Biochemistry Core Concepts: Proteins, Enzymes, and Carbohydrates

Proteins: Structure and Function

Primary Function of Myoglobin & Hemoglobin

Myoglobin and hemoglobin are essential oxygen-binding proteins found in animals. Their primary roles involve oxygen transport and storage.

• Myoglobin: Stores oxygen in muscle tissues, facilitating oxygen supply during intense muscular activity.

• Hemoglobin: Transports oxygen from the lungs to tissues via the bloodstream.

• Example: Myoglobin provides oxygen reserve for diving mammals; hemoglobin enables efficient oxygen delivery in humans.

Sigmoid Oxygen-Binding Curve of Hemoglobin

Hemoglobin exhibits cooperative binding to oxygen, resulting in a sigmoidal (S-shaped) oxygen dissociation curve.

• Cooperativity: Binding of one oxygen molecule increases the affinity for subsequent oxygen molecules.

• Physiological Importance: Enhances oxygen delivery to tissues under varying oxygen concentrations.

• Equation: The Hill equation describes cooperativity: $Y = \frac{pO_2^n}{P_{50}^n + pO_2^n}$

Bohr Effect

The Bohr effect describes how changes in pH and CO2 concentration affect hemoglobin's oxygen affinity.

• Lower pH (higher H+): Decreases oxygen affinity, promoting oxygen release in tissues.

• Higher CO2: Also decreases oxygen affinity.

• Equation: $HbO_2 + H^+ \leftrightarrow HHb + O_2$

Primary Function of Immunoglobulins (Antibodies)

Immunoglobulins are specialized proteins that recognize and bind to foreign antigens, facilitating immune defense.

• Antigen Recognition: Each antibody binds specifically to a unique antigen.

• Immune Response: Neutralizes pathogens and marks them for destruction.

• Example: IgG antibodies protect against bacterial and viral infections.

Enzyme Structure and Mechanism

Active Site Role

The active site is the region of an enzyme where substrate binding and catalysis occur.

• Specificity: The active site is shaped to fit specific substrates.

• Catalysis: Facilitates chemical reactions by lowering activation energy.

Michaelis-Menten Equation

The Michaelis-Menten equation models the rate of enzymatic reactions as a function of substrate concentration.

• Equation: $v = \frac{V_{max}[S]}{K_m + [S]}$

• Definitions: $V_{max}$ is the maximum reaction rate; $K_m$ is the substrate concentration at half $V_{max}$.

• Application: Used to characterize enzyme kinetics.

Lineweaver-Burk Plot

The Lineweaver-Burk plot is a double reciprocal graph used to linearize the Michaelis-Menten equation.

• Equation: $\frac{1}{v} = \frac{K_m}{V_{max}} \cdot \frac{1}{[S]} + \frac{1}{V_{max}}$

• Purpose: Facilitates determination of $K_m$ and $V_{max}$ from experimental data.

Inhibitors

Enzyme inhibitors are molecules that decrease or block enzyme activity.

• Types: Competitive, noncompetitive, and uncompetitive inhibitors.

• Competitive Inhibition: Inhibitor competes with substrate for active site.

• Noncompetitive Inhibition: Inhibitor binds elsewhere, altering enzyme function.

• Example: Methotrexate inhibits dihydrofolate reductase.

Serine Protease

Serine proteases are a class of enzymes that use a serine residue for peptide bond hydrolysis.

• Mechanism: Serine acts as a nucleophile in the active site.

• Examples: Trypsin, chymotrypsin, elastase.

HIV Protease

HIV protease is an aspartyl protease essential for viral maturation.

• Function: Cleaves viral polyproteins into functional units.

• Clinical Relevance: Target for antiretroviral drugs.

Hexokinase

Hexokinase catalyzes the phosphorylation of glucose to glucose-6-phosphate, the first step in glycolysis.

• Reaction: $\text{Glucose} + \text{ATP} \rightarrow \text{Glucose-6-phosphate} + \text{ADP}$

• Importance: Traps glucose inside the cell for metabolism.

Phosphorylation

Phosphorylation is the addition of a phosphate group to a molecule, often regulating protein function.

• Enzymes: Kinases catalyze phosphorylation; phosphatases remove phosphate groups.

• Role: Key regulatory mechanism in signal transduction and metabolism.

Carbohydrates: Structure and Specificity

Carbohydrates (General)

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, serving as energy sources and structural components.

• Monosaccharides: Simple sugars like glucose and fructose.

• Disaccharides: Two monosaccharides linked (e.g., sucrose).

• Polysaccharides: Long chains of monosaccharides (e.g., starch, cellulose).

Stereospecificity

Stereospecificity refers to the property of enzymes and receptors to distinguish between different stereoisomers of a substrate.

• Importance: Biological systems often utilize only one enantiomer (e.g., D-glucose).

• Example: Hexokinase acts specifically on D-glucose, not L-glucose.

Polysaccharide

Polysaccharides are large carbohydrate molecules formed by the polymerization of monosaccharide units.

• Structure: Can be linear or branched.

• Examples: Starch (energy storage in plants), glycogen (energy storage in animals), cellulose (structural component in plants).

Additional Topics

Sliding Filament Theory

The sliding filament theory explains muscle contraction via the interaction of actin and myosin filaments.

• Mechanism: Myosin heads bind to actin, pulling filaments past each other using ATP.

• Result: Muscle shortening and force generation.

• Additional info: While primarily a physiology topic, actin and myosin are proteins relevant to biochemistry.

pH Effect

pH can influence protein structure, enzyme activity, and biochemical reactions.

• Enzyme Activity: Each enzyme has an optimal pH for activity.

• Protein Structure: Extreme pH can denature proteins.