Introduction to Proteins

• Proteins are intricate nitrogenous molecules composed of variable numbers of amino acid residues.
• Amino acid are bonded by covalent peptide bonds.
• There are 20 known amino acids found in all proteins, which are known as standard amino acids.

Protein Functions

• Catalysis is preformed by enzymes
• Keratin provides structural support
• Hemoglobin transports molecules
• Na+/K+ channels facilitate transmembrane transport
• Snake venom contains toxins
• Actin and myosin enable contractile function
• Insulin is a hormone
• Seeds and eggs contain storage proteins
• Antibodies are defensive proteins

Protein Structure

• Proteins are crucial biological macromolecules constructed from amino acid polymers.
• Biochemists recognize multiple levels of structural organization in proteins.
• The levels of protein structure are primary, secondary, tertiary, and quaternary.
• Loss of structure results in loss of function.
• Primary structure is the sequence of a chain of amino acids

Secondary Structure

• Secondary structure involves local folding of the polypeptide chain into alpha-helices or beta-sheets.
• The polypeptide chain can adopt different shapes or patterns, which are referred to as the secondary protein structure.
• Protein folding results in two types of secondary structures, alpha-helix and beta-pleated sheets
• Alpha helix is a helix.
• Hydrogen bonds between amino acids stabilize secondary structure
• The coiled shape of an alpha helix is maintained by hydrogen bonds between amide and carbonyl groups of amino acids along the chain.
• Beta-sheets are formed from multiple side-by-side beta-strands.
• Beta sheets exist in parallel and anti-parallel configurations, with anti-parallel sheets being more stable.

Tertiary Structure

• Tertiary structure is the three-dimensional shape of a protein.
• It features a single polypeptide chain "backbone" with one or more protein secondary structures, the protein domains.
• Amino acid side chains can interact and bond

Quaternary Structure

• Quaternary structure arises when multiple amino acid chains combine to form a protein complex.
• Proteins with quaternary structure include hemoglobin, DNA polymerase, ion channels, and antibodies.
• Quaternary structure contains two or more tertiary subunits in its structure.
• Hemoglobin, for example, contains two alpha and two beta chains.
• The heme group in each subunit binds oxygen to transport in the blood to the tissues.

Protein Classification

• One polypeptide chain is a monomeric protein
• More than one polypeptide chain is a multimeric protein
• A homomultimer has all one kind of chain
• A heteromultimer has two or more different chains
• Hemoglobin, with its two alpha and two beta chains, exemplifies a heterotetramer.
• Simple proteins consist of only amino acid residues.
• Conjugated proteins contain prosthetic groups, such as metal ions, co-factors, lipids, and carbohydrates, with hemoglobin (Heme) as an example.

Diseases

• Alterations in protein structure can lead to diseases.
• A single amino acid change in hemoglobin is related to sickle cell anemia.
• A single amino acid change in collagen protein causes joint damage to cause osteoarthritis

Protein Classification by Shape

• Fibrous proteins feature polypeptides arranged in long strands, are water-insoluble due to hydrophobic amino acids, are strong and flexible, and provide structural support like keratin and collagen
• Globular proteins have polypeptide chains folded into spherical shapes, are water-soluble, contain various secondary structures, and perform diverse functions as enzymes and regulatory proteins

Classification by Function

• Catalytic proteins include alkaline phosphatase and alkaline transaminase
• Regulatory or hormonal proteins include insulin and growth hormone
• Transport proteins include hemoglobin and transferrin
• Immune proteins include immunoglobulins
• Contractile proteins include actin and myosin