chp 11

Protein Structure, Chemistry, and Amino Acid Properties

Amino Acid Architecture & R-Group Classification

Proteins are linear polymers constructed from 20 standard amino acids linked sequentially. Every amino acid shares a common central structural core centered on an alpha carbon ($C_\alpha$), to which four distinct chemical groups are covalently attached:

1. A positively charged amino group ($^+H_3N-$)

2. A negatively charged carboxyl group ($COO^-$)

3. A hydrogen atom ($H$)

4. A variable radical group or side chain ($R$)

The chemical identity, size, charge, and hydrophobic or hydrophilic nature of an amino acid are defined entirely by its specific side chain ($R$ group).

The 20 amino acids are categorized into distinct chemical classes based on side chain characteristics:

Polar Amino Acids

• Negatively Charged (Acidic):

  • Aspartic acid (Asp / D)

  • Glutamic acid (Glu / E)

• Positively Charged (Basic):

  • Arginine (Arg / R)

  • Lysine (Lys / K)

  • Histidine (His / H)

• Uncharged Polar:

  • Asparagine (Asn / N)

  • Glutamine (Gln / Q)

  • Serine (Ser / S)

  • Threonine (Thr / T)

  • Tyrosine (Tyr / Y)

Nonpolar (Hydrophobic) Amino Acids

• Aliphatic & Nonpolar:

  • Alanine (Ala / A)

  • Glycine (Gly / G)

  • Valine (Val / V)

  • Leucine (Leu / L)

  • Isoleucine (Ile / I)

  • Proline (Pro / P)

  • Phenylalanine (Phe / F)

  • Methionine (Met / M)

  • Tryptophan (Trp / W)

  • Cysteine (Cys / C)

Aromatic Amino Acids

• Phenylalanine (Phe / F)

• Tyrosine (Tyr / Y)

• Tryptophan (Trp / W)

Nonpolar amino acids are strongly hydrophobic. In aqueous cytosolic environments, folded proteins routinely bury hydrophobic side chains within their internal core to minimize thermodynamically unfavorable interactions with water molecules.

Peptide Bond Formation and Polypeptide Backbone

Amino acids are joined end-to-end via condensation (dehydration synthesis) reactions. The carboxyl group ($COO^-$) of one amino acid reacts with the amino group ($^+H_3N-$) of another, forming a covalent peptide bond (amide linkage) with the concomitant removal of a water molecule ($H_2O$).

Repetition of this reaction produces a polypeptide chain with a repeating, polar backbone ($-N-C_\alpha-C-$). The peptide backbone possesses inherent polarity due to partial charge distribution:

• Carbonyl oxygen atoms carry a partial negative charge ($\delta^-$)

• Amide hydrogen atoms carry a partial positive charge ($\delta^+$)

• Amide nitrogen atoms carry a partial negative charge ($\delta^-$)

• Carbonyl carbon atoms carry a partial positive charge ($\delta^+$)

Polypeptide chains possess explicit directional polarity, defined by an amino terminus (N-terminus) at the beginning and a carboxyl terminus (C-terminus) at the end. Functional side chains ($R$ groups) project outward from this repeating central backbone.

Secondary, Tertiary, and Domain Organization

Protein structure is organized hierarchically:

• Secondary Structure: Local folding patterns resulting from hydrogen bonding along the peptide backbone, predominantly forming individual $\alpha$ helices and $\beta$ sheets.

• Domains: A domain is a specific, compact grouping of $\alpha$ helices and $\beta$ sheets within a polypeptide chain that folds independently and confers a distinct biological function. Polypeptides can contain a single domain or multiple functional domains.

Examples of structural domains include:

• Bacterial Transcription Factor: Contains two distinct domains—a cyclic AMP (cAMP) binding domain (comprising $\alpha$ helices and anti-parallel $\beta$ sheets) and a DNA-binding domain. Together, these domains allow the single protein to perform dual regulatory roles.

• G-Protein Coupled Receptors (GPCRs): Receptors such as the dopamine receptor characteristically contain seven transmembrane domains that anchor the protein across the lipid bilayer.

• Green Fluorescent Protein (GFP): Originally isolated from jellyfish, GFP adopts a cylindrical $\beta$-barrel structure composed of antiparallel $\beta$ sheets surrounding a central chromophore. When excited by specific wavelengths of light, GFP fluoresces bright green.

Unstructured Regions and Structural Flexibility

Polypeptide chains frequently contain intrinsically unstructured regions or linker strands between defined secondary structures and domains.

Functions and characteristics of unstructured regions:

• They act as flexible bridges connecting distinct structural domains, allowing proper alignment and spatial orientation.

• They enable conformational adaptability, permitting domains to shift relative to one another during ligand binding or catalytic events.

• While unstructured regions do not maintain rigid secondary geometry, specific amino acid residues within them are vital; mutations in linkers can disrupt the overall folding pathway, causing neighboring $\beta$ sheets or $\alpha$ helices to unravel and rendering the protein nonfunctional.

Structural Comparison and Protein Families

Proteins sharing evolutionary origins and functional structural motifs are grouped into protein families. Members of a family often exhibit highly conserved three-dimensional backbone structures while possessing distinct amino acid side chain compositions that adapt them to specific substrates.

For example, elastase and chymotrypsin belong to the serine protease family. Ribbon and wireframe models reveal nearly identical overall tertiary folding patterns and active site geometry between the two enzymes. However, precise differences in key amino acid side chains within their substrate-binding pockets confer distinct cleavage specificities, resulting in completely different enzymatic functions.

Protein Folding, Misfolding, and Cellular Regulation

Protein Misfolding and Neurodegenerative Pathologies

The correct three-dimensional folding of a protein is mandatory for maintaining cellular function and tissue integrity. Under pathological conditions, disease-causing unfolded or misfolded proteins fail to fold correctly, exposing hydrophobic side chains that are normally buried.

Exposed hydrophobic regions cause misfolded proteins to adhere to one another, forming insoluble protein aggregates. These sticky aggregates act like cellular sinkholes or whirlpools, sequestering other functional cellular proteins and cellular factors.

Pathologies directly associated with specific misfolded proteins and aggregate formation include:

• Alzheimer's disease

• Huntington's disease

• Creutzfeldt-Jakob disease

Protein aggregates disrupt cellular homeostasis long before actual neuronal cell death occurs by impairing protein trafficking, organelle function, and metabolic pathways.

Cellular Mechanisms of Protein Activity Regulation

Cells maintain optimal homeostasis by precisely regulating protein activity, abundance, and spatial localization through several primary mechanisms:

1. Gene Expression Control: Up-regulating or down-regulating transcription and translation adjusts the rate of protein synthesis according to physiological demand.

2. Targeted Degradation: Misfolded, damaged, or surplus proteins are selectively targeted for enzymatic degradation to prevent toxic accumulation.

3. Subcellular Compartmentalization: Confining proteins to specific cellular regions or organelles ensures they interact only with appropriate targets (e.g., keeping nuclear transcription factors inside the nucleus and away from cytosolic components).

Dynamic Protein Interactions and Binding Kinetics

Cellular processes rely on dynamic, reversible protein-protein and protein-ligand interactions. Macromolecular complexes undergo continuous assembly, conformational rearrangement, and disassembly.

Protein interaction kinetics are governed by dynamic equilibrium:

• $K_{on}$ represents the rate constant for association (binding).

• $K_{off}$ represents the rate constant for dissociation (unbinding).

Binding pockets are specialized molecular cavities formed by complex tertiary folding that bring non-adjacent amino acid side chains into spatial proximity. When a ligand enters a binding pocket, noncovalent bonds (such as hydrogen bonds, electrostatic attractions, and van der Waals interactions) stabilize the complex, altering the system's free energy and driving conformational changes that dictate protein function.

Plasma Membrane Structure and Transmembrane Protein Architecture

Transmembrane Alpha Helices and Energy Minimization

Because the internal core of a lipid bilayer is composed of hydrophobic fatty acid tails, membrane-spanning proteins must adopt structural conformations that shield their polar peptide backbones from the surrounding hydrophobic lipids.

The standard structural motif traversing a membrane is a single $\alpha$ helix containing approximately $20\,-\,30\text{ nonpolar (hydrophobic) amino acids}$:

• The polar peptide backbone forms internal hydrogen bonds with itself along the central axis of the helix, satisfying its hydrophilic potential away from the lipid tails.

• The nonpolar hydrophobic side chains project outward from the helix axis, directly contacting and stably interacting with the hydrophobic phospholipid tails.

Examples of proteins possessing long structural $\alpha$-helices include human keratin Type I, hemagglutinin, tropomyosin, and keratin Type II.

Amphipathic Alpha Helices and Aqueous Pores

Proteins that form channels or pores across the plasma membrane utilize multiple transmembrane $\alpha$ helices arranged in a circular bundle.

These transmembrane $\alpha$ helices are amphipathic:

• Nonpolar hydrophobic side chains line the outer face of the helix bundle, interacting with the phospholipid tails of the lipid bilayer.

• Polar hydrophilic side chains line the inner face of the helix bundle, creating a water-filled central channel (aqueous pore) that permits the passive diffusion of hydrophilic molecules or ions across the membrane.

Proton Transport: The Bacteriorhodopsin Model

Bacteriorhodopsin is a specialized transmembrane transport protein found in haloarchaea that acts as a light-driven proton pump.

Key structural features of bacteriorhodopsin:

• It spans the plasma membrane using seven transmembrane $\alpha$ helices.

• It contains a covalently bound light-absorbing chromophore called retinal.

• When retinal absorbs a photon of light, it undergoes a conformational change that alters the protein structure, driving a unidirectional pathway of proton ($H^+$) translocation from the cytosol across the membrane to the extracellular space.

Membrane Protein Mobility, Restriction, and Structural Cortex

Mechanisms Restricting Lateral Mobility

While the plasma membrane is fluid, cells restrict the lateral diffusion of specific membrane proteins to establish distinct functional membrane domains. Lateral mobility is restricted via four primary mechanisms:

1. Tethering to the Cell Cortex: Proteins are anchored to the internal sub-membrane protein network.

2. Tethering to the Extracellular Matrix: Proteins bind to stable extracellular structural components outside the cell.

3. Intercellular Protein Binding: Membrane proteins anchor to complementary proteins on the surface of neighboring cells.

4. Diffusion Barriers: Specialized intercellular junctions restrict membrane proteins to distinct membrane regions.

Diffusion Barriers and Epithelial Polarity

In polarized epithelial cells lining the intestine, the plasma membrane is segregated into distinct functional domains:

• Apical Plasma Membrane: Faces the intestinal lumen and contains transport proteins (e.g., Protein A) specialized for nutrient uptake.

• Lateral Plasma Membrane: Contacts adjacent epithelial cells.

• Basal Plasma Membrane: Rests upon the basal lamina and contains distinct transport proteins (e.g., Protein B) for exporting nutrients into underlying tissues.

Lateral movement of proteins between the apical and basolateral domains is blocked by tight junctions. Tight junctions form continuous, watertight diffusion barriers encircling the cell apex, preserving cell polarity.

The Red Blood Cell Cortex

The shape and mechanical resilience of red blood cells (erythrocytes) are maintained by the cell cortex, a dense sub-membrane protein network anchored to the cytosolic face of the plasma membrane.

Components of the erythrocyte cortex:

• Spectrin Dimers: Long, flexible rod-like proteins forming a hexagonal meshwork.

• Actin Filaments: Connect spectrin dimers into a continuous structural lattice.

• Attachment Proteins: Anchor the spectrin-actin network to specific transmembrane proteins embedded in the lipid bilayer.

Genetic defects affecting cortical structural proteins or transmembrane anchors impair membrane stability, resulting in pathologies such as Epidermolysis bullosa (EB)—a severe genetic condition characterized by skin fragility, blistering, and tissue tearing upon minor mechanical trauma.

Experimental Methods for Studying Membrane Protein Dynamics

Fluorescence Recovery After Photobleaching (FRAP)

Fluorescence Recovery After Photobleaching (FRAP) is an ensemble optical technique used to measure the lateral mobility and diffusion rates of large populations of membrane proteins.

Procedure for FRAP:

1. Membrane proteins on living cell surfaces are uniformly labeled with fluorescent markers or GFP tags.

2. A small, focused patch of the membrane is irradiated with an intense, high-power laser beam to irreversibly bleach the fluorescence in that defined region.

3. The bleached region is monitored over time using low-intensity fluorescence microscopy.

4. Unbleached fluorescent proteins diffuse laterally into the bleached zone while bleached proteins diffuse out, resulting in fluorescence recovery. The rate and extent of signal recovery directly reflect the lateral diffusion coefficient and mobile fraction of the protein population.

Single-Particle Tracking (SPT)

While FRAP measures average population dynamics, Single-Particle Tracking (SPT) enables the observation of individual protein trajectories to detect local membrane barriers, tethering, or transient confinement.

Procedure for SPT:

1. Individual membrane proteins are tagged using highly specific antibodies attached to gold nanoparticles or quantum dots.

2. The motion of individual gold-tagged particles is recorded over time using high-speed video microscopy.

3. Tracking analysis reveals distinct diffusion patterns:

   • Unrestricted Random Diffusion (A): The protein moves freely across large membrane areas.

   • Transient Confinement (B): The protein is temporarily caged within small sub-micron membrane domains created by the underlying cell cortex.

   • Restricted / Immobile Motion (C): The protein is tethered to structural elements, remaining immobilized within a tiny area ($<1\,\mu\text{m}$).

Detergent Solubilization and Liposome Reconstitution

To analyze the biochemical properties and transport kinetics of integral membrane proteins in isolation, proteins must be extracted from native lipid bilayers without loss of functional tertiary structure.

Reconstitution Workflow:

1. Solubilization: Amphipathic detergent molecules (forming detergent micelles) are added to cell membranes. Detergent monomers disrupt the lipid bilayer, creating soluble lipid-detergent micelles and detergent-bound solubilized membrane proteins.

2. Purification: The protein of interest is isolated from other cellular components using chromatographic techniques.

3. Reconstitution: Purified proteins are mixed with excess detergent-solubilized phospholipids.

4. Detergent Removal: Detergents are selectively removed, driving the spontaneous self-assembly of phospholipids into artificial lipid vesicles (liposomes) containing functional, reconstituted membrane proteins.

Cell Surface Carbohydrates and Immune Cell Extravasation

The Carbohydrate Layer (Glycocalyx)

The extracellular surface of eukaryotic plasma membranes is heavily coated with carbohydrates, forming a protective outer layer termed the glycocalyx.

Components of the glycocalyx:

• Transmembrane Glycoproteins: Proteins bearing short, branched oligosaccharide chains.

• Adsorbed Glycoproteins: Extracellular glycoproteins bound noncovalently to surface receptors.

• Transmembrane Proteoglycans: Membrane proteins linked to long glycosaminoglycan polysaccharide chains.

• Glycolipids: Membrane lipids covalently linked to sugar residues.

Functions of the glycocalyx:

• Provides mechanical protection against mechanical and chemical stress.

• Prevents unwanted cell-cell adhesion and creates a lubricated surface layer.

• Facilitates specific cell-cell recognition events, immune responses, and cell adhesion.

Neutrophil Extravasation and Lectin-Mediated Cell Adhesion

During infection or tissue inflammation, white blood cells called neutrophils must exit blood vessels and migrate into infected tissues via a process termed extravasation.

Step-by-step mechanism of neutrophil extravasation:

1. In response to inflammatory signals from an infection site, vascular endothelial cells lining blood vessels express cell-surface carbohydrate-binding proteins called lectins.

2. Transmembrane lectins on endothelial cells specifically bind to oligosaccharides displayed on the surface glycocalyx of circulating neutrophils.

3. This low-affinity lectin-carbohydrate binding causes circulating neutrophils to slow down, adhere weakly, and roll along the inner blood vessel wall.

4. Secondary high-affinity protein-protein interactions trigger the neutrophil to arrest rolling, flatten against the endothelial surface, and crawl through the blood vessel wall (diapedesis) into infected tissue toward the pathogen source.