chapter 9

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Questions and Answers

What role does the membrane skeleton play in the cell?

It controls cell division.

It helps define cell shape. (correct)

It promotes protein synthesis.

It facilitates energy production.

Which protein accounts for 75% of the erythrocyte membrane skeleton?

Tropomyosin

Ankyrin

Spectrin (correct)

Actin

What structural feature connects the C-terminal helix of one repeat in spectrin?

A beta-sheet

A disulfide bridge

A helical bundle

A 5 residue helical linker (correct)

What is the function of ankyrin within the erythrocyte membrane skeleton?

To bind to integral membrane ion channel proteins Signup and view all the answers

What is the conformation of spectrin chains in the membrane skeleton?

They arrange as antiparallel polypeptides. Signup and view all the answers

How does stretching the erythrocyte membrane skeleton facilitate observation?

It reveals clear images due to reduced packing. Signup and view all the answers

What are the tandem repeats found in ankyrin primarily responsible for?

Mediating protein-protein interactions Signup and view all the answers

What type of microscopy is used to monitor the recovery of fluorescence in bleached areas?

Fluorescence microscopy Signup and view all the answers

How does cholesterol affect membrane fluidity?

It decreases membrane fluidity. Signup and view all the answers

What is the primary structure of glycophorin A?

A structure that completely spans the membrane. Signup and view all the answers

What distinguishes glycophorin A^M^ from glycophorin A^N^?

They vary in their first five amino acid residues. Signup and view all the answers

What feature is characteristic of the lipid bilayer structure at temperatures below the transition temperature?

The lipid molecules form a gel-like solid. Signup and view all the answers

What method was used to determine the structure of bacteriorhodopsin?

Electron crystallography Signup and view all the answers

What is the role of retinal in bacteriorhodopsin?

It is the protein's light-absorbing group. Signup and view all the answers

What is the primary arrangement of the helical rods in bacteriorhodopsin?

They span the lipid bilayer perpendicular to the bilayer plane. Signup and view all the answers

How does the structure of integral membrane proteins differ from peripheral proteins?

Integral proteins penetrate the lipid bilayer. Signup and view all the answers

What is the primary function of SNAREs in synaptic vesicle fusion?

They mediate vesicular fusion. Signup and view all the answers

Which components are part of the SNARE complex during vesicle fusion?

1 R-SNARE and 2 Q-SNAREs Signup and view all the answers

What structural change occurs in hemagglutinin (HA) during the fusion process?

It undergoes conformational change. Signup and view all the answers

What is the role of hemagglutinin (HA) in the influenza virus?

It facilitates entry into host cells. Signup and view all the answers

What is the effect of low pH on the structural configuration of hemagglutinin?

It causes the fusion peptide to protrude above the membrane. Signup and view all the answers

What role do neurotoxins play in relation to SNARE proteins?

They cleave SNARE proteins, inhibiting neurotransmitter release. Signup and view all the answers

What are the two peptides that comprise hemagglutinin (HA)?

HA1 and HA2 Signup and view all the answers

Which structural form does hemagglutinin take after proteolytic removal of its transmembrane helix?

BHA monomer only Signup and view all the answers

What key function do lipids serve in biological membranes?

Function as signaling molecules Signup and view all the answers

Which fatty acid is considered polyunsaturated?

Alpha-linolenic acid Signup and view all the answers

What type of lipid predominantly acts as an energy reservoir in animals?

Triacylglycerols Signup and view all the answers

Which statement accurately describes saturated fatty acids?

They can assume a wide range of conformations. Signup and view all the answers

Why are fatty acids usually found in esterified form?

To facilitate their storage in cells Signup and view all the answers

How does the structure of Mycobacterium tuberculosis help it evade the immune system?

Due to its thick cell wall containing mycolic acid Signup and view all the answers

Which configuration do double bonds in common biological fatty acids exhibit?

Cis configuration Signup and view all the answers

What characterizes the biosynthesis of most fatty acids?

Utilizes C<del>2</del> units via catenation Signup and view all the answers

What is the most abundant steroid in animals?

Cholesterol Signup and view all the answers

What is a major component of animal plasma membranes?

Cholesterol Signup and view all the answers

What type of hormones are glucocorticoids primarily involved with?

Stress response Signup and view all the answers

How does active vitamin D increase serum calcium levels?

By promoting intestinal absorption of dietary calcium Signup and view all the answers

Which vitamin D form is produced from the photolytic action of UV light on ergosterol?

Vitamin D2 Signup and view all the answers

What disease is characterized by stunted growth and deformed bones in children due to vitamin D deficiency?

Rickets Signup and view all the answers

What characterizes the type of steroid hormones that influence sexual development and function?

Androgens and estrogens Signup and view all the answers

What component gives cholesterol its weak amphiphilic character?

Polar OH group Signup and view all the answers

What happens to lipid molecules above the transition temperature?

They are highly mobile in the bilayer. Signup and view all the answers

How does cholesterol affect membrane fluidity?

Decreases fluidity by causing rigidity. Signup and view all the answers

What structural feature characterizes glycophorin A?

Spans the membrane completely and bears o-linked oligosaccharides. Signup and view all the answers

What is the primary function of bacteriorhodopsin?

Creating a proton concentration gradient across the cell membrane. Signup and view all the answers

What unique structural element does glycophorin A contain?

Nineteen predominantly hydrophobic residues. Signup and view all the answers

What is the primary molecular shape of bacteriorhodopsin?

A bundle of seven α-helical rods. Signup and view all the answers

Which characteristic defines the extracellular domain of glycophorin A?

Contains 15 O-linked oligosaccharides and one N-linked. Signup and view all the answers

What type of analysis was used to determine the structure of bacteriorhodopsin?

Electron crystallography. Signup and view all the answers

What indicates the recovery rate of fluorescence in a bleached area?

The diffusion of fluorophore labeled molecules Signup and view all the answers

Which protein is primarily responsible for binding integral membrane ion channel proteins?

Ankyrin Signup and view all the answers

What forms the structural basis of the erythrocyte membrane skeleton?

Spectrin heterotetramers and associated proteins Signup and view all the answers

What effect does osmotic lysis have on erythrocytes?

It leads to the formation of membranous particles Signup and view all the answers

Which structural component connects two spectrin heterodimers?

Triple-helical bundles Signup and view all the answers

What characterizes the structural arrangement of ankyrin repeats?

Tandem, repeat-based configuration Signup and view all the answers

What does a positive value on a hydropathy plot indicate about the peptide segment?

It is hydrophobic and requires energy for transfer to water. Signup and view all the answers

Which amino acid residues are most commonly associated with prenylation?

Cysteine and an aliphatic residue Signup and view all the answers

What is the molecular weight of the α-subunit of spectrin?

280 kDa Signup and view all the answers

How are the spectrin chains configured to form a functional membrane skeleton?

In antiparallel arrangement as heterodimers Signup and view all the answers

What is a characteristic feature of β-barrels in membrane proteins?

They are commonly found in porins. Signup and view all the answers

What role do lipids have when covalently attached to membrane-associated proteins?

They anchor the protein to the membrane. Signup and view all the answers

Which of these best describes the structure of bacteriorhodopsin?

It consists of seven TM helices. Signup and view all the answers

What type of linkage connects the prenyl group to the protein during prenylation?

Thioether linkage Signup and view all the answers

How many transmembrane helices does glycophorin A contain?

One Signup and view all the answers

What common isoprenoid groups are associated with prenylation?

C<del>15</del> farnesyl and C<del>20</del> geranylgeranyl residues Signup and view all the answers

Study Notes

Lipids: Structure and Function

Lipids are an essential group of molecules found in all cells.
While not polymeric like other macromolecules, lipids aggregate and perform crucial biological functions.
These functions include:
- Biological membrane components: Lipids with hydrocarbon chains are essential for forming biological membranes.
- Energy storage: Lipids act as energy reserves in cells.
- Cellular signaling: Lipids play key roles in various signaling pathways.
Mycobacterium tuberculosis, the causative agent of tuberculosis, possesses a thick cell wall enriched with a unique lipid called mycolic acid, aiding its avoidance of the human immune system and antibiotics.
Lipids are extracted using organic solvents like chloroform and methanol.
They are separated using chromatography and identified through mass spectrometry.

Classes of Lipids: Fatty Acids

Fatty acids are long-chain carboxylic acids with hydrocarbon side groups.
They typically exist in esterified forms as major components of various lipids.
Common biological fatty acids include:
- Palmitic acid (C16)
- Oleic acid (C18)
- Linoleic acid (C18)
- Stearic acid (C18)
Fatty acids with an even number of carbon atoms are prevalent due to their biosynthesis by the catenation of C2 units.
Unsaturated fatty acids contain double bonds; many are polyunsaturated (with two or more double bonds).
ω-3 and ω-6 fatty acids are crucial polyunsaturated fatty acids.
Saturated fatty acids are fully reduced, highly flexible, and can adopt numerous conformations.

Triacylglycerols: Energy Reservoirs

Triacylglycerols, also known as triglycerides, are predominantly found in plants and animals.
These nonpolar, water-insoluble substances are fatty acid triesters of glycerol.
They function as energy reservoirs in animals and are the most abundant class of lipids in this group.
The three fatty acid residues in triacylglycerols can vary, creating diverse structures like 1-Palmitoleoyl-2-linoleoyl-3-stearoylglycerol.

Steroids: Cholesterol and Hormones

Steroids are lipids derived from cyclopentanoperhydrophenanthrene, a compound with four fused nonplanar rings.
Cholesterol, the most abundant steroid in animals, is classified as a sterol due to its C3-OH group.
It is a major component of animal plasma membranes, contributing 30-40% to its composition.
Its polar OH group imparts weak amphiphilic character.
Cholesterol can be esterified to long-chain fatty acids, forming cholesterol esters like cholesterol stearate.
While plants contain minimal cholesterol, they synthesize other sterols.

Steroid Hormones: Regulating Function

Cholesterol is the metabolic precursor for steroid hormones in mammals.
These hormones are classified based on their physiological effects.
Glucocorticoids like cortisol regulate carbohydrate, lipid, and protein metabolism, influence inflammatory reactions, and aid in stress responses.
Aldosterone and other mineralocorticoids control salt and water excretion by the kidneys.
Androgens and estrogens regulate sexual development and functions.
Testosterone is an androgen (male sex hormone), and β-estradiol is an estrogen (female sex hormone).

Vitamin D: Sunlight and Calcium Absorption

Vitamin D2 (ergocalciferol) is formed nonenzymatically in animal skin through the photolytic action of UV light on ergosterol, a plant sterol commonly added to milk.
Vitamin D3 (cholecalciferol) is similarly formed from 7-dehydrocholesterol.
Both vitamin D2 and D3 are inactive; their active forms are produced by adding an OH group by liver and kidney enzymes, yielding 1α,25-Dihydroxycholecalciferol.
Active vitamin D elevates serum Ca2+ by promoting intestinal absorption of dietary Ca2+.
It also enhances the absorption of Ca2+ in bones and teeth.

Vitamin D Deficiency: Rickets

Vitamin D deficiency in children leads to rickets, a disease characterized by stunted growth and deformed bones.

Membrane Fluidity and Cholesterol

Cholesterol does not form a bilayer independently.
It decreases membrane fluidity because its rigid steroid ring system hinders the movement of fatty acid side chains in other membrane lipids.

Membrane Proteins: Aquaporin-O and Glycophorin A

The integral membrane protein aquaporin-O (AQPO) is associated with lipids.
It forms channels facilitating water transport through the membrane.
Glycophorin A is a transmembrane (TM) protein completely spanning the erythrocyte membrane.
It bears 15 O-linked oligosaccharides and one N-linked oligosaccharide in its extracellular domain.
Its hydrophobic transmembrane region consists of 19 sequential amino acid residues.
The terminal portion, located on the cytoplasmic face, is rich in anionic, cationic, and polar residues.

Glycophorin A Polymorphisms

Two common genetic variants of glycophorin A exist.
Glycophorin A^M^ has Ser and Gly at positions 1 and 5, whereas glycophorin A^N^ has Leu and Glu at these positions.

Bacteriorhodopsin: Generating Proton Gradient

Bacteriorhodopsin, an integral membrane protein produced by the archaebacterium Halobacterium salinarum, uses electron crystallography for structure determination.
Its structure consists of seven α-helical rods spanning the lipid bilayer perpendicularly.
It generates a proton concentration gradient across the cell membrane, driving ATP synthesis.
Retinal, a covalently bound molecule, acts as the light-absorbing group in the protein.

Identifying Transmembrane Helices: Hydropathy Plots

Hydropathy plots help identify transmembrane helices in proteins.
They plot the average hydropathy indices of amino acid residues within a sliding window against the position of the first amino acid.
A positive value indicates a hydrophobic peptide segment, suggesting a transmembrane helix.
Segments longer than 20 hydrophobic residues are likely to form TM helices.
Glycophorin A has only one TM helix, while bacteriorhodopsin possesses seven.

β-Barrels: Porins

Porins, transmembrane proteins, can form β-barrels.
These structures facilitate the transport of specific molecules through the membrane.
The X-ray structure of E. coli OmpF porin, a 16-stranded monomer, reveals its β-barrel architecture.

Prenylation: Anchoring Proteins to Membranes

Some membrane-associated proteins have covalently attached lipids that anchor them to the membrane.
Prenylated proteins have lipids built from isoprene units.
The most common isoprenoid groups are the C15 farnesyl and C20 geranylgeranyl residues.
Prenylation is a post-translational modification where lipids are added to proteins.
The most frequent prenylation site is a C-terminal tetrapeptide C-X-X-Y, where C is cysteine, X is often an aliphatic amino acid, and Y influences the type of prenylation.
The prenyl group is enzymatically linked to the Cys sulfur atom via a thioether linkage.

Membrane Fluidity: Lateral Diffusion

The rate at which a bleached area recovers its fluorescence, as measured by fluorescence microscopy, indicates the rate of lateral diffusion of fluorophore-labeled molecules in the membrane.

Membrane Skeleton: Erythrocyte Ghost

The membrane skeleton helps define cell shape.
Erythrocyte ghosts, obtained by osmotic lysis, represent the membranous particles lacking cytoplasm.
Spectrin, a major protein in erythrocyte ghosts, constitutes 75% of the erythrocyte membrane skeleton.
Spectrin has two polypeptide chains: a 280-kDa α-subunit and a 246-kDa β-subunit.
Each polypeptide contains multiple 106-residue repeats, forming flexibly connected triple-helical bundles.
Two heterodimers join head-to-head to create an (αβ)2 heterotetramer.

Spectrin and Erythrocyte Membrane Skeleton

The X-ray structure of three consecutive repeats of chicken brain α-spectrin reveals an up-down-up triple-helical bundle.
Electron micrographs illustrate the stretched erythrocyte membrane skeleton, revealing its structure.
A model of the erythrocyte membrane skeleton shows its relationship to the intact cell.
Junctions between spectrin tetramers include actin, tropomyosin, and band 4.1 protein.

Ankyrin: Connecting Spectrin and Ion Channels

Spectrin is associated with the 1880-residue protein ankyrin, which binds to integral membrane ion channel proteins.
Ankyrin's N-terminal 798-residue segment consists primarily of 24 tandem ankyrin repeats.
The X-ray structure of human ankyrin repeats 13-24 shows its structure.

Membrane Protein Mobility

The interaction between membrane components and the underlying cytoskeleton explains the various mobilities of membrane proteins within the membrane.
A model illustrates how the cytoskeleton influences membrane protein mobility.

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Test your knowledge on the membrane skeleton of erythrocytes and its structural components. This quiz covers key proteins, microscopy techniques, and the influence of cholesterol on membrane fluidity. Dive deep into the specifics of membrane structure and function.