Proteins and Enzymes: Structure, Function, and Regulation (Ch. 19 & 20 Study Guide)

Proteins: Structure and Function

Functions of Proteins

Proteins are essential biomolecules that perform a wide variety of functions in living organisms. Their diverse roles are determined by their unique structures.

• Structural proteins: Provide support and shape to cells and tissues (e.g., collagen in connective tissue).

• Enzymatic proteins: Catalyze biochemical reactions (e.g., amylase in saliva).

• Transport proteins: Carry molecules across membranes or through the bloodstream (e.g., hemoglobin transports oxygen).

• Regulatory proteins: Control cellular processes (e.g., insulin regulates blood glucose).

• Defensive proteins: Protect against disease (e.g., antibodies).

• Contractile proteins: Enable movement (e.g., actin and myosin in muscles).

Amino Acids: Structures and Properties

Amino acids are the building blocks of proteins. Each amino acid contains an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a unique side chain (R group) attached to a central carbon.

• General structure: $\mathrm{H_2N{-}CHR{-}COOH}$

• Classification: Based on the properties of the R group, amino acids can be nonpolar, polar, acidic, or basic.

• Peptide bond: Amino acids are linked by peptide bonds formed via condensation reactions.

Conformations of Proteins

Proteins have four levels of structural organization, each contributing to their function.

• Primary structure: The linear sequence of amino acids in a polypeptide chain.

• Secondary structure: Local folding patterns such as alpha helices and beta sheets, stabilized by hydrogen bonds.

• Tertiary structure: The overall three-dimensional shape of a single polypeptide, determined by interactions among side chains (hydrophobic interactions, ionic bonds, disulfide bridges).

• Quaternary structure: The arrangement of multiple polypeptide subunits in a protein complex (e.g., hemoglobin).

Denaturing Proteins

Denaturation is the process by which a protein loses its native structure due to external stress, such as heat, pH changes, or chemicals. This disrupts the protein's function.

• Causes: Heat, strong acids or bases, organic solvents, heavy metals.

• Effects: Loss of biological activity, often irreversible.

• Example: Cooking an egg denatures the albumin protein, causing it to solidify.

Enzymes: Structure, Mechanism, and Regulation

Nomenclature of Enzymes

Enzymes are biological catalysts that speed up chemical reactions. They are named based on the substrate they act on or the type of reaction they catalyze, often ending in "-ase" (e.g., lipase breaks down lipids).

• Systematic naming: Based on the type of reaction (e.g., oxidoreductases, transferases, hydrolases).

• Common names: Often derived from the substrate (e.g., sucrase hydrolyzes sucrose).

Lock & Key and Induced Fit Models

Enzyme specificity is explained by two models:

• Lock & Key model: The enzyme's active site is a rigid structure that fits the substrate exactly.

• Induced Fit model: The enzyme's active site is flexible and molds itself around the substrate upon binding.

• Example: Hexokinase changes shape when binding glucose, illustrating the induced fit model.

Enzyme Inhibitors

Enzyme activity can be regulated by inhibitors, which decrease or stop the enzyme's function.

• Competitive inhibitors: Bind to the active site, competing with the substrate.

• Noncompetitive inhibitors: Bind to a different site, altering the enzyme's shape and reducing activity.

• Reversible inhibitors: Can dissociate from the enzyme, allowing activity to resume.

• Irreversible inhibitors: Permanently inactivate the enzyme (e.g., nerve gases).

Factors Influencing Enzyme Activity

Several factors affect how efficiently enzymes catalyze reactions.

• Temperature: Enzyme activity increases with temperature up to an optimum, then decreases due to denaturation.

• pH: Each enzyme has an optimal pH range; extreme pH can denature the enzyme.

• Substrate concentration: Activity increases with substrate concentration until saturation is reached.

Enzyme Catalysis and Kinetics

Enzymes lower the activation energy required for reactions, increasing reaction rates.

• Michaelis-Menten equation:

$V = \frac{V_{max}[S]}{K_m + [S]}$

• V: Reaction rate

• Vmax: Maximum rate

• [S]: Substrate concentration

• Km: Michaelis constant (substrate concentration at half-maximal velocity)

Regulation of Enzyme Activity

Cells regulate enzyme activity to control metabolic pathways.

• Feedback inhibition: End product of a pathway inhibits an earlier enzyme, preventing overproduction.

• Allosteric enzymes: Enzyme activity is regulated by molecules binding to sites other than the active site, causing conformational changes.

• Proenzymes (zymogens): Inactive precursors activated by specific modifications (e.g., pepsinogen to pepsin).

• Protein modification: Covalent addition or removal of groups (e.g., phosphorylation) can activate or deactivate enzymes.

Type of Regulation	Mechanism	Example
Feedback Inhibition	End product inhibits pathway	ATP inhibits phosphofructokinase in glycolysis
Allosteric Regulation	Effector binds non-active site	Hemoglobin oxygen binding
Proenzyme Activation	Proteolytic cleavage	Trypsinogen to trypsin
Protein Modification	Covalent addition/removal	Phosphorylation of enzymes

Additional info: Expanded explanations and examples have been added for clarity and completeness.