Protein Function and Ligand Binding: Oxygen Transport, Regulation, and Immune Response

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Protein Function

Principles of Protein-Ligand Interaction

Proteins perform their functions by interacting dynamically with other molecules, often through reversible binding. These interactions are fundamental to biological processes, including catalysis, signaling, and transport.

Ligand: A molecule reversibly bound by a protein. Ligands can be small molecules, ions, or other proteins.
Binding Site: The region of the protein complementary to the ligand in size, shape, charge, and hydrophobic/hydrophilic character.
Specificity: Proteins selectively bind specific ligands, maintaining cellular order.
Flexibility: Proteins can undergo subtle or dramatic conformational changes, essential for function.
Induced Fit: Ligand binding often induces a conformational change, increasing binding affinity.
Multisubunit Effects: In multisubunit proteins, conformational changes in one subunit can affect others.
Regulation: Protein-ligand interactions may be regulated by allosteric mechanisms.

Proteins interacting with ligands

Oxygen-Binding Proteins

Heme Prosthetic Group and Oxygen Binding

Oxygen transport and storage in vertebrates rely on proteins containing a heme prosthetic group, which binds oxygen reversibly. Heme consists of a protoporphyrin ring with a central Fe2+ ion.

Heme Structure: Four nitrogen atoms in the porphyrin ring coordinate Fe2+, with two additional perpendicular coordination sites.
Oxygen Binding: One perpendicular site binds a histidine residue (proximal His), the other binds O2. Fe2+ binds O2 reversibly; Fe3+ does not.

Heme structure and coordination bonds Coordination bonds of iron in heme Edge view of heme binding with proximal His and O2

Globin Family: Myoglobin and Hemoglobin

Globins are a widespread protein family with a highly conserved tertiary structure. Myoglobin and hemoglobin are the primary oxygen-binding globins in mammals.

Myoglobin: Monomeric, facilitates O2 diffusion in muscle tissue, contains a single heme group.
Hemoglobin: Tetrameric, transports O2 in blood, consists of two α and two β chains, each with a heme group.

Myoglobin structure with heme group Structural conservation of globins Low sequence similarity, high structural similarity in globins

Quantitative Description of Protein-Ligand Binding

The reversible binding of a protein (P) to a ligand (L) is described by equilibrium expressions and constants.

Association Constant (Ka): Measures affinity of ligand for protein. Higher Ka = higher affinity.
Dissociation Constant (Kd): Reciprocal of Ka. Lower Kd = higher affinity.
Fractional Occupancy (Y): Fraction of binding sites occupied by ligand.

Key Equations:

Ligand binding curve and equation Oxygen binding curve for myoglobin

Protein	Ligand	Kd (M)
Avidin (egg white)	Biotin	1 × 10-15
Insulin receptor (human)	Insulin	1 × 10-9
Anti-HIV immunoglobulin (human)	gp41 (HIV-1 surface protein)	1 × 10-10
Nickel-binding protein (E. coli)	Ni2+	3 × 10-7
Calmodulin (rat)	Ca2+	1 × 10-6

Protein dissociation constants table

Partial Pressure and Oxygen Binding

Oxygen binding is often described in terms of partial pressure (pO2), with P50 representing the pO2 at which half the binding sites are occupied.

Oxygen binding curve with partial pressure

Protein Structure and Ligand Binding

Protein conformation affects ligand binding. For example, carbon monoxide (CO) binds free heme much more strongly than O2 due to differences in orbital structure and binding geometry.

CO and O2 binding to heme Myoglobin's distal His increases heme's affinity for O2

Hemoglobin: Structure and Cooperative Binding

Hemoglobin exhibits cooperative binding, where the binding of O2 to one subunit increases the affinity of other subunits. This is reflected in its sigmoidal binding curve.

T state: Low affinity for O2, stabilized by ion pairs.
R state: High affinity for O2, favored upon O2 binding.
Cooperativity: Conformational changes in one subunit affect others.

Hemoglobin T and R states Ion pairs stabilizing T state Conformational change in hemoglobin T to R transition in hemoglobin Changes in conformation near heme

Quantitative Description of Cooperative Binding

Cooperative binding is described by the Hill equation, which relates ligand concentration to fractional occupancy.

Hill coefficient (nH): Indicates degree of cooperativity. nH = 1 (non-cooperative), nH > 1 (positive cooperativity), nH < 1 (negative cooperativity).

Hill plot for myoglobin and hemoglobin

Models of Cooperative Binding

Two models explain cooperative binding in hemoglobin:

MWC (Concerted) Model: All subunits are in the same conformation; ligand binding shifts equilibrium toward R state.
Sequential Model: Each subunit can change conformation independently; ligand binding progressively favors R state.

MWC model for cooperative binding Sequential model for cooperative binding

Regulation of Oxygen Binding: Bohr Effect and BPG

Hemoglobin's affinity for O2 is regulated by pH, CO2, and 2,3-bisphosphoglycerate (BPG).

Bohr Effect: Lower pH and higher CO2 favor the T state, promoting O2 release.
BPG: Binds to hemoglobin, stabilizes T state, reduces O2 affinity, and increases at high altitudes.

Bohr effect: pH and O2 binding 2,3-Bisphosphoglycerate structure Effect of BPG on O2 binding BPG binding to deoxyhemoglobin BPG increases at high altitudes BPG binds to cavity between beta subunits

Hemoglobin Variants and Disease

Fetal hemoglobin (α2γ2) has higher O2 affinity due to lower BPG binding. Sickle cell anemia results from a single amino acid substitution, causing hemoglobin polymerization and cell deformation.

Immune System and Immunoglobulins

Immune Response and Antibody Structure

The immune system distinguishes self from nonself and eliminates pathogens. Antibodies (immunoglobulins) are proteins that bind antigens with high specificity and affinity.

Antigen: Molecule capable of eliciting an immune response.
Antibody: Protein produced by B cells, binds antigens at specific sites (epitopes).
Immunoglobulin G (IgG): Major class, consists of two heavy and two light chains, each with variable and constant domains.
Induced Fit: Antibody-antigen binding involves conformational changes for tight interaction.

Structure of immunoglobulin G Variable domain of immunoglobulin G IgM pentamer structure

Antibody-Antigen Interaction and Analytical Applications

Antibodies bind antigens tightly and specifically, forming the basis for analytical techniques such as Western blotting.

Polyclonal Antibodies: Mixture recognizing different parts of a protein.
Monoclonal Antibodies: Homogeneous, recognize a single epitope.

Antibody-antigen interaction Western blot assay

Muscle Contraction: Actin, Myosin, and Molecular Motors

Major Proteins of Muscle

Muscle contraction is driven by transient interactions between actin and myosin filaments, powered by ATP hydrolysis.

Myosin: Forms thick filaments, consists of two heavy and four light chains, with a coiled-coil tail and globular head.
Actin: Monomeric G-actin forms F-actin (thin filaments), each monomer binds and hydrolyzes ATP.
Thick Filaments: Aggregated myosin.
Thin Filaments: F-actin, troponin, and tropomyosin.

Myosin structure Actin structure Thick filaments Thin filaments

Muscle Fiber Organization and Sarcomere Structure

Muscle fibers contain myofibrils organized into sarcomeres, the fundamental contractile units.

Sarcomere: Contains thick and thin filaments, defined by A band, I band, Z disk, and M line.

Structure of a sarcomere

Regulation of Muscle Contraction

Muscle contraction is regulated by tropomyosin and troponin, which control access to myosin-binding sites on actin.

Tropomyosin: Blocks myosin-binding sites on actin.
Troponin: Binds Ca2+, causes conformational change, exposes myosin-binding sites.

Summary: Protein function is governed by dynamic, reversible interactions with ligands, regulated by structural changes, allosteric effects, and environmental factors. These principles underpin oxygen transport, immune response, and muscle contraction.