Proteins and Enzymes: Structure, Function, and Regulation

Introduction to Proteins

Proteins are essential macromolecules in all living organisms, composed of amino acids linked by peptide bonds. They perform a wide variety of functions, including catalysis, structural support, transport, and regulation.

• Amino acids are the building blocks of proteins. There are 20 standard amino acids found in physiological systems.

• Peptide bonds (amide bonds) link amino acids together, forming polypeptides and proteins.

• Proteins can be classified by size: polypeptides (26-50 amino acids), proteins (up to 250 amino acids), and macromolecules (over 2000 amino acids).

• Chirality: Most amino acids are levo-rotatory (rotate plane-polarized light to the left), except glycine, which is achiral.

Example: The formation of a dipeptide from two α-amino acids involves the removal of a water molecule and the creation of a peptide bond.

Types of Amino Acids

Amino acids can be grouped based on their side chains and properties:

• Polar/charged amino acids: Aspartic acid (Asp), Glutamic acid (Glu), Lysine (Lys), Arginine (Arg), Histidine (His)

• Polar/uncharged amino acids: Serine (Ser), Threonine (Thr), Glutamine (Gln), Asparagine (Asn), Tyrosine (Tyr)

• Nonpolar/hydrophobic amino acids: Alanine (Ala), Valine (Val), Leucine (Leu), Isoleucine (Ile), Methionine (Met), Phenylalanine (Phe), Tryptophan (Trp)

• Special amino acids: Glycine (Gly), Proline (Pro), Cysteine (Cys)

Additional info: Cysteine can form disulfide bonds, stabilizing protein structure; proline introduces kinks in polypeptide chains.

Primary Functions of Proteins

Proteins serve diverse roles in biological systems:

• Antibodies: Bind to foreign particles (e.g., viruses, bacteria) for immune defense.

• Enzymes: Catalyze biochemical reactions (e.g., pepsin, trypsin).

• Messenger proteins: Transmit signals (e.g., hormones like growth hormone).

• Structural proteins: Provide support (e.g., actin, collagen).

• Transport/storage proteins: Carry molecules (e.g., ferritin).

• Maintenance: Tissue development and repair (e.g., growth factors).

• Energy source: Used for energy if not needed for other functions.

Regulation of Protein Function

Protein activity is regulated by several factors:

• 3D geometry and structural shape

• Post-translational modifications (PTM)

• Co-factors

• Binding partners

• pH of the medium

• Solvation (solvent interaction)

• Regulators

• Other factors

Classification of Proteins by Solubility and Location

• Soluble proteins: Found in cytoplasm or secreted.

• Insoluble proteins: Structural components.

• Membrane-bound proteins: Embedded in cell membranes.

Membrane Proteins: Structure and Function

Types of Membrane Proteins

Membrane proteins are crucial for cellular communication, transport, and structure. They are classified into three main types:

Additional info: Integral proteins often function as channels or transporters; peripheral proteins may act as enzymes or signaling molecules.

Studying Membrane Proteins

• Freeze-fracture analysis: Reveals distribution of integral proteins in membranes.

• Detergent solubilization: Used to isolate and study membrane proteins.

• Hydropathy plots: Identify transmembrane domains by analyzing stretches of hydrophobic amino acids.

• Site-directed mutagenesis: Replaces specific amino acids to study protein structure and function.

• Electron paramagnetic resonance (EPR) spectroscopy: Monitors conformational changes in proteins.

Enzymes: Function and Mechanism

Enzyme Basics

Enzymes are biological catalysts that accelerate chemical reactions in living organisms. They are highly specific for their substrates and operate under optimal temperature and pH conditions.

• Active site: Region where substrate binds and reaction occurs.

• Substrate: Molecule upon which the enzyme acts.

• Enzyme-substrate complex: Temporary association during catalysis.

• Enzyme reaction:

• Enzymes are not consumed in the reaction and can be reused.

Example: Digestive enzymes break down food molecules; DNA ligases join DNA fragments during replication and repair.

Types of Enzymes

• Proteases (Proteinases): Cleave peptide bonds in proteins.

• Ligases: Join two molecules together (e.g., DNA ligase).

• Other enzymes: Oxidoreductases, transferases, hydrolases, etc. (not detailed in the notes)

Proteases: Classification and Mechanism

Proteases are enzymes that hydrolyze peptide bonds in proteins. They are classified as:

• Endoproteases: Cleave internal peptide bonds within a protein chain.

• Exoproteases: Remove amino acids from the ends of protein chains.

Proprotein/Prohormone Theory: Endoproteases generate bioactive peptides from inactive precursors.

Protease Families

Proteases are grouped based on their catalytic residues and mechanism:

Additional info: Serine proteases are the largest class and are involved in digestion, blood coagulation, and immune response.

Regulation of Protease Activity

Protease function is tightly regulated by activators and inhibitors:

• Activators: Accelerate proteolysis at specific sites.

• Inhibitors: Prevent unwanted protease activity; can be natural (e.g., antitrypsin) or synthetic (drugs).

• Each protease may have a specific inhibitor/regulator.

Bioengineered Proteases

Artificial enzymes designed by mutation and produced recombinantly are used in drug design and biochemical research.

Summary Table: Key Protein and Enzyme Concepts

Key Equations

• General enzyme reaction:

• Peptide bond formation: