Summary
Highlights
Proteins are the building blocks of cells, performing most cellular functions. They come in various types, including enzymes (catalyze reactions), structural proteins (mechanical support), transport proteins (carry molecules), motor proteins (generate movement), storage proteins, transcription regulators, and receptor proteins (detect signals). Understanding proteins is crucial to comprehending body functions, genetics, and muscle contraction.
Proteins are polymers of amino acids linked by covalent peptide bonds. Each of the 20 amino acids contains an alpha carbon, an amino group, a carboxyl group, and a unique side chain (R group) that determines its properties. Peptide bonds form via a condensation reaction between the carboxyl group of one amino acid and the amino group of another, forming a polypeptide backbone with projecting side chains that dictate the protein's unique properties.
Proteins have four levels of structural organization: primary, secondary, tertiary, and quaternary. The primary structure is the unique sequence of amino acids, determined by inherited genetic information. This sequence dictates the subsequent folding into higher-order structures.
The secondary structure involves coiled (alpha helices) or folded (beta pleated sheets) patterns in the polypeptide backbone. These patterns arise from hydrogen bonding between atoms in the backbone, excluding side chains. Alpha helices are coils held by hydrogen bonds every fourth peptide bond, while beta sheets form from hydrogen bonds between adjacent polypeptide segments, creating a rigid structure.
The tertiary structure is the overall three-dimensional shape of a polypeptide, resulting from interactions between the side chains. These interactions include weak non-covalent bonds (hydrogen bonds, electrostatic attractions, van der Waals forces, hydrophobic clustering) and covalent disulfide bridges. Hydrophobic side chains cluster in the protein's interior, while polar side chains arrange on the exterior to interact with water. The final folded conformation minimizes free energy and is energetically favorable.
The 3D structure of a protein is encoded in its amino acid sequence, as demonstrated by renaturation experiments. Protein folding in cells is often assisted by chaperone proteins, which guide polypeptides to fold efficiently and prevent aggregation. Misfolded proteins, called prions, can form amyloid structures that damage cells and contribute to neurodegenerative disorders like Alzheimer's and Huntington's disease, and can be infectious.
Proteins perform their functions by binding specifically to other molecules called ligands. The binding site, formed by specific amino acid side chains, allows for selective and tight binding through weak non-covalent interactions. The complementary fit between a protein and its ligand ensures strong binding and prevents incorrect associations. Examples include enzymes binding to substrates and antibodies binding to antigens.
Antibodies are Y-shaped immunoglobulin proteins produced by the immune system to recognize and bind tightly to specific foreign molecules (antigens). Each antibody molecule has identical antigen-binding sites, formed by variable loops of polypeptide chains, enabling a wide variety of specific binding capabilities.
Enzymes are a crucial class of proteins that act as catalysts, speeding up chemical reactions without being consumed. They bind to specific ligands called substrates, transforming them into products, and can repeat this process many times.
Cells tightly control and regulate protein activity to maintain an optimal state, ensuring appropriate substrate and product levels without wasting energy. Regulation can occur at several levels: gene expression, protein degradation rate, subcellular localization, or direct modulation of protein activity.
Many enzymes are allosteric proteins, meaning they can exist in two conformations (active and inactive) with different catalytic activities. Ligands binding to a specific regulatory site can stabilize either the active or inactive form, effectively turning the enzyme on or off. This allows for both negative regulation (feedback inhibition by a product later in a pathway) and positive regulation (stimulation by a regulatory molecule).
Phosphorylation is a common mechanism for controlling protein activity, involving the covalent attachment of a phosphate group to specific amino acid side chains. This can either stimulate or inhibit protein activity by causing a conformational change. Protein kinases add phosphate groups (from ATP), while protein phosphatases remove them, providing a reversible on/off switch for many proteins in eukaryotic cells.