Membrane Proteins and Carbohydrates

Give the 3 types of membrane protein:

1) integral/ intrinsic membrane protein

2) lipid-linked membrane protein

3) peripheral/ extrinsic membrane protein

What are the 4 groups of integral proteins?

1) monotopic

2) bitopic

3) polytopic

4) oligomeric

What are monotopic membrane proteins?

intrinsic proteins that insert into the membrane but do not span it

What are bitopic membrane proteins?

intrinsic proteins with a single helix that span the layer once

What are type 1 bitopic membrane proteins?

bitopic proteins where the N terminal is facing outside the cell

What are type 2 bitopic membrane proteins?

bitopic proteins where the N terminal is facing inside the cell

What are polytopic proteins?

intrinsic proteins that have segments spanning the membrane and are connected by loopS

What is another term for polytopic proteins?

type 3 membrane proteins

What are oligomeric proteins?

instrinsic proteins that result when multiple proteins oligomerize

What does oligomerize mean?

form a few repeating units

Give 5 broad functions of membrane proteins:

1) transport

2) enzymatic activity

3) signal transduction

4) cell-cell interactions

5) attachment to the cytoskeleton

Describe the role of membrane proteins in cell-cell interactions:

glycoproteins are recognised by proteins by other cells

Describe the role of membrane proteins in the attachment to the cytoskeleton:

intrinsic proteins can non-covalently bind to proteins in the cell's cytoskeleton which regulates cell shape

Describe how the transmembrane domains of integral proteins allow them to embed in the membrane:

the amino acids have hydrophobic side chains which allows them to be embedded in the hydrophobic membrane environment

Describe the 3 domains of glycophorin A (integral membrane protein):

1) hydrophilic domain which is glycosylated and resides on the extracellular side

2) hydrophobic transmembrane domain

3) hydrophilic domain that resides in the cytosol

How many amino acids make up glycophorin A?

131

How many transmembrane helices make up bacteriorhodopsin?

7

Give two examples of intrinsic membrane proteins that are used in transport:

1) GLUT

2) Ion channels

What is the role of GLUT?

To transport glucose into cells

How many alpha helices make up GLUT?

12

Describe how GLUT transports glucose across the plasma membrane:

glucose binds to the receptor causing a conformational change that moves glucose across the membrane

Give 6 stimuli that can cause ion channels to open:

1) ligand binding

2) electric potential

3) pH

4) temperature

5) pressure

6) lipids

Describe the structure of K+ ion channels:

4 subunits each with 2 transmembrane helices and a pore half helix

What is the role of the selectivity filter on K+ ion channels?

to remove any H2O on K+ ions

What are mechanosensitive channels?

channels that are activated by the application of mechanical stress to the membrane allowing cells to respond to touch, sound, pressure and gravity

What are Beta barrels?

A type of integral protein that commonly functions as a channel

How many strands to beta-barrels have?

8-22

What organelles/ organisms are beta-barrels abundant in?

mitochondria, chloroplasts, bacteria

Describe the amphipathic property of beta-barrels:

the polar amino acids face the interior of the barrel and the hydrophobic amino acids face the hydrophobic bilayer

How do peripheral membrane proteins attach to membranes?

using lipid binding domains

Give two ways peripheral membrane proteins attach to lipid binding domains:

1) via anionic lipid head groups

2) hydrophobic interactions with lipid acyl chains

True or false: the binding of a peripheral membrane protein can cause lipid rearrangement

true

True or false: cytoskeleton proteins are not regarded as peripheral membrane proteins

false

Describe how cytoskeleton proteins are peripheral membrane proteins:

they attach to cytosolic domains of integral membrane proteins

Why to RBC membranes need to be highly elastic?

as it undergoes large deformations and has to rapidly respond to fluid stress

What causes hereditary elliptocytosis?

a mutation in spectrin alpha or beta genes

What is the name of the blood condition where erythrocytes have elliptical shapes?

hereditary elliptocytosis

What are the symptoms of hereditary elliptocytosis?

fragmented cells can cause anaemia or be asymptomatic

What is the name of the blood condition where erythrocytes have spherical shapes?

hereditary spherocytosis

What causes hereditary spherocytosis?

deficiencies in spectrin, ankyrin and band 3 proteins causing a loss of cohesion between the plasma membrane and cytoskeleton due to defective anchoring of the cytoskeleton network to the membrane

What lipids do glycine residues of membrane proteins covalently link to?

myristoyls

What lipids do cystine and serine residues of membrane proteins covalently link to?

palmitoyls

What is the name of the anchor that can be added to proteins to help them anchor in the outer leaflet of the membrane after translation?

GPI anchors

Where does Alzheimer's start?

hippocampus

Give 4 symptoms of Alzheimer's:

1) amnesia

2) aphasia

3) agnosia

4) apraxia

What is agnosia?

inability to recognize objects

What is apraxia?

difficulty with motor planning

What causes Alzheimer's?

senile plaques made of amyloid beta peptide derived from membrane bound amyloid precursor protein accumulate in the nervous tissue

What membrane bound protein do Alzheimer's plaques come from?

amyloid precursor protein

Describe how Alzheimer's is linked to cholesterol:

cholesterol rich lipid rafts process amyloid precursor protein which produces the toxic amyloid beta peptide

How are membrane proteins removed from membranes for research?

detergents have hydrophobic parts that associate with hydrophobic regions of the protein and displace the lipids while the hydrophilic region of detergent creates a protein-detergent complex

Give the name of the computational method used to analyse the motion of membrane proteins in their biological environments:

molecular dynamics simulations of membrane proteins

Where are carbohydrates found in lipids?

glycolipids and glycoproteins

What are O-linked glycoproteins?

membrane proteins with short carbohydrate chain attached to the oxygen atom in the side chain of serine or threonine

What are N-linked glycoproteins?

membrane proteins with long carbohydrate chains attached to the amide N in the side chain of asparagine amino acids

Fill in the gaps: An asparagine residue can only accept an __________ if the residue is part of a ___-X-___ or ___-X-___ sequence in which X can be any residue in N-linked glycoproteins

oligosaccharide, Asn, Ser, Asn, Thr

Where do carbohydrates join to in N-linked glycoproteins?

the amide N in the side chain of asparagine amino acids

Where do carbohydrates join to in O-linked glycoproteins?

he oxygen atom in the side chain of serine or threonine

True or false: glycoproteins form part of the extracellular matrix

true

What is the role of glycoproteins?

to allow for cell-cell recognition and to stabilise proteins