Protein Tertiary Structure and Interior Side-Chain Interactions

Protein Folding Thermodynamics and Structural Architecture

Proteins fold spontaneously into a thermodynamically stable three-dimensional native conformation dictated by their primary amino acid sequence. In an aqueous solvent, the driving force for protein folding is governed by the Gibbs free energy equation \n\Delta G = \Delta H - T\Delta S\n where the overall change in free energy $\Delta G$ must be negative for folding to occur spontaneously.

The primary thermodynamic driving force for the folding of water-soluble globular proteins is the hydrophobic effect. In an unfolded state, surrounding water molecules form highly ordered, ice-like clathrate cages around nonpolar side chains. When nonpolar side chains cluster together in the interior core of the protein, these structured solvent cages are disrupted, releasing water molecules into the bulk solvent. This yields a significant increase in solvent entropy ($\Delta S_{\text{solvent}} > 0$), which compensates for the decrease in conformational entropy of the polypeptide chain ($\Delta S_{\text{protein}} < 0$).

Detailed Classification of Side-Chain Interactions in Tertiary Structure

The tertiary structure of a protein is stabilized by non-covalent interactions and covalent cross-links occurring between the side chains (R-groups) of amino acid residues. These are categorized into five distinct interaction types:

Interaction 1 represents a hydrogen bond between a polar hydroxyl group ($-\text{CH}_2-\text{OH}$, such as that on a serine or threonine residue) and a carbonyl oxygen ($\text{O}=\text{C}-$) on the peptide backbone or another side chain. Hydrogen bonding is an electrostatic attraction between a hydrogen atom covalently bonded to an electronegative atom and a neighboring electronegative electron donor.

Interaction 2 represents a hydrogen bond formed between a polar side-chain hydroxyl group ($-\text{CH}_2-\text{OH}$) and a polar carbonyl-containing group of an adjacent side chain. Polar non-charged side chains prefer interacting with other polar groups or water molecules.

Interaction 3 represents hydrophobic interactions and van der Waals forces occurring among nonpolar, aliphatic, or aromatic amino acid side chains, such as leucine, isoleucine, valine, alanine, phenylalanine, and tryptophan. In aqueous solutions, these nonpolar groups associate to exclude water molecules.

Interaction 4 represents a covalent disulfide bond ($-\text{CH}_2-\text{S}-\text{S}-\text{CH}_2-$). Disulfide linkages are formed through the oxidation of two thiol ($-\text{SH}$) groups on cysteine residues. With a covalent bond energy of approximately $250\,\text{kJ}\,\text{mol}^{-1}$, disulfide bonds are much stronger than non-covalent interactions and provide exceptional structural stability.

Interaction 5 represents an ionic interaction, commonly called a salt bridge or ion pair, formed between a positively charged amino acid side chain (such as the protonated amino group of lysine, $-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{NH}_3^+$, or arginine) and a negatively charged side chain (such as the carboxylate group of aspartate or glutamate, $-\text{OOC}-\text{CH}_2-\text{CH}_2-$).

Spatial Distribution of Side Chains in Soluble Globular Proteins

The spatial distribution of side chains within a folded globular protein depends on their chemical polarity. Water-soluble proteins establish a structural orientation consisting of a buried hydrophobic core surrounded by a hydrophilic surface shell.

The hydrophobic interior core is composed predominantly of nonpolar side chains sequestered away from the surrounding aqueous medium by hydrophobic interactions (Interaction 3). Covalent disulfide bonds (Interaction 4) are frequently situated within this interior framework or bridging structural elements, providing internal covalent cross-links that stabilize the tertiary architecture.

Conversely, polar and charged side chains are located mainly on the exterior surface shell exposed to solvent. Hydroxyl groups forming hydrogen bonds (Interactions 1 and 2) and charged groups forming salt bridges (Interaction 5) interact favorably with surrounding water molecules ($\text{H}_2\text{O}$) and dissolved electrolytes. Placing uncompensated charges or unpaired polar groups in the nonpolar interior is energetically unfavorable due to the high energetic cost of desolvation.

Comparative Evaluation of Interior Core Interactions

When evaluating which labeled interactions are present in the interior of a protein soluble in water, nonpolar hydrophobic clusters (3) and covalent disulfide bridges (4) fit the structural criteria. Nonpolar interactions (3) assemble inside the core to drive the hydrophobic effect, while disulfide bonds (4) covalently lock the internal structural elements in place.

By contrast, interactions 1, 2, and 5 consist of polar and charged functional groups that form favorable electrostatic interactions with water. These hydrophilic groups are situated on the protein surface to maximize solubilization in the aqueous phase. Consequently, interactions 3 and 4 are the interactions characteristically observed in the interior core of a water-soluble protein.