Protein Structure and Function Quiz

Chapter 21: Amino Acids, Proteins, and Enzymes

• Proteins have various functions, including structure, catalysis, movement, transport, hormones, protection, storage, and regulation.
• Structure: Collagen and keratin are key components of skin, bone, hair, and nails. Keratin also forms the cortex of hair.
• Catalysts: Proteins called enzymes catalyze virtually all reactions in living systems.
• Movement: Muscles are made of myosin and actin proteins.
• Transport: Hemoglobin transports oxygen from lungs to cells, and other proteins transport molecules across cell membranes.
• Hormones: Many hormones are proteins, like insulin, oxytocin, and human growth hormone.
• Protection: Blood clotting involves the protein fibrinogen, and antibodies fight disease.
• Storage: Casein stores nutrients in milk, ovalbumin in eggs, and ferritin in the liver stores iron.
• Regulation: Certain proteins control gene expression, and when gene expression takes place.

Amino Acids

• An amino acid is a compound with both an amino group and a carboxyl group.
• The amino group is attached to the α-carbon adjacent to the carboxyl group.
• The R group, or side chain, determines the identity of the amino acid.
• A-amino acids are typically written in the zwitterion (internal salt) form at physiological pH.
• Glycine is the simplest amino acid (R = H).
• There are 20 common naturally occurring amino acids.
• Essential amino acids are marked with an asterisk (*).

Chirality of α-Amino Acids

• Except for glycine, all protein-derived amino acids are chiral, having at least one stereocenter (the α-carbon).
• Most α-amino acids have an L-configuration at the α-carbon.

Peptides

• Peptides are short polymers of amino acids joined by peptide bonds.
• A peptide bond is the amide bond between the α-carboxyl group of one amino acid and the α-amino group of another.
• Polypeptides are long chains of amino acids joined by peptide bonds.
• Proteins are biological macromolecules consisting of 30-50 or more amino acids joined by peptide bonds.
• Individual amino acid units in a protein are called residues.
• The amino acid with a free -NH3+ group is called the N-terminal amino acid and written on the left.
• The amino acid with a free -COO- group is called the C-terminal amino acid and written on the right.
• The amino acids Ala and Ser can combine by reacting functional groups.

Enzyme Catalysis

• Enzymes are biological catalysts that increase the rate of reactions.
• Enzymes have high specificity and efficiency in catalyzing reactions.

Enzyme Terminology

• Substrate: The compound or compounds whose reaction an enzyme catalyzes.
• Active site: The specific portion of the enzyme to which a substrate binds during a reaction.

Enzyme Specificity

• Lock-and-key model: The active site is a rigid cavity; a substrate must exactly match the shape of the active site to react.
• Induced-fit model: The active site has a flexible shape that adjusts to fit a variety of substrate shapes. The open cavity of the active site adjusts around the substrate for a tighter fit.

Enzyme Activity

• Enzyme activity is a measure of how much a reaction rate is increased over a non-catalyzed reaction.
• Enzyme activity is affected by temperature and pH.

Allosteric Control, Activators, and Inhibitors of Enzyme Activity

• Allosteric control is the binding of a regulator to a site on an enzyme other than the active site, affecting the enzyme's ability to bind a substrate at its active site.
• Activators are substances that initiate or increase enzyme activity.
• Inhibitors are substances that make an enzyme less active or inactive.
• Reversible inhibitors: bind to an enzyme but the enzyme activity is restored when the inhibitor is released.
• Irreversible inhibitors: covalently bind to an enzyme, permanently destroying its activity.

Enzymes in Different Classes

• Enzymes are classified into six major classes according to the types of reactions they catalyze.
• Common examples of enzymes in different classes are mentioned.

Protein Hydrolysis

• Hydrolysis is the breaking of peptide bonds via treatment with aqueous acid, base, or certain enzymes.
• Examples discussed include enzymes involved in protein digestion.

Denaturation of Proteins

• Denaturation is the process of destroying the native conformation of a protein.
• Denaturing agents include heat, urea, surface-active agents, and alcohols.
• Denaturation does not typically break amide bonds responsible for primary structure (peptide bonds).

Cofactors

• Cofactors are metal ions or organic molecules needed for enzyme-catalyzed reactions.
• A common cofactor is, for example, NAD+.

Levels of Protein Structure

• Primary structure: The sequence of amino acids in a polypeptide chain.
• Secondary structure: The 3D arrangement of localized regions within a protein. Common types include α-helices and β-pleated sheets.
• Tertiary structure: The overall 3D conformation of a protein.
• Quaternary structure: Some proteins have more than one polypeptide chain; this structure describes the arrangement of multiple folded polypeptide chains.

Examples of Proteins

• Hemoglobin and myoglobin are common globular proteins.
• Collagen and a-keratins are fibrous proteins.