PROTEINS AND MEMBRANES

Proteins:Recall protein secondary structures: -helices and ß-sheets

Proteins in membranes

Integral: membrane-spanning (generally N- terminal out in plasma membrane) Peripheral: associated with membrane surface (often associated with membrane- spanning protein or membrane- intercalating lipid)

Proteins in membranes

Integral: membrane-spanning (generally N-terminal out in plasma membrane) • -helix: 19 m ino cids, hydrophobic side chins • m ulti- -helix (bcteriorhodopsin) • m ulti-ß-shee t (m ltoporin)

Bacteriorhodopsin: note the seven membrane-spanning -helices

Maltoporin: a ß-barrel protein, with membrane-spanning ß sheets

Although they are not common, there are several ß-barrel proteins known--primarily trans-membrane transport proteins.

Unilateral membrane embedding may occur through hydrophobic amino acids

Or connection to glycolipids

Folding of membrane protein during synthesis is complex:

(Science 339:398, 25 January 2013)

Folding oflactosepermeasedepends onthe lipidcomposition of the membrane.

Folding ofaquaporininvolves changes in the orientationof alpha-helices.

Membrane “rafts” areaccumulations of proteins, stabilized byglycosphingolipids andcholesterol.

(Science 327:46, 1 January 2010)

Membranes are heterogeneous:

How molecules cross membranes

1. Dissolving in lipid layer Small non-polar molecules (benzene, ethanol, O2, CO2) Works for artificial lipid bilayers (i.e., no proteins)

2. Pores in lipid layer Small polar molecules (H2O, NH3) Also works for lipid bylayers Postulate transient pores 3. Channels, Carriers, Pumps Specific materials transported Specific membranes Regulated in time and sometimes direction Proteins

The purpose of membranes is to control transport into and out ofthe cell or from one cell compartment to another.

Channels ♦ Protein comp lex form ing controlled hole for rpid

flow ♦ "Downhill” (long free -energy grdient) ♦ “Gted” (opens, closes in response to stim ulus)

♦ Chnnels known for N+, K+, Cl-, C2+, nd others, including H2O (quporins)!

Example of a channel:A nerve impulse involves depolarization, followed by re-polarization.Nerve cells’ electrical polarity results from coupled Na+ efflux and K+ influx.Depolarization results from Na+ influx (opened Na+ channels).Re-polarization results from K+ efflux (opened K+ channels).

Potassium channel: responsible for re-polarizing nerve cellsafter a nerve impulse.

Note the 3-angstrom constriction with negatively charged groups.

How does this channel control K+ movement, and why is it specific for K+?

This is the slide used to explain the ability of enzymes to catalyze chemicalreactions: does this apply to the operation of channels?

Enzymes bind tosubstrates, so G(ES) < G(E+S).

However, if all theydid was to bind, thenDG for the reactionwould not be reduced.

So when they bind thesubstrate, they stressIt in some way, raisingG(ES) and reducing DG(ES*)(=Ea).

The specificity of the K+ channel for K+, relative to Na+, depends on desolvation and resolvation energy.

What would explain the specificity of the Na+ channel?

From Science, 3/12/2010: “Pain’s in the genes”:

“Subtle changes to a certain gene seem to determine how sensitive people are to pain, according to new research. In the past 5 years, researchers have discovered that three rare but serious pain disorders are caused by mutations in a gene called SCN9A. In nerve cells that relay painful sensations in the body's tissues to the central nervous system,…”

How many of you are particularly sensitive to pain?…particularly insensitive?

What do you think the mutation affected? (a) A membrane lipid(b) A channel(c) A carrier(d) A pump

In nerve cells that relay painful sensations in the body's tissues to the central nervous system, SCN9A encodes instructions for sodium channels that help the cells fire. In two of the disorders, people carry faulty versions of the gene and suffer crippling pain because their sodium channels open too easily or can't close. In the third disorder, which leaves patients unable to feel pain at all, SCN9A produces a protein that can't function.

"We wondered if more common, apparently harmless [changes] in the gene might give rise to an altered degree of pain threshold," says Geoffrey Woods, a medical geneticist at Cambridge University in the U.K., who discovered the genetic reason for this third disorder. One [genetic variant], found in 10% of the subjects, caused the greatest increase in reported pain between those who had it and those who didn't. When the team applied heat stimuli to 186 healthy women, they found that those with the rare version were more likely to have lower pain thresholds. It was as if the normal subjects had taken an ibuprofen, but the subjects with the rare SNP hadn't.

Mechanosensitive channels in bacteria:these open in response to high tension

(Science 321:1166, 29 August 2008)

Summary

•Intrinsic membrane proteins generally cross the membranewith an -helix

•Some membrane pore proteins use a ß-barrel structure•Membrane proteins can associate in “rafts” stabilized by

sterols and sphingolipids•Hydrophilic molecules, including ions, cross membranes through

channels and pumps•Nerve function (and other functions) depend on control of

channel opening•The specificity of the potassium channel depends on solvation-

desolvation energy•Mutations in channel proteins influence ease of nerve

stimulation