CBIO 3400 Exam 3 Review

Functions of Cellular Membranes

• Structure

• Gradients

• Compartalization

Components of Membranes

• Phospholipids

• Cholesterol

• Transporters

• Receptors

• Glycoproteins

What parts make up the hydrophilic head of phospholipids

• Choline

• Phospate

What parts make up the hydrophobic tail of phospholipids

• Glycerol

• Hydrocarbon Tail

Saturated Fatty Acid

• no double bond

• more rigid

• pack tightly

Unsaturated Fatty Acids

• Double Bonds in tails

• more fluid and flexible

Phospholipids and Cholesterol are both ________ molecules

amphiphilic

Influence of the Hydrophobic Effect

• Head face outward

• Tails face inward

• Forms a self assembled bilayer

Cholesterol _____ membrane flexibility and permeability

reduces

Cholesterol _______ membrane thickness

increases

Neutrally charged phospholipids are responsible for

interactions in the extracellular space

Negatively charged phospholipids are responsible for

cell signaling pathways within the cell

What is the primary form of phospholipid movement within the membrane

Lateral Diffusion: Side to Side within same leaflet

Which phospholipid movement requires enzymes for help

Transverse Diffusion: one leaflet to another

• rare

Types of Membrane Proteins

• Transmembrane

• Multipass Transmembrane

• Peripheral

• Attached via linkages

• Attached via another protein

Hydropathy Plots tell us

How many transmembrane proteins are present

What is FRAP

Fluorescence Recovery After Photobleaching

How does FRAP work

1. Lipids tagged with GFP

2. Area is bleached

3. watch for recovery/response of lipids or tagged molecule

Basic Function of Rough ER

• translocation of proteins

• Ca2+ storage

Basic Function of Smooth ER and Golgi

• lipid production

Basic Function of Lysosomes

lipid degradation

Basic Function of Endosomes

uptake of material into the cell

What type of molecules easily cross membranes

small non-polar

• O2, Steroids, Hormones, CO2

What molecules have low permeability to membrane

sugars (glucose and sucrose)

Membranes are impermeable to

ions

Whats required to allow transport of molecules across membranes

transporters and channels

Porin

beta barrel protein that allows for passive diffusion

Single Membrane Organelles

• Lysosomes

• Endoplasmic Reticulum

• Golgi Apparatus

Double Membrane Organelles

• Chloroplast

• Nucleus

• Mitochondria

Characteristics of Transmembrane Proteins

• Positively charged inside

• Alpha Helices

• 20-25 amino acids spanning the bilayer

Primary Protein Structure

sequence of amino acids in a peptide

Secondary Protein Structure

• Beta Pleated Sheets

• Alpha Helices

Tertiary Protein Structure

3 dimensional shape of polypeptide chain

Quaternary Protein Structure

multiple polypeptide chains working to form a single unit

How to remember transmembrane protein functions

JET RAT

JET RAT

• Junction: connect join cells together

• Enzymes: localize metabolic pathways

• Transport: facilitated and active diffusion

• Recognition: marker for identification

• Anchorage: attachment points for cytoskeleton and ECM

• Transduction: receptors for peptide hormones

In Alpha Helices, the H+ bond forms between

carbonyl oxygen and amide hydrogen of amino acid residue

In Beta Pleated sheets, H+ bonds form between

carbonyl oxygen and amide hydrogen of amino acid residue on adjacent strand

Facilitated Diffusion requires

transmembrane protein

Uniporter

single solute across membrane due to gradient. Undergoes conformational changes

Symporter

 Movement of two molecule in same direction

• one down its gradient

• one against its gradient

Antiporter

movement of two molecules

• using gradient of one molecule to transport the other

• opposite directions

What mediates the passive movement of a solute in a transporter

conformational changes

What provides the energy for solutes to cross the membrane

concentration gradients

Glucose Transport involves what kind of transporters

1. Symporter

2. Uniporter

3. ATPase Pump

4. Uniporter

Transporter used first in Glucose transport into the blood stream

Na+/Glucose Symporter

Transporter used second in Glucose transport into the blood stream

GLUT2 Uniporter

Transporter used third in Glucose transport into the blood stream

Na+/K+ ATPases Pump

Transporter used fourth in Glucose transport into the blood stream

K+ uniporter

What defines Vmax

• concentration of transporter

• substrate affinity

What is used to transport glucose inside the cell against its concentration gradient

energy from the electrochemical gradient of Na+

4 Classes of ATP Active Transporters

• P Type Pump

• ABC Transporter

• V Type Pump

• F Type Pump

Importance of Phosphate in Na+/K+ pump

binding causes the conformational change

When ATP is bound to Na+/K+ Pump

• K+ exits

• Na+ enters

Addition of phosphate in Na+/K+ Pump causes

• Na+ to exit

• conformational change

• K+ enters

3 Types of Eukaryotic Channels

1. Gap Junctions

2. Aquaporins

3. Ion Channels

Gap Junctions

directly link the cytoplasm of two cells allowing exchange of ions, small molecules, and signaling molecules

How does K+ ion channel remain selective

selects K+ over Na+ due to size

Similarities Between Channels and Transporters

• Amphiphilc

• Passage of molecules

• Use Gradients, both chemical and electro

How do channels differ from transporters

• highly specific

• faster

• select molecules based on size

• change conformation based on ions, ligands, mechano, and light

How do transporters differ from channels

• Change conformation after every transport event

• allow for transport of many molecules

Secretory Pathway

Used to sort proteins destined for other organelles or outside of the cell

Organelles involved in the secretory pathway

• ER

• Golgi

How are the Rough and Smooth ER Dynamic

• easily respond to changes in stress

Signal Sequence Hypothesis

1. Secreted Proteins have signal sequence

2. Signal Sequence direct proteins into the ER

3. Signal Sequences are cleaved in the ER

New Discoveries of Signal Sequence Hypothesis

• proteins destined for Golgi, Lysosomes, and Plasma Membrane have Signal Sequences

• Must enter the ER first

Signal Sequence

amino acid sequence marking a protein for transport

the signal sequence must be read _____ times

2

What reads and recognizes the signal sequence twice

1. SRP

2. Translocator

SRP

Signal Sequence Binding Pocket

Function of the SRP

• recognize signal sequence

• slow translation by binding to ribosome

• induce conformational change that pulls hinge and stops translocation

What does the SRP prevent by binding to ribosome

• early protein folding

• early protein release

What transfers the ribosome to translocon

SRP and SRP Receptors

Translocon

protein complex in ER that associates with SRP Complex, moving peptide chain across membrane

Sec61

Translocon in the ER

Sec61 requires

high concentration of calcium

Signal Peptidase

cleaves the signal sequence after it exits via lateral gate of translocon

Protein Modification is carried out by the

ER Resident Proteins

Protein Glycosylation

covalent addition of sugar to the protein

What type of Glycosylation occurs only in the ER

N-Linked

What type of Glycosylation occurs mainly in the golgi

N-Linked

Protein Glycosylation promotes

• proper folding

• stability by stopping early degradation

Precursor Synthesis

Initial step of protein glycosylation that occurs on the cytosolic side, where sugar attach

Role of Oligosaccharide Transferase

adds precurson to protein at the Asn-X-Thr/Ser in the ER lumen

ER Glycosidases trimming leads to

Man8GluNac2

What is Man8(GluNAC)2

exit signal

ER Chaperones Protein Folding

bind to polypeptides as they enter the ER

• recognize misfolded and unfolded proteins

Role of BI/GRP78

• prevent protein aggregation

• Regulate ER Stress Response

Role of Calnexin and Calreticulin

• recruit proteins and chaperones to correct protein folding

• best known quality control mechanism

• Lectins

Calnex - Calreticulin Cycle

1. Glucosyl Transferase binds to misfolded proteins and reglycosylates them

2. Calnexin rebinds

3. Glucosidase removes glucose from correctly folded protein

UPR is

Unfolded Protein Response

Goal of the UPR

reinstate homeostasis in the ER by reducing stress and misfolded proteins

Main mediators upon UPR activation

• IRE1

• ATF6

• PERK

IRE 1 (UPR Activation)

• EDIT

• increases folding and degrades misfolded proteins

ATF6 (UPR Activation)

• Activate

• makes more chaperones and folding material

PERK (UPR Activation)

• PAUSE

• reduces protein synthesis

Function of the Smooth ER

• Lipid Synthesis

• Drug Detoxification

• CalciumHomeostasis

Anterograde Vesicular Movement

• Forward movement

• ER → Golgi

Retrograde Vesicular Movement

• backward movement

• Golgi → ER

Steps to Vesicular Transport

1. Vesicle Side Formation

2. Scission

3. Uncoating

4. Vesicular Movement

5. Tethering and Fusion