Protein Structure and Function

Introduction

Proteins are essential biological macromolecules that perform a vast array of functions in living organisms. Their function is intimately related to their structure, which is organized hierarchically into four levels: primary, secondary, tertiary, and quaternary structures. Understanding these structural levels is fundamental to biochemistry.

Primary Structure of Proteins

Definition and Importance

• Primary structure refers to the linear sequence of amino acids in a polypeptide chain, read from the N-terminal (amino end) to the C-terminal (carboxyl end).

• This sequence is unique for each protein and determines its three-dimensional conformation and biological function.

• Even a single amino acid change can significantly alter protein function, as seen in diseases like sickle-cell anemia (caused by a single amino acid substitution in hemoglobin).

• Primary structure is routinely determined using biochemical techniques such as Edman degradation or mass spectrometry.

Example: The one-letter notation is often used to represent the amino acid sequence of proteins for simplicity and clarity.

Secondary Structure of Proteins

Overview and Types

Secondary structure refers to the local three-dimensional arrangements of the polypeptide backbone, stabilized mainly by hydrogen bonds. The two most common types are the α-helix and the β-pleated sheet.

• α-Helix: A right-handed coil where each backbone N-H group forms a hydrogen bond with the C=O group of the amino acid four residues earlier.

• β-Pleated Sheet: Composed of two or more polypeptide chains (strands) lying side by side, stabilized by hydrogen bonds between backbone atoms in adjacent strands. Strands can be parallel or antiparallel.

• Turns and Loops: Short regions that connect α-helices and β-sheets, often containing glycine (for flexibility) and proline (for rigidity).

Torsion Angles and Planarity

• The peptide bond has partial double-bond character due to resonance, making it planar and restricting rotation.

• Rotation is possible around the bonds adjacent to the α-carbon: the φ (phi) and ψ (psi) angles.

• Allowed combinations of φ and ψ angles are depicted in a Ramachandran plot.

Equation:

= angle between N–Cα bond = angle between Cα–C bond

α-Helix Structure

• There are 3.6 amino acids per turn of the helix.

• The pitch (distance per turn) is 5.4 Å.

• Side chains project outward from the helix, minimizing steric hindrance.

• All peptide bonds are in the s-trans configuration and planar.

• Hydrogen bonds are nearly parallel to the helical axis.

Disruption of α-Helix

• Proline disrupts α-helices due to its rigid cyclic structure and lack of an N-H group for hydrogen bonding.

• Electrostatic repulsion between like-charged side chains (e.g., Lys and Arg, Glu and Asp) can destabilize the helix.

• Steric hindrance from bulky side chains (e.g., Val, Ile) can also disrupt the helix.

• Glycine provides high conformational flexibility, which can destabilize the helix.

β-Pleated Sheet Structure

• Polypeptide chains (strands) align side by side, forming hydrogen bonds between backbone atoms.

• Strands can be parallel (same direction) or antiparallel (opposite direction).

• Side chains alternate above and below the plane of the sheet.

• β-sheets are often found in the core of globular proteins.

Tertiary Structure of Proteins

Definition and Stabilizing Interactions

Tertiary structure refers to the overall three-dimensional arrangement of all atoms in a single polypeptide chain. It results from interactions between secondary structural elements and side chains.

• Hydrogen bonds between polar side chains (e.g., Ser, Thr).

• Hydrophobic interactions among nonpolar side chains (e.g., Val, Ile).

• Electrostatic attractions between oppositely charged side chains (e.g., Lys and Glu).

• Electrostatic repulsions between like-charged side chains.

• Covalent disulfide bonds between cysteine residues.

Classification

• Fibrous proteins: Long, parallel polypeptide chains forming fibers or sheets; usually insoluble in water; provide structural support (e.g., keratin, collagen).

• Globular proteins: Compact, roughly spherical; usually soluble in water; perform dynamic functions (e.g., enzymes, myoglobin).

Quaternary Structure of Proteins

Definition and Examples

Quaternary structure is the association of two or more polypeptide chains (subunits) into a functional protein complex. Subunits may be identical or different.

• Stabilized by non-covalent interactions (hydrogen bonds, hydrophobic interactions, electrostatic forces) and sometimes covalent bonds (e.g., disulfide bridges).

• Examples: Hemoglobin (α2β2 tetramer), ribosomes (complexes of proteins and RNA).

Protein Folding, Denaturation, and Chaperones

Folding and Stability

• The amino acid sequence (primary structure) determines the three-dimensional shape (native conformation) of a protein.

• The hydrophobic effect drives folding, causing nonpolar residues to be buried in the protein core and polar residues to be exposed to the aqueous environment.

• Folding is a spontaneous process, but can be assisted by chaperone proteins to prevent misfolding and aggregation.

Denaturation and Refolding

• Denaturation is the loss of structural order (secondary, tertiary, or quaternary structure) and biological activity, caused by factors such as extreme pH, detergents, urea, guanidine, or reducing agents (e.g., β-mercaptoethanol).

• Some proteins can refold spontaneously if the denaturing agent is removed, demonstrating that all information for folding is encoded in the primary structure.

Ligand Binding and Cooperativity

Binding Equilibria

• Proteins can bind small molecules (ligands) reversibly at specific binding sites, forming a protein-ligand complex.

• The fraction of bound protein () is given by:

• is the dissociation constant; lower indicates higher affinity.

• At , half of the protein binding sites are occupied ().

Myoglobin and Hemoglobin: Oxygen Binding

• Myoglobin: A monomeric protein with a single heme group; stores O2 in muscle; exhibits a hyperbolic O2 binding curve.

• Hemoglobin: A tetramer (α2β2) with four heme groups; transports O2 in blood; exhibits sigmoidal O2 binding curve due to cooperativity.

• Cooperativity means that binding of O2 to one subunit increases the affinity of the remaining subunits for O2.

Cooperativity and the Hill Equation

• The degree of cooperativity is described by the Hill coefficient ():

• : No cooperativity (e.g., myoglobin).

• : Positive cooperativity (e.g., hemoglobin).

• : Negative cooperativity.

Allosteric Regulation and the Bohr Effect

• Hemoglobin exists in two states: T (tense, low affinity) and R (relaxed, high affinity). O2 binding shifts equilibrium toward the R state.

• The Bohr effect: Decreased pH (increased H+) or increased CO2 reduces hemoglobin's O2 affinity, facilitating O2 release in tissues.

• 2,3-Bisphosphoglycerate (BPG) stabilizes the T state, promoting O2 release.

Summary Table: Comparison of Myoglobin and Hemoglobin

Additional info: Some explanations and context have been expanded for clarity and completeness, including the summary table and equations.