Amino Acids, Proteins, and Enzymes: Study Guide

Protein Basics

Definition and Structure of Proteins

Proteins are essential biological macromolecules composed of long chains of amino acids. They perform a wide variety of functions in living organisms, including catalyzing biochemical reactions, providing structural support, and regulating processes.

• Proteins as Polymers: Proteins are polymers made up of repeating units called amino acids.

• Dehydration Synthesis Reaction: Proteins form when amino acids are joined together by dehydration synthesis (also called a condensation reaction), in which a molecule of water is removed as a peptide bond forms between the amino group of one amino acid and the carboxyl group of another.

• Peptide Bonds: The covalent bond linking two amino acids is called a peptide bond.

• Definition of a Protein: A protein is a polypeptide (chain of amino acids) that has folded into a specific three-dimensional structure and is biologically active.

Amino Acids: Structure and Classification

• General Structure: All amino acids contain an amine group (–NH2), a carboxylic acid group (–COOH), a hydrogen atom, and a unique R group (side chain) attached to a central carbon atom (the α-carbon).

• R Group: The R group is different for each amino acid and determines its chemical properties (e.g., polarity, charge, hydrophobicity).

• Classification: Amino acids are classified based on the properties of their R groups, such as nonpolar, polar, acidic, or basic.

Types of Proteins (by Function)

• Structural Proteins: Provide support (e.g., collagen in connective tissue).

• Enzymatic Proteins: Catalyze biochemical reactions (e.g., amylase).

• Transport Proteins: Carry substances (e.g., hemoglobin transports oxygen).

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

• Regulatory Proteins: Regulate biological processes (e.g., insulin).

• Contractile Proteins: Involved in movement (e.g., actin and myosin in muscles).

Example: Collagen is a structural protein found in skin, tendons, and bones.

Protein Structure

Levels of Protein Structure

Proteins have four levels of structure, each contributing to their final shape and function.

• Primary Structure: The unique sequence of amino acids in a polypeptide chain, held together by peptide bonds.

• Secondary Structure: Local folding of the polypeptide chain into structures such as α-helix, β-pleated sheet, and triple helix (as in collagen), stabilized by hydrogen bonds.

• Tertiary Structure: The overall three-dimensional shape of a single polypeptide chain, stabilized by interactions such as hydrogen bonds, ionic bonds, disulfide bridges, and hydrophobic interactions.

• Quaternary Structure: The association of two or more polypeptide chains (subunits) to form a functional protein. Not all proteins have quaternary structure.

• Primary Structure and Function: The primary structure determines the protein's biological function.

• Tertiary Structure and Activity: Tertiary structure is always necessary for a protein to be active.

• Quaternary Structure: Only some proteins require quaternary structure to be active (e.g., hemoglobin).

Example: Collagen has a triple helix secondary structure.

Hydrolysis and Denaturation

Hydrolysis of Proteins

Hydrolysis is a chemical reaction in which water is used to break peptide bonds, resulting in the breakdown of proteins into smaller peptides or amino acids.

• Role in Digestion: Hydrolysis is the first reaction in the digestion of proteins and all food.

• Effect on Structure: Hydrolysis breaks the primary structure of a protein.

Denaturation of Proteins

Denaturation is the process by which a protein loses its native structure due to the disruption of non-covalent interactions, resulting in loss of function.

• Effect on Structure: Denaturation disrupts secondary, tertiary, and/or quaternary structure, but not the primary structure.

• Loss of Activity: When a protein is denatured, it unfolds and is no longer biologically active.

• Causes of Denaturation: Factors include heat, extreme pH, organic solvents, heavy metals, and agitation.

Example: Cooking an egg denatures the proteins in the egg white, causing it to solidify.

Enzymes

Definition and Function

Enzymes are biological catalysts, usually proteins, that speed up chemical reactions in living organisms without being consumed in the process.

• Active Site: The region on the enzyme where the substrate binds and the reaction occurs is called the active site.

• Enzyme Specificity: Enzymes are highly specific for their substrates.

• Enzyme Unchanged: An enzyme is not changed or consumed by the reaction it catalyzes.

Models of Substrate Binding

• Lock-and-Key Model: The substrate fits exactly into the active site of the enzyme, like a key into a lock.

• Induced Fit Model: The enzyme changes shape slightly to accommodate the substrate, enhancing the fit.

Factors Affecting Enzyme Activity

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

• pH: Each enzyme has an optimal pH; activity decreases at pH values above or below this optimum.

• Substrate Concentration: Activity increases with substrate concentration up to a maximum (saturation point).

Enzyme Inhibition

Enzyme inhibitors are substances that decrease or prevent enzyme activity. There are three main types:

Type of Inhibitor	Mechanism	Overcome by Increasing Substrate?
Competitive	Binds to the active site, blocking substrate	Yes
Noncompetitive	Binds to a site other than the active site, changing enzyme shape	No
Irreversible	Forms a covalent bond with the enzyme, permanently inactivating it	No

Example: Many drugs act as enzyme inhibitors, such as penicillin inhibiting bacterial cell wall synthesis enzymes.