Biochemistry Study Notes: Muscle Contraction, Peptide Bond Resonance, and Enzyme Cofactors

Muscle Contraction and the Role of ATP

The Cycle of Actin-Myosin Association and Dissociation

Muscle contraction is driven by the interaction between the proteins actin and myosin, regulated by the hydrolysis of ATP (adenosine triphosphate). The process involves a cyclical association and dissociation of myosin heads with actin filaments, which is essential for muscle movement.

• ATP Binding: When ATP binds to myosin, it induces a conformational change that causes myosin to release actin.

• Hydrolysis of ATP: The hydrolysis of ATP to ADP and inorganic phosphate (Pi) energizes the myosin head, allowing it to move into a "high-energy" state.

• Rebinding to Actin: The energized myosin head binds to a new position on the actin filament, closer to the Z disk.

• Power Stroke: Release of Pi triggers the power stroke, where the myosin head pivots, pulling the actin filament toward the center of the sarcomere.

• Release of ADP: After the power stroke, ADP is released, and the myosin remains tightly bound to actin until another ATP molecule binds, restarting the cycle.

Example: This cycle is fundamental to skeletal muscle contraction, enabling movements such as walking, running, and lifting objects.

Peptide Bond Resonance and Structure

Resonance Structure of the Peptide Bond

The peptide bond is a covalent bond formed between the carboxyl group of one amino acid and the amino group of another. The resonance structure of the peptide bond imparts partial double-bond character, restricting rotation around the C–N bond and stabilizing protein structure.

• Resonance: The electrons are delocalized between the carbonyl oxygen and the amide nitrogen, resulting in a planar structure.

• Partial Double-Bond Character: The resonance prevents free rotation around the C–N bond, which is crucial for maintaining the specific three-dimensional structure of proteins.

• Structural Implication: The rigidity of the peptide bond contributes to the formation of secondary structures such as alpha helices and beta sheets.

Example: The resonance structure is illustrated below, showing the delocalization of electrons:

H2N–C(=O)–NH–C(=O)–NH–C(=O)–NH–C(=O)–NH–C(=O)–NH2

Enzyme Cofactors and Enzyme Activity

Definitions: Cofactor and Coenzyme

Enzymes often require additional non-protein molecules to function efficiently. These molecules are classified as cofactors and coenzymes.

• Cofactor: A chemical component required for enzyme activity. Cofactors can be organic molecules (called coenzymes) or inorganic ions (such as metal ions).

• Coenzyme: A specific type of organic cofactor, often derived from vitamins, that assists in enzyme-catalyzed reactions.

Example: NAD+ (nicotinamide adenine dinucleotide) is a coenzyme involved in redox reactions.

Enzyme-Catalyzed vs. Uncatalyzed Reactions

Enzymes accelerate biochemical reactions by lowering the activation energy required for the reaction to proceed.

• With Enzyme: The reaction occurs rapidly and efficiently under physiological conditions.

• Without Enzyme: The reaction may proceed extremely slowly or not at all, making enzyme catalysis essential for life.

Equation:

$\text{Rate}_{\text{enzyme}} \gg \text{Rate}_{\text{uncatalyzed}}$

Example: The hydrolysis of peptide bonds in proteins is extremely slow without protease enzymes.

Summary Table: Cofactors and Coenzymes

Term	Definition	Example
Cofactor	Non-protein chemical required for enzyme activity; includes coenzymes and inorganic ions	Mg2+ in DNA polymerase
Coenzyme	Organic molecule serving as a cofactor, often derived from vitamins	NAD+, FAD

Additional info: The notes above expand on the brief points in the original material, providing definitions, examples, and context for each topic relevant to biochemistry students.