Protein Structure and Enzymes: Structure, Function, and Regulation

Protein Structure and Enzymes

Protein Structure

Proteins are essential biomolecules that perform a wide variety of functions in living organisms. They are composed of amino acids linked by peptide bonds, and their structure determines their function.

• Enzymatic proteins: Accelerate chemical reactions (act as catalysts).

• Storage proteins: Store amino acids.

• Transport proteins: Transport substances.

• Structural proteins: Provide support.

Amino Acids

• Proteins are polymers constructed from 20 different amino acids.

• Amino acids are linked by peptide bonds to form polypeptides.

• When folded correctly, a polypeptide forms a functional protein.

General Structure of an Amino Acid:

• Central carbon (α-carbon)

• Amino group (-NH2)

• Carboxyl group (-COOH)

• Hydrogen atom

• R group (side chain): determines the properties and identity of the amino acid

Classification of Amino Acids:

• Nonpolar side chains: hydrophobic

• Polar side chains: hydrophilic

• Electrically charged side chains: acidic or basic

Peptide Bond Formation

A peptide bond forms between the carboxyl group of one amino acid and the amino group of another, releasing a water molecule (dehydration reaction):

• Polypeptide backbone: Repeating sequence of atoms (excluding side chains).

Levels of Protein Structure

• Primary structure: Unique sequence of amino acids in a polypeptide.

• Secondary structure: Coiling or folding of the polypeptide backbone (e.g., α-helix, β-pleated sheet) stabilized by hydrogen bonds.

• Tertiary structure: Overall 3D shape of a polypeptide, determined by interactions among side chains (hydrophobic interactions, ionic bonds, hydrogen bonds, disulfide bridges).

• Quaternary structure: Association of two or more polypeptide subunits.

Denaturation

• Loss of protein structure due to changes in pH, salt concentration, temperature, etc.

• Denatured proteins lose their function.

Protein Structure Determination

• X-ray crystallography: Common method to determine 3D structure of proteins.

Enzymes and Their Function

Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy required for the reaction to proceed.

• Activation energy (Ea): The energy required to start a chemical reaction.

• Enzymes bind substrates at their active site, forming an enzyme-substrate complex.

• Enzyme action is often described by the induced fit model, where the enzyme changes shape to better fit the substrate.

Factors Affecting Enzyme Activity

• Temperature

• pH

• Substrate concentration

• Presence of inhibitors or activators

Enzyme Inhibition

• Competitive inhibitors: Bind to the active site, blocking substrate binding.

• Noncompetitive inhibitors: Bind elsewhere on the enzyme, altering its shape and function.

Regulation of Enzyme Activity

Enzyme activity can be regulated by various mechanisms to ensure proper cellular function.

• Allosteric regulation: Regulatory molecules bind to a site other than the active site, stabilizing either the active or inactive form of the enzyme.

• Many enzymes consist of multiple polypeptide chains called subunits.

• Allosteric enzymes often function as metabolic pathway regulators, controlling the flow of biochemical reactions.

• Cooperativity: A form of allosteric regulation where substrate binding to one subunit increases the activity of other subunits.

Summary Table: Levels of Protein Structure

Level	Description	Bonds/Interactions
Primary	Sequence of amino acids	Peptide bonds
Secondary	Local folding (α-helix, β-sheet)	Hydrogen bonds
Tertiary	3D shape of polypeptide	Hydrophobic interactions, ionic bonds, hydrogen bonds, disulfide bridges
Quaternary	Association of multiple polypeptides	Same as tertiary (between subunits)

Key Equations

• Peptide bond formation (dehydration reaction):

$\text{Amino acid}_1 + \text{Amino acid}_2 \rightarrow \text{Dipeptide} + H_2O$

• Enzyme-catalyzed reaction:

$E + S \rightleftharpoons ES \rightarrow E + P$

Where E = enzyme, S = substrate, ES = enzyme-substrate complex, P = product.

Additional info: The notes have been expanded to include definitions, examples, and a summary table for clarity and completeness.