Summarizing Proteins

Summary of lessons so far

Proteins are a class of macromolecules that perform a diverse range of functions for the cell. They help in metabolism by providing structural support and by acting as enzymes, carriers, or hormones.

The building blocks of proteins (monomers) are amino acids. Each amino acid has a central carbon that linked to an amino group, a carboxyl group, a hydrogen atom, and an R group or side chain. There are 20 commonly occurring amino acids, each of which differs in the R group. Each amino acid links to its neighbors by a peptide bond. We refer to a long chain of amino acids as a polypeptide.

The organization of proteins occurs at four levels: primary, secondary, tertiary, and (optional) quaternary. The primary structure is the unique sequence of amino acids. The local folding of the polypeptide to form structures such as the α-helix and β-pleated sheet constitutes the secondary structure. The overall three-dimensional structure is the tertiary structure. When two or more polypeptides combine to form the complete protein structure, the configuration is known as the quaternary structure of a protein (see image below).

protein-structure

Note that the primary structure here illustrates the amino acids using abbreviations of their names. For example, Lys stands for Lysine, Gly stands for Glycine, and so on. Image Attribution: “Holger87” / Wikimedia Commons (CC BY-SA 3.0)

Protein shape and function are intricately linked. In fact, any change in shape caused by changes in temperature or pH may lead to protein denaturation and a loss in function.

Glossary of Words

Alpha-helix structure (α-helix)

type of secondary structure of proteins formed by folding of the polypeptide into a helix shape with hydrogen bonds stabilizing the structure

Amino acid

monomer of a protein; has a central carbon or alpha carbon to which an amino group, a carboxyl group, a hydrogen, and an R group or side chain is attached; the R group is different for all 20 amino acids

Beta-pleated sheet (β-pleated)

secondary structure found in proteins in which “pleats” are formed by hydrogen bonding between atoms on the backbone of the polypeptide chain

Chaperone

(also, chaperonin) protein that helps nascent protein in the folding process

Denaturation

loss of shape in a protein as a result of changes in temperature, pH, or exposure to chemicals

Enzyme

catalyst in a biochemical reaction; usually a complex or conjugated protein

Hormone

chemical signaling molecule, usually protein or steroid, secreted by endocrine cells that act to control or regulate specific physiological processes

Peptide bond

bond formed between two amino acids by a dehydration reaction

Polypeptide

long chain of amino acids linked by peptide bonds

Primary structure

linear sequence of amino acids in a protein

Protein

biological macromolecule composed of one or more chains of amino acids

Quaternary structure

association of discrete polypeptide subunits in a protein

Secondary structure

regular structure formed by proteins by intramolecular hydrogen bonding between the oxygen atom of one amino acid residue and the hydrogen attached to the nitrogen atom of another amino acid residue

Tertiary structure

three-dimensional conformation of a protein, including interactions between secondary structural elements; formed from interactions between amino acid side chains

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