Introduction to Human Biology - Week 3, Module 3B: Proteins
Learning Objectives for this Week
Overview: This week focuses on the structure and functions of proteins, following a foundational understanding of gene expression and protein synthesis.
Key Concepts:
Amino acids: The building blocks of proteins
Characteristics of proteins
Structure of proteins
Classification of proteins
Functions of proteins
Enzymes and enzyme regulation
Amino Acids - The Building Blocks of Proteins
Definition: Amino acids are organic compounds characterized by both an amino group (-NH2) and a carboxyl group (-COOH).
Amino Acid Structure Features:
An R group, known as the amino acid side chain, varies among different amino acids.
Differences in side chains include variations in size, shape, charge, acidity, functional groups, and chemical reactivity.
Categories of Amino Acids:
Non-polar amino acids:
Composition: One carboxyl group and one non-polar side chain.
Nature: Hydrophobic.
Polar neutral amino acids:
Composition: One carboxyl group and one neutral polar side chain.
Nature: Hydrophilic.
Polar acidic amino acids:
Composition: One amino group and two carboxyl groups; the second carboxyl group belongs to the side chain.
Polar basic amino acids:
Composition: Two amino groups and one carboxyl group; the second amino group belongs to the side chain.
Characteristics of Proteins
Definition: Proteins are polymers made up of amino acids, typically containing at least 40 amino acid monomers connected via peptide bonds.
Amino Acids Count: There are 20 standard amino acids used to form proteins; 9 of these are categorized as essential amino acids.
Essential Amino Acids:
Definition: Amino acids that must be obtained through dietary sources because the body cannot synthesize them in sufficient quantities.
Complete dietary proteins: Proteins that contain all essential amino acids in the needed proportions, typically derived from animal sources.
Incomplete dietary proteins: Proteins lacking adequate amounts of one or more essential amino acids, generally found in plant-based foods.
Structure of Proteins
Four Levels of Structural Organization:
Primary structure: Sequence of amino acids.
Secondary structure: Local folding patterns (alpha helices and beta sheets).
Tertiary structure: Overall three-dimensional shape of a protein.
Quaternary structure: Complex formed by multiple polypeptide chains.
Classification of Proteins
Basis of Classification: Proteins can be categorized based on their chemical properties, shape, and functions.
Features: Generally simple and straight, hard to denature.
Macromolecular structure formation, e.g., hair and nails, often reinforced by cross-linkages.
Properties: Strong, stable, less flexible, often age-related fragility.
Functionality: Waterproofing and serving as physical barriers.
Globular Proteins:
Solubility: Varied solubility in water; typically soluble.
Structure: Spherical shape, generally involved in metabolic processes.
Features: Weaker secondary and tertiary structures, more prone to denaturation.
Functionality: Suitable for transport within blood or body fluid, involved in catalysis, transport, and regulation.
Examples of Common Proteins in the Body
Myoglobin: A globular protein responsible for oxygen transport in muscles.
Keratin: A fibrous protein present in hair and nails.
Collagen: A fibrous protein abundant in tendons, bones, and connective tissues.
Insulin: Globular protein that regulates blood glucose levels.
Fibrin: A fibrous protein that helps in blood clotting.
Transport proteins: E.g., Transferrin, globular proteins involved in transport.
Actin: Exists in two forms (G-Actin and F-Actin); critical for muscle contraction.
Immunoglobulins: Globular proteins aiding in immune responses.
Albumin: A globular protein that helps in regulating blood pH.
Enzymes
Definition: Enzymes act as biological catalysts that accelerate chemical reactions in the body, predominantly consisting of globular proteins.
Characteristics:
Highly specific for substrates.
Efficient in catalysis.
Subject to various forms of cellular control.
Naming: Enzymes are named based on the type of reaction they catalyze or the substrates they interact with, often ending in the suffixes -ase or -in.
Enzyme Structure and Function
Types of Enzymes:
Simple enzymes: Comprised solely of proteins.
Conjugated enzymes: Composed of two parts: an apoenzyme (the protein part) and a cofactor (the non-protein part).
Holoenzyme: The active form that results from the union of an apoenzyme and its cofactor.
Cofactors Types:
Metal ions: E.g., Zinc (Zn2+), Magnesium (Mg2+), Iron (Fe2+, Fe3+), Copper (Cu+, Cu2+); can be tightly or loosely associated with the enzyme.
Coenzymes: Organic molecules derived from nutrients (often from vitamin B), can attach temporarily or permanently to enzymes, and may be reused.
Enzyme Action
Process:
When an enzyme binds to a substrate, they form an enzyme-substrate complex.
The substrate is transformed into products, forming an enzyme-product complex before the products are released.
Lock and Key vs. Induced Fit Models
Induced fit: Some enzymes possess an active site able to make minor conformational adjustments to accommodate similar substrates while retaining specificity, albeit at a slightly reduced efficiency.
Lock and Key: The active site is rigidly shaped; only a substrate with a complimentary fit can bind, allowing a specific reaction before releasing products for subsequent substrate binding.
Enzyme Specificity
Levels of Specificity:
Absolute specificity: Enzymes act on a singular substrate, e.g., Catalase converts H2O2 to O2 and H2O.
Group specificity: Enzymes act on a group of substrates, e.g., Carboxypeptidase degrades peptide chains involving amino acids.
Linkage specificity: Enzymes break specific types of chemical bonds, e.g., Phosphatases target phosphate-ester bonds.
Active Site Role: Determinants of specificity.
Factors Affecting Enzyme Activity
Temperature:
Reaction rate increases with temperature until reaching denaturation, after which activity declines sharply.
pH:
Maximum enzymatic reaction is feasible only within a narrow pH range; deviations lead to denaturation and reduced activity.
Concentration of Substance:
Reaction rates climb with rising substrate concentration until saturation is reached.
Concentration of Enzyme:
Reaction rate increases with higher enzyme concentrations, provided that substrate levels are sufficient.
Enzyme Inhibition
Definition: Inhibitors are substances inhibiting normal enzyme catalysis by binding to it.
Types of Inhibition:
Reversible competitive inhibition: Competes with the substrate for binding to the active site.
Reversible non-competitive inhibition: Binds to an allosteric site, altering enzyme function.
Irreversible inhibition: Permanently inactivates the enzyme.
Allosteric Enzyme Regulation
Concept: Allosteric enzymes regulate cellular functions with multiple subunits and distinct binding sites; binding substances at regulatory sites can modulate structural conformations affecting active site fitting.
Proteolytic Enzymes and Zymogens
Regulation Mechanism: Inactive enzymes are produced but are activated only when necessary by cleavage of peptide fragments, changing their structure and active site shape.
Examples: Proteolytic enzymes and zymogens serve essential physiological roles.