Membrane Proteins 2

Overview of Membrane Proteins

Membrane proteins are specialized proteins that interact with biological membranes. Their structure and orientation are dictated by the distinct environments of the lipid bilayer.

Membrane Architecture and Protein Placement

• Exterior Face: The side of the membrane facing the outside of the cell, often containing carbohydrate side chains.

• Cytosolic Face: The side of the membrane facing the interior of the cell (cytoplasm).

• Lipid Monolayers: The membrane is composed of an Outer Monolayer and an Inner Monolayer.

• Protein Types:     * Integral Membrane Proteins: These proteins are embedded within the lipid bilayer, spanning from one side to the other or deeply penetrating the hydrophobic core.     * Peripheral Membrane Proteins: These proteins are associated with the surface of the membrane via electrostatic or hydrogen bonding interactions with lipid headgroups or integral proteins.     * Glycoprotein: Proteins with attached Carbohydrate side chains, typically found on the exterior face of the membrane.

Energetics of Membrane Protein Folding

A critical challenge for membrane proteins is the energetically favorable burial of peptide sequences within the hydrophobic environment of the lipid bilayer.

The Peptide Bond Challenge

The peptide backbone consists of repeating units ($N-H$ and $C=O$) that are inherently polar.

Energetics of Inserting a Peptide Bond into a Lipid Bilayer

The transfer free energy ($\Delta G_{\text{tr}}$) from water to an alkane environment (representing the lipid core) depends heavily on hydrogen bonding state:

• Non-H bonded N-H/CO: $\Delta G_{\text{tr}} = +6.4\,kcal\,mol^{-1}$

• H-bonded N-H/CO: $\Delta G_{\text{tr}} = +2.1\,kcal\,mol^{-1}$

To minimize the energetic penalty of placing polar backbone groups in a hydrophobic environment, membrane proteins maximize internal hydrogen bonding, resulting in regular secondary structures like $\alpha$-helices and $\beta$-sheets.

Side Chain Transfer Energetics

Research by Radzicka, A. and Wolfenden, R. (1987) published in Biochemistry (27) 1664 quantified the $\Delta G$ for the transfer of amino acid side chains from octanol (hydrophobic) to water. This data is essential for predicting which residues will reside within the membrane.

Amino Acid Distribution in Transmembrane Domains

According to Liu, Engelman, and Gerstein (2002) in Genome Biology, amino acids are not distributed equally in transmembrane (TM) domains. Their frequency reflects the hydrophobic nature of the environment.

Distribution Statistics

• Highly Abundant (Hydrophobic):     * Leucine (Leu): $16\%$     * Isoleucine (Ile): $10\%$     * Alanine (Ala): $9\%$     * Valine (Val): $8\%$     * Phenylalanine (Phe): $8\%$     * Glycine (Gly): $8\%$

• Moderately Abundant:     * Serine (Ser): $7\%$     * Threonine (Thr): $7\%$     * Methionine (Met): $4\%$     * Proline (Pro): $4\%$

• Low Abundance (Polar/Charged):     * Tryptophan (Trp): $3\%$     * Tyrosine (Tyr): $3\%$     * Asparagine (Asn): $3\%$     * Cysteine (Cys): $2\%$     * Histidine (His): $2\%$     * Arginine (Arg): $1\%$     * Aspartic Acid (Asp): $1\%$     * Glutamic Acid (Glu): $1\%$     * Glutamine (Gln): $1\%$     * Lysine (Lys): $1\%$

Secondary Structure in Membrane Proteins

To satisfy the hydrogen bonding requirements of the peptide backbone, membrane proteins adopt two primary structural motifs.

The $\alpha$-Helix

In an $\alpha$-helix, the peptide backbone is coiled such that every carbonyl oxygen ($C=O$) forms a hydrogen bond with the amide hydrogen ($N-H$) of a residue four positions down the chain. This maximizes internal H-bonding.

• Side Chain Orientation: $\text{R}$-groups (side chains) point outward from the helical axis into the lipid environment.

The $\beta$-Sheet

In $\beta$-sheets, hydrogen bonds form between the backbones of adjacent strands. In the context of the membrane, these usually form a closed structure known as a $\beta$-barrel to avoid exposing "unpaired" backbone charges at the edges of the sheet.

$\alpha$-Helical Transmembrane Domains

Geometry and Dimensions

• Standard Rise: Each amino acid in an $\alpha$-helix contributes a vertical rise of $1.5\,Å$.

• Bilayer Thickness: The average hydrophobic thickness of a lipid bilayer is approximately $36\,Å$.

• Required Length: To span the entire bilayer, a transmembrane helix requires at least $24$ residues ($\frac{36\,Å}{1.5\,Å/\text{residue}} = 24\,\text{residues}$). In practice, helical domains typically range from $19$ to $24$ residues.

Examples of Helical Membrane Proteins

• Receptors: Bacteriorhodopsin, Rhodopsin, Histidine Kinase Receptors.

• Channels: KcsA (Potassium channel), ELIC, LGICs, Aquaporins, $H^{+}/Cl^{-}$ Channels.

• Transporters: LacY (Lactose Permease).

• Energy Converters: ATPases.

Sequence Example and Helical Wheels

A typical TM sequence like VVLALLTLTSSAFLLFQL can be visualized using a helical wheel projection to show the distribution of residues around the helix.

• Color Key for Wheel Projection:     * Nonpolar: Hydrophobic residues (e.g., Val, Leu, Ala).     * Polar, Uncharged: (e.g., Ser, Thr).     * Acidic: Negatively charged.     * Basic: Positively charged.

Positional Preference of Amino Acids in Helices

Research by Von Heijne (1994) (Ann. Rev. Biophys. Biomol. Struc. 23) highlights specific positional preferences within the membrane.

The Hydrophobic Core

Hydrophobic amino acids like Alanine, Valine, Leucine, and Isoleucine (represented as fILV) exhibit a high frequency (approaching $0.9$ probability density) in the central region of the membrane ($10$ to $25$ residues deep into the sequence).

Aromatic Residues as Buffers

Aromatic amino acids—Phenylalanine (Phe), Tyrosine (Tyr), and Tryptophan (Trp)—are predominantly located at the water/bilayer interface.

• Reasoning: Aromatic residues exhibit both polar and apolar characteristics. This allows the protein to interact with the charged environment of the lipid headgroups/aqueous phase while simultaneously engaging with the hydrophobic lipid core. They act as a physical and chemical "buffer" between the lipid and water (e.g., as seen in Lac Permease).

The Positive-Inside Rule

Arginine (Arg) and Lysine (Lys) residues are typically found on the cytoplasmic side of the membrane. This statistical observation is a primary determinant of protein orientation (topology) within the bilayer.

Helix Packing and Identification

Helix Packing

Helices do not span the membrane in isolation; they pack together with specific geometries:

• Crossing Angles: Typically defined at $-37^{\circ}$, $+83^{\circ}$, and $+22^{\circ}$.

• TM Domain Packing: Helical packing in transmembrane domains (TMD) normally occurs at angles between $+5^{\circ}$ and $+25^{\circ}$.

• Orientation: Antiparallel packing is generally preferred.

• Promoters of Interaction:     * Small side chains: Motifs like GXXXG (Glycine-any-any-any-Glycine).     * Polar residues: Specific motifs like SxxSSxxS (Ser-any-any-Ser-Ser-any-any-Ser).

Identification: Kyte-Doolittle Hydropathy Plots

To identify potential TM domains, biologists use the Kyte-Doolittle Hydropathy Scale, which assigns a score based on the hydrophobicity of each amino acid.

Hydropathy Scores:

• Isoleucine (I): $4.5$

• Valine (V): $4.2$

• Leucine (L): $3.8$

• Phenylalanine (F): $2.8$

• Cysteine (C): $2.5$

• Methionine (M): $1.9$

• Alanine (A): $1.8$

• Glycine (G): $-0.4$

• Threonine (T): $-0.7$

• Serine (S): $-0.8$

• Tryptophan (W): $-0.9$

• Tyrosine (Y): $-1.3$

• Proline (P): $-1.6$

• Histidine (H): $-3.2$

• Glutamic acid (E): $-3.5$

• Glutamine (Q): $-3.5$

• Aspartic acid (D): $-3.5$

• Asparagine (N): $-3.5$

• Lysine (K): $-3.9$

• Arginine (R): $-4.5$

Analysis Method: A "sliding window" moves along the protein sequence, calculating the average hydropathy. High positive peaks (above an defined upper cutoff) indicate potential transmembrane domains. Examples include Phospholamban and Rhodopsin.

$\beta$-Barrel Proteins

Architecture and Stability

• Structure: Anti-parallel $\beta$-sheets rolled into a closed cylinder.

• Efficiency: Because the $\beta$-strand is more extended than an $\alpha$-helix, only $7$ to $11$ residues are needed to cross the bilayer.

• Geometry: Strands are typically slanted at a $45^{\circ}$ angle.

• Stability: The structure is extremely stable due to extensive hydrogen bonding. Calculation: $10\,\text{amino acids per strand} \times 8\,\text{strands} \times 0.5\,kcal/mol \approx 40\,kcal/mole$.

• Pore Size: The internal diameter is determined by the number of strands.

Examples of $\beta$-Barrel Proteins

• Maltoporin (Sugar transport)

• Fe-siderophore transporter

• OmpLA (phospholipase)

• TolC

• $\alpha$-hemolysin

Loop Connections

• Extracellular Loops: Tends to be long with high sequence variability. They often cause constriction in the pore and contribute to selectivity (e.g., OmpF distribution of charged residues favors cation uptake).

• Periplasmic Loops: Typically short and formed by tight loops or $\beta$-turns (e.g., Type-I and Type-II beta turns).

Residue Distribution in $\beta$-Barrels

• The surface of the barrel is composed of aliphatic side chains forming a hydrophobic ring.

• There is an aliphatic ribbon lined by two girdles of aromatic residues (Phe, Tyr, Trp) that buffer the interface between lipid and water, similar to $\alpha$-helical proteins.

Structural Biology of Membrane Proteins

Summary of Major Classes

1. Receptors: Bacteriorhodopsin, Rhodopsin, Halorhodopsin, Sensory rhodopsin.

2. Channels: GAP Junction, Aquaporin, KcsA, ClC Chloride channel.

3. Transporters: $F_1F_0$-ATPase, $Ca^{2+}$ ATPase, Lac Permease, $Fe^{2+}$ transporter, ABC-transporter.

4. Electron Transfer: Cytochrome c oxidase, Cytochrome bc1, PS-I/PS-II, Bacterial Reaction Centre.

5. Outer Membrane Proteins: Porins, Maltoporin, Glycerol channel.

The Scarcity Challenge

Despite their biological importance, integral membrane proteins represent less than $1\%$ of all known protein structures.

Obstacles in Structure Determination

• Expression: Difficult to produce in high quantities. Eukaryotic proteins lack proper post-translational machinery when expressed heterologously in bacteria or yeast.

• Solubilization/Purification: Requires detergents to extract from the membrane without denaturing the protein. Reconstitution into a stable environment is complex.

• Analysis Techniques:     * X-ray Crystallography: Difficult to grow high-quality crystals.     * Electron Microscopy (2D diffraction): Often low to medium resolution; requires 2D crystal formation.     * NMR Spectroscopy:         * Solution state NMR: Used for structures in micellar systems.         * Solid state NMR: Used for structures directly within the lipid bilayer.

Final Summary

• Lipids form a hydrophobic barrier around the cell.

• Membrane proteins facilitate information and material transfer across this barrier.

• The structure is determined by the lipid environment: minimize charge transfer and maximize hydrogen bonding.

• Two major classes: $\alpha$-helical transmembrane domains and $\beta$-barrel structures.

• Studying these proteins remains one of the greatest challenges in biochemistry due to their unique environmental requirements.