Membranes & Membrane Proteins

drug-resistant pathogenic fungi

enzyme target for antifungals: chitin → chitobiosidase, endochitinase

Selective disruption of viral membranes

Structured Regions

Freely diffusing proteins in membranes probably would not give efficient communication or biological processes

Lipid rafts: thicker membranes, cholesterol rich, glycolipid rich, GPI-anchored proteins

Enriched in signaling proteins

Peripheral membrane proteins

not strongly bound to the membrane; dissociated with mild detergent treatment or with high salt concentrations

Integral membrane proteins

strongly imbedded in the bilayer; can only be removed from the membrane by denaturing the membrane (organic solvents, or strong detergents); Often transmembrane but not necessarily; Polar side chains are usually not found in membrane-spanning regions

ex. Glycophorin, bacteriorhodopsin

Lipid-anchored membrane proteins

covalently bonded to lipid

• Amide-linked myristoyl anchors

• Thioester-linked fatty acyl anchors

• Thioether-linked prenyl anchors

• Glycosyl phosphatidylinositol anchors

Hydropathy Plots

graphical means of describing the hydrophobicity of amino acid sequence segments is used to assess the  likelihood of membranelocalization and topology.

Why don’t we see loops, or disordered regions within the membrane?

nonpolar environment; No H-bond donors or acceptors provided by lipids in the hydrophobic interior of membrane; H-bond pairing of peptide backbone is satisfied by alpha-helix structure; An isolated helix is stable in membrane context (unlike a helix in aqueous environment)

Transporters” or “permeases”

Membrane proteins assist the transport of polar compounds; lower the energy barrier to transmembrane passage by providing a hydrophilic passage, allowing for facilitated diffusion.

Passive transport

no energy required

Active transport

energy required, can “pump” against concentration gradient

Membrane channels common architecture

-strands surrounding a large channel (d ~ 3 nm)

Porins are passive membrane transport channels (-barrels).

Nonpolar residues face membrane polar residues face channel

Electrochemical/concentration gradients of ions

These gradients are maintained by ATP-dependent pumps and channels - very specific

K+/Na+ gradient that is established can provide free energy to export other ions such as Cl-

Membrane Potential (ΔΨ or ΔV )

primary active transporters

one molecule up gradient; phosphorylated or ATp hydrolysis

secondary active transporters

one up, one down = use down transport to energize up

Antiporter

opposite directions of secondary active transport

Symporter

same directions of secondary active transport

Na+–K+ ATPase

P-type Primary Transporter; Responsible for maintaining an electrochemical gradient across animal cell membranes 

Binding of ATP to the N domain leads to phosphorylation of the P domain and subsequent conformational changes in the A domain

Aquaporins

protein family of passive transport channels; Transmembrane channels that facilitate passage of water (and glycerol or urea), Particularly important in erythrocytes, renal ducts, and plant vacuoles, Associated with multiple disease states

tetrameric protein complexes; Selectivity of aquaporins is

mediated by a “constriction point”

Glucose transporter family (GLUT)

Enhances diffusion of glucose by 50,000 fold; Highly specific for D-glucose over L-glucose, mannose, galactose

Kinetics of glucose transport are enzyme-like; Rate as a function of solute concentration can be saturated. Suggests a distinct binding site and mechanism of transport.

alternating access model (GLUT1)

Receptor Proteins

Information Gatekeepers of the Cell; generation of second messengers by an upstream signaling protein results in signal amplification through the activation of one or more downstream target proteins