Protein Function: Binding, Hemoglobin, and Allostery (Chapter 5 Study Notes)

Protein Function

Overview of Protein Functions

Proteins play diverse roles in biological systems, often by binding other molecules reversibly. The molecule that a protein binds is called a ligand, and the specific region on the protein where this interaction occurs is the binding site. In the case of enzymes, the ligand is referred to as the substrate, and the site of chemical transformation is the active site.

• Ligand: Any molecule that binds specifically to a protein.

• Binding Site: The location on the protein where the ligand interacts.

• Active Site: For enzymes, the region where substrate conversion occurs.

• Reversible Binding: Most protein-ligand interactions are reversible, allowing dynamic biological regulation.

• Example: Hemoglobin binding oxygen; enzyme-substrate interactions.

Induced Fit Model

Conformational Changes Upon Binding

The induced fit model describes how ligand binding often causes changes in protein structure, enhancing the interaction. This conformational change typically increases binding affinity or catalytic efficiency.

• Induced Fit: Structural adaptation of a protein upon ligand binding.

• Facilitation: Enhanced binding or catalysis due to conformational change.

• Example: Enzyme active sites adjusting to fit substrates.

Oxygen Binding Proteins

Role and Mechanism

Proteins such as hemoglobin and myoglobin bind and transport oxygen using a heme prosthetic group. Oxygen binding is reversible, allowing these proteins to release oxygen when needed.

• Heme Group: A planar, conjugated ring structure (porphyrin) with a central iron (Fe) atom.

• Reversible Binding: Oxygen can bind and dissociate from heme, enabling transport.

• Example: Hemoglobin in blood, myoglobin in muscle.

Heme Structure and Iron Coordination

Porphyrin Ring and Iron Bonds

The heme group consists of a porphyrin ring that coordinates an iron atom. Iron (Fe) forms six bonds: four to the nitrogen atoms of the ring, one to a histidine residue, and one to oxygen.

• Porphyrin Ring: Cyclic structure found in heme and chlorophyll.

• Iron Coordination: Four bonds to ring nitrogens, one to protein histidine, one to oxygen.

• Protein Environment: Protein structure positions heme for optimal oxygen binding.

Binding Equilibria and Equations

Mathematical Description of Binding

Protein-ligand binding is a reversible equilibrium, described by association and dissociation constants. The fraction of occupied binding sites can be calculated using these constants.

• Association Constant (Ka): Measures binding affinity.

• Dissociation Constant (Kd): Inverse of Ka; commonly used to describe binding strength.

• Fraction Occupied (θ or q): Proportion of binding sites occupied by ligand.

Key Equations:

• Equilibrium: $[PL] \leftrightarrow [P][L]$

• Association constant: $K_a = \frac{[PL]}{[P][L]}$

• Dissociation constant: $K_d = \frac{[P][L]}{[PL]} = \frac{1}{K_a}$

• Fraction occupied: $\theta = \frac{[L]}{[L] + K_d}$

Affinity and Kd

Interpreting Dissociation Constant

The dissociation constant (Kd) is inversely proportional to binding affinity. A low Kd indicates strong binding (high affinity), while a high Kd indicates weak binding (low affinity). Kd is the ligand concentration at which half the binding sites are occupied.

• High Affinity: Small Kd value.

• Low Affinity: Large Kd value.

• Graphical Representation: θ vs. [L] curve; θ = 0.5 at [L] = Kd.

Measuring Binding

Experimental Methods

Binding interactions can be quantified using various techniques, including spectroscopy, surface plasmon resonance (SPR), biolayer interferometry (BLI), and isothermal titration calorimetry (ITC).

• Spectroscopy: Measures changes in absorbance or fluorescence upon binding.

• SPR/BLI: Real-time measurement of binding kinetics.

• ITC: Measures heat changes during binding.

Structure-Function Relationship

Protein Structure and Binding

Protein structure determines binding capability. Specific residues, such as histidine, coordinate the heme group and facilitate optimal oxygen binding.

• Histidine Coordination: Positions heme for oxygen interaction.

• Hydrophobic Interactions: Drive oligomerization and stabilize protein structure.

Hemoglobin: Structure and Function

Tetrameric Organization and Oxygen Transport

Hemoglobin is a tetrameric protein with two α-chains and two β-chains, each binding a heme group. The subunits are structurally similar but differ in sequence. Hemoglobin transports oxygen in the blood and undergoes conformational changes upon oxygen binding.

• Tetrameric Structure: 2 α-chains + 2 β-chains.

• Heme Binding: Each subunit binds one heme group.

• Oligomerization: Driven by hydrophobic interactions.

Hemoglobin Conformational States

R-State and T-State

Hemoglobin exists in two major conformations: the R-state (relaxed, high affinity for O₂) and the T-state (tense, low affinity for O₂). Oxygen binding induces a shift from T-state to R-state, affecting both individual subunits and the overall quaternary structure.

• R-State: High oxygen affinity, closed cavity.

• T-State: Low oxygen affinity, open cavity.

• Cooperative Binding: Binding of O₂ to one subunit increases affinity in others.

Allostery and Cooperative Binding

Mechanisms of Allosteric Regulation

Allosteric interactions are changes in protein structure induced by ligand binding. Homotropic allostery occurs when binding of a ligand affects affinity for the same ligand (e.g., O₂ in hemoglobin). Heterotropic allostery involves one ligand affecting affinity for a different ligand.

• Homotropic: Ligand binding increases affinity for same ligand.

• Heterotropic: Ligand binding increases affinity for a different ligand.

• Example: Hemoglobin's response to O₂ and H⁺/CO₂.

Phosphofructokinase (PFK)

Allosteric Enzyme Example

Phosphofructokinase is an allosteric enzyme involved in glycolysis. It is regulated by substrate and product binding, which induce conformational changes and affect enzyme activity.

• Substrate Binding: Induces structural changes.

• Allosteric Regulation: ATP and magnesium binding enhance activity.

• Oligomerization: Exists as dimer/tetramer, affecting function.

Quantitation of Cooperative Binding

Hill Equation and Hill Plot

Cooperative binding is quantified using the Hill equation and Hill plot. The Hill coefficient (nH) indicates the degree of cooperativity.

• Hill Equation: $\log\left(\frac{q}{1-q}\right) = n \log(L) - \log K_d$

• Hill Coefficient (nH): Slope of Hill plot; nH > 1 indicates positive cooperativity, nH < 1 negative cooperativity.

• Models: Concerted (MWC) and sequential models describe cooperative binding mechanisms.

Hemoglobin and Physiological Regulation

Transport of H⁺ and CO₂

Hemoglobin also transports protons (H⁺) and carbon dioxide (CO₂) away from tissues. Affinity for O₂, H⁺, and CO₂ is regulated by blood pH and allosteric effects.

• pH Dependence: Lower pH (higher H⁺) stabilizes T-state, promoting O₂ release.

• CO₂ Binding: CO₂ binds to N-terminus as carbamate, forming salt bridges and stabilizing T-state.

• Bohr Effect: Physiological mechanism for O₂ delivery and CO₂ removal.

Sickle Cell Anemia

Hemoglobin Differences

Normal Hemoglobin binds oxygen reversibly. When sickle cell hemoglobin (HG S) is deoxygenated, it becomes insoluble and forms fibrils in the RBCS's causing deformation

E6V -> Glutamate at position 6 mutated to Valine

Protein Interactions

Basis for the Immune System

Immune System has 2 components

• Humoral/Innate

  • Designed to destroy invaders (bacteria/viruses) before they get a foothold

• Cellular

  • Designed to destroy infected cells

Antigen: any molecule that elicits an immune response

Antibodies/immunoglobulins: large proteins at the heart of the humoral immune system

• Produced by B-Cells

• Macrophages and some B-Cells will engulf invading cells and viruses

• Antibodies bind to foreign invaders and help target them for destruction

Antibody Structure

2 domains:

• Constant

  • The constant domains have the same structure, the immunoglobulin fold

• Variable

Types of Constant Regions

Heavy Chains

• IgG – Acts against invading pathogens

• IgD - located on B-cells and binds antigens

• IgE – Binds allergens and triggers histamine release

• IgA – tears, milk and saliva. Prevents colonization by pathogens on mucosa

• IgM – Found on B-cells and secreted. Like IgG

Classes of Antibodies

There are different types of constant regions

• Light chains have both a constant and a variable region

• The variable region defines binding specificity

• Light Chains

Antibody Specificity

• Antibodies are designed to bind with VERY high specificity and VERY high affinity

• Polyclonal antibodies recognize different regions of the same antigen

• Monoclonal antibodies recognize the same region of an antigen

• Antibodies are widely used as an analytical tool in biochemistry due to their high specificity and affinity

Antibody Applications

Protein Detection

• SDS-PAGE/Western Blot

• ELISA

Protein Purification

• Affinity chromatography

• Immunotherapy

T-cells are the main player in the cellular immune system

• T-cells must discriminate between normal and infected cells

• A protein-protein interaction between cell surface proteins allows this discrimination

• This discrimination is via the interaction of a T- cell protein (T-cell receptor or TCR) and cellular proteins called MHCs (Major Histocompatability Complexes)

MHCs

• MHC-I

• MHC-II

MHC-I are found on the surface of most normal cells

MHC-II complexes are found on macrophages and B-cells (the cells that engulf invaders)

MHCs display small peptides derived from digested cellular proteins

Actin-Myosin interactions

• Muscles use the interaction of actin and myosin to create force

• Myosin is a heterohexamer with 2 heavy chains and 4 light chains which are arranged into superstructural “thick filaments”

  • Myosin “walks” along actin to cause contraction of muscles

• Actin is a fibrous protein complex that forms the basis of “thin filaments”

Summary Table: Key Equations and Concepts

Concept	Equation	Description
Association Constant (Ka)	$K_a = \frac{[PL]}{[P][L]}$	Affinity of protein for ligand
Dissociation Constant (Kd)	$K_d = \frac{[P][L]}{[PL]}$	Inverse of Ka; lower Kd = higher affinity
Fraction Occupied (θ)	$\theta = \frac{[L]}{[L] + K_d}$	Proportion of binding sites occupied
Hill Equation	$\log\left(\frac{q}{1-q}\right) = n \log(L) - \log K_d$	Quantifies cooperativity; n = Hill coefficient

Additional info: These notes expand on the original slides by providing definitions, equations, and context for protein-ligand binding, hemoglobin function, and allosteric regulation, suitable for biochemistry exam preparation.