Download Biological membranes: the basics and why they are

Biological membranes: the basics
and why they are important in IBBE
“Thinking about membranes
could improve your bioprocess”
Membrane function: an evolutionary
perspective
• First arose as a free energy barrier between ‘outside’
and ‘inside’ (diffusion)
• Keeps cell contents from leaking out and unwanted
chemicals getting in
• Evolved to permit and regulate the transfer of nutrients
and waste products (channels)
• Acquired the ability to achieve these functions against a
concentration gradient (transporters)
• Later developments: conversion of membrane potential
to do work; signalling from outside to inside; cell
recognition; movement of molecules in eukaryotic
vesicles; compartmentalization (eukaryotes)
Membrane engineering: opportunities
for enhanced productivity
Rosanese et al. (1999) J. Cell Biol.
“Thinking about membranes
could improve your bioprocess”
Membrane composition: lipids and
proteins
• Phospholipid
bilayer
• Hopanoids act
as stiffening
agents
• False
impression:
> half the volume
of membrane is
protein
Fluid mosaic models
• Flexible phospholipid
bilayer interspersed with
protein molecules
• Fluid – some parts move
freely, if not anchored by
other cell components
• Mosaic – patchwork
networks of proteins
Chemical toxicity: historical
perspective
• Damage to cell membrane function implicated in chemical toxicity
• Toxicity correlated to hydrophobicity (partition into membrane
and lower surface tension)
• Membrane fluidity increases
Stolte et al. (2007) Green Chem. 9, 1170.
The barrier: phospholipids
• PE is the major E. coli
phospholipid
Variation in fatty acid components:
stiffening the membrane under stress
• Adaptation to starvation,
temperature increase and
acid stress is associated with
membrane stiffening
• cis unsaturated FAs have
‘kinked’ chains and pack less
well than saturated FAs –
increased fluidity
Modifying lipid profiles: enhanced
tolerance to toxic chemicals
• Evolved strains gain tolerance by changing lipid profiles
• Maintain optimal membrane fluidity under stress
• Unpredictable: E. coli - isobutanol tolerance increased
U:S ratio; n-butanol tolerance decreased U:S ratio
• Clostridium: solvent tolerance more consistent
decreased U:S ratio (abolished U production)
• Engineered strain
• Pseudomonas cti
gene into E. coli
Modifying lipid profiles: enhanced
tolerance to toxic chemicals
Octanoic acid tolerance
‘sweet-spot’
Modifying lipid profiles: enhanced
tolerance to toxic chemicals
Not only increased
tolerance but
increased production
of octanoic acid and
styrene
More radical adaptations: engineering
membrane fluidity
Control gene expression and integrate
into cell regulatory networks
Crossing Biological Membranes
Crossing Biological Membranes
Primary transporters
Secondary transporters
Transporter engineering: threonine
producing E. coli
Transporter engineering: threonine
producing E. coli
parent
thrA*
Dtdh
DtdcC::rhtA23
sstT
Lee et al. (2009) Microb Cell Fact 8:2
Energetics: membrane potential is
crucial
Thinking about membranes could
improve your bioprocess
Engineering the
cell-environment
interface to
improve process
efficiency