Lecture 1: Structure of Biological Membranes, Function of Membrane Proteins

What are the two main constituents of the plasma membrane?

lipid and protein

What are the two regions of the phospholipids?

hydrophilic (head group) and hydrophobic (backbone)

What does amphiphathic mean?

a molecule has both hydrophobic and hydrophilic regions

If there is a lower concentration of phospholipids, which form will the molecule be in?

monolayer

If there is a higher concentration of phospholipids, which form will the molecule be in?

bilayer

What do the physical characteristics of the lipid bilayer depend on?

the chemical composition of the phospholipids that make it up

What does the width of the plasma membrane depend on?

the length of the fatty-acid tails

What does the density of the plasma membrane depend on?

how closely packed the phospholipids are in each leaflet

True or False: Phospholipid bilayer is a fluid structure. Individual phospholipid molecules are free to diffuse, but must stay within same leaflet.

true

What is the backbone of the phospholipid?

glycerol

Which group distinguishes phospholipids from sphingolipids?

sphingolipids contain a sphingosine group

True or False: Phospholipids can move laterally, rotate, or flex. Rarely do they flip to the other leaflet.

true

Cholesterol aids in ____ the membrane and can flip ____.

stiffening; easily

Is the bilayer asymmetrical or symmetrical?

asymmetrical

Which phospholipids are enriched on the extracellular side?

choline-containing phospholipids (PC), sphingolipids

Which phospholipids are enriced on the intracellular side?

amine-containing phospholipids (PS and PE)

How does the distribution of the phospholipids across the bilayer form?

vesicle formation

Where does phospholipid synthesis occur?

the endoplasmic reticulum/golgi complex

Describe the vesicle formation process.

1. a piece of the ER/golgi membrane buds off

2. protein complex forms a bridge to bring vesicle bilayer into close proximity to plasma membrane bilayer

3. fusion of hydrophobic bilayer cores

Which type of proteins are embedded in the vesicles?

integral proteins

What is the inside of the vesicle?

the lumen

The lumine becomes which side of the membrane?

the extracellular

The cytosol becomes which side of the membrane?

the intracellular

Where do enzymes that catalyze the synthesis of PE and PC arise from?

the cytosolic side of ER

Where do enzymes that catalyze the synthesis of PS and sphingolipid?

the luminal surface of the ER or golgi

How do PCs end up on the extracellular side, and the PSs on the intracellular side?

trans bilayer transporters

What moves PSs?

flippase (PSs move in)

What moves PCs?

floppase (PCs move out)

Which enzyme moves cholesterol?

floppase

What type of transporter are floppases and flippases?

ATP-dependent transporters

What do ATP-dependent transporters do?

transport phospholipids against their concentration gradients

What are peripheral proteins?

proteins that are noncovalently bonded with integral proteins or to phospholipids through electrostatic or hydrophobic interactions

True or False: Most integral membrane proteins have membrane-spanning a-helical domains of about 20 amino acids.

true

What are integral proteins?

proteins that are embedded within (but not necessarily spanning) or covalently attached to the lipid bilayer

How do membrane-spanning portions stay stably associated with bilayer?

• most membrane-spanning segments composed mainly of amino acids with nonpolar side chains

• bilayer phospholipids protect membrane-spanning portions of transmembrane proteins from aqueous environments

• amino acid sequences of most membrane-spanning segments form a (alpha) helices

True or False: Passive protein diffusion is slow. Even faster proteins diffuse 1000X more slowly than typical phospholipids.

true

What are lipid rafts?

membrane microdomains enriched in sphingolipids and cholesterol

True or False: Proteins ride along with moving phospholipids, enabling faster charges in protein localization in membrane.

true

Describe how ligand-binding receptors function.

1. presence of extracellular water-soluble ligands must be made known to intracellular elements to modulate their behavior

2. interaction of ligand with extracellular portion of receptor

3. conformational changes in receptor protein

4. intracellular domain can become enzymatically active or interact with cytoplasmic proteins

What are the two types of adhesion (integral) molecules?

• cell-matrix adhesion molecules

• cell-cell adhesion molecules

What do cell-matrix adhesion molecules do?

forms physical contacts with surrounding extracellular matrix

What is an example of cell-matrix adhesion molecules?

integrins

What are cell-cell adhesion molecules?

attach cells to each other

What are some examples of cell-cell adhesion molecules?

cadherins (Ca2+-dependent) and neural cell adhesion molecules (NCAMs, Ca2+-independent)

Which integral proteins move water-soluble substances?

• pores and channels

• carriers

• pumps

What are pores and channels?

conduits for passive water/ion/protein flow

What are carriers?

facilitate transport of specific molecules across membrane

What are the pumps?

uses energy of ATP hydrolysis to drive transport against energy gradient

True or False: Integral proteins allow hydrophilic substances to cross the membrane by creating a hydrophilic pathway in the bilayer.

true

What type of enzymes are pumps?

ATPases

Membrane-bound enzymes break down other molecules. List a few examples.

• intestine: polysaccharides broken down into simple sugars

• polypeptides into shorter peptides or amino acids

• nervous system and neuromuscular junction: breakdown of neurotransmitters

True or False: Integral proteins attach to plasma membrane by covalently attaching to fatty acids or prenyl groups that insert into lipid bilayer. Many integral proteins are involved in relaying signals received at the cell surface to effector machinery in cell interior.

true

What are the two types of peripheral protein membrane associations?

• ionic interactions with phospholipid head groups

• direct binding to extracellular or cytoplasmic surfaces of integral membrane proteins

What is the sub membranous cytoskeleton and ankyrin important for?

• provides plasma membrane with strength and resilience

• organizes plasma membrane into morphologically and functionally distinct domains

What is ankyrin?

a peripheral membrane protein that anchors spectrin-actin meshwork to integral membrane proteins

True or False: Transmembrane proteins in a wide variety of cells bind ankyrin.

true