Vocab The Carbon Compounds of Life

Protein

A macromolecule built as a polymer of amino acid monomers; performs vital functions including structural support, enzymatic activity, movement, transport, recognition/receptor roles, regulation, hormonal signaling, defense, and storage.

Amino acid

The monomer subunit of proteins, containing both an amino group and a carboxyl group; 20 common types exist, grouped into five classes by their side chains (R groups).

Nonpolar amino acid

A class of amino acid with a hydrophobic side chain; includes alanine, valine, leucine, isoleucine, glycine, phenylalanine, tryptophan, methionine, and proline.

Polar uncharged amino acid

A class of amino acid with an uncharged but polar side chain; includes serine, threonine, tyrosine, asparagine, glutamine, and cysteine.

Negatively charged (acidic) amino acid

A class of amino acid with a negatively charged side chain at physiological pH; includes aspartic acid and glutamic acid.

Positively charged (basic) amino acid

A class of amino acid with a positively charged side chain at physiological pH; includes lysine, arginine, and histidine.

Proline

A nonpolar amino acid with a unique ring structure including the central carbon, bonded to a —COOH group on one side and an imino (=NH) group on the other.

Cysteine

A polar uncharged amino acid containing a sulfhydryl group (—SH) that can form disulfide linkages, helping stabilize protein 3-D shape.

Peptide bond

A covalent bond linking amino acids into polypeptide chains, formed by dehydration synthesis between the —NH2 group of one amino acid and the —COOH group of another.

Polypeptide

A chain of amino acids linked by peptide bonds; the subunit of a protein.

N-terminal end

The end of a polypeptide chain with a free amino group.

C-terminal end

The end of a polypeptide chain with a free carboxyl group; new amino acids are added only here during synthesis.

Primary structure

The precise, linear sequence of amino acids joined by peptide bonds in a polypeptide chain; determines all higher levels of protein structure.

Secondary structure

Coiled or folded regions in a polypeptide chain (such as alpha helices and beta strands) resulting from hydrogen bonding within particular amino acid sequences.

Alpha (α) helix

A secondary structure in which the amino acid chain is twisted into a regular right-handed spiral, stabilized by regularly spaced hydrogen bonds, forming rigid rod-like structures.

Beta (β) strand/sheet

A secondary structure in which the amino acid chain zigzags in a flat plane; strands align side by side (same or opposite directions) and are stabilized by hydrogen bonds.

Random coil

An irregularly folded arrangement in a protein that provides flexible "hinge" sites, allowing α-helical or β-strand segments to bend or move relative to one another.

Tertiary structure

The overall three-dimensional folding (conformation) of a polypeptide chain, determined by secondary structures, disulfide linkages, hydrogen bonds, and charge/polarity interactions; determines a protein's function and solubility.

Disulfide linkage

A covalent bond (—S—S—) formed when the —SH groups of two cysteine amino acids are oxidized and joined, helping stabilize a protein's tertiary structure.

Quaternary structure

The arrangement of multiple polypeptide chains in a protein that contains more than one chain (e.g., hemoglobin, antibodies); held together by hydrogen bonds, polar/nonpolar attractions, and disulfide linkages.

Denaturation

The unfolding of a protein from its active conformation, causing loss of structure and function, due to chemicals, pH changes, or high temperature.

Renaturation

The reversible refolding of a denatured protein back into its functional conformation.

Anfinsen's experiment

A study showing that chemically breaking the disulfide linkages of the protein ribonuclease caused it to unfold and lose enzyme activity, demonstrating the role of disulfide bonds in tertiary structure.

Chaperone protein (chaperonin)

A "guide" protein that binds temporarily with newly synthesized proteins, directing them toward correct tertiary structure and inhibiting incorrect folding.

Functional domain

A large subdivision of a protein produced by folding of the amino acid chain(s); proteins with multiple functions often have those functions located in different domains.

Motif

A highly specialized region within or between protein domains, produced by the 3-D arrangement of amino acid chains.

Lipoprotein

A molecule formed when proteins link with lipids; forms part of cell membranes.

Glycoprotein

A molecule formed when proteins link with carbohydrates; functions as enzymes, antibodies, recognition/receptor molecules, and extracellular support structures.

Nucleoprotein

A molecule formed when proteins link with nucleic acids; forms structures such as chromosomes.

Structural protein

A protein that provides support (e.g., microtubules, microfilaments, collagen, cell wall proteins).

Enzymatic protein

A protein that increases the rate of biological reactions (e.g., DNA polymerase, rubisco, digestive enzymes).

Membrane transport protein

A protein that speeds the movement of substances across biological membranes (e.g., ion transporters, glucose transporters, aquaporins).

Motile protein

A protein that produces cellular movement (e.g., myosin, kinesin).

Regulatory protein

A protein that promotes or inhibits the activity of other cellular molecules (e.g., nuclear regulatory proteins, protein kinases).

Receptor protein

A protein that binds molecules at the cell surface or within the cell, sometimes triggering internal responses (e.g., hormone receptors, cellular adhesion molecules).

Hormonal protein

A protein that carries regulatory signals between cells (e.g., insulin, growth hormone).

Defensive protein

A protein that defends against invading molecules and organisms (e.g., antibodies).

Storage protein

A protein that holds amino acids or other substances in stored form (e.g., ovalbumin, apolipoproteins).