complexes
Various different outer membrane systems that interact with all proteins that come through Sec complexes
how is maltose taken up in gram positive bacteria
don’t need outer membrane part of system
sugars can diffuse down to surface of cytoplasmic membrane
still picked up by proteins in pseduoperiplasm
