Macromolecules II: Proteins and Nucleic Acids

Proteins are polymers of amino acid monomers, of which living things use twenty different kinds, each differing only in one side chain (the "R-group") that gives it distinct chemical properties. Amino acids link by peptide bonds into a polypeptide chain. A protein's shape is described in four levels: primary structure is simply the exact sequence of amino acids, read like a word spelled out in a twenty-letter alphabet; secondary structure describes local coiling or folding patterns, especially the alpha helix and the beta pleated sheet, held together by hydrogen bonds between nearby parts of the backbone; tertiary structure is the overall three-dimensional shape the whole chain folds into, driven by interactions between the R-groups; and quaternary structure applies only to proteins built from more than one polypeptide chain joined together, such as hemoglobin.

Shape determines function almost completely -- an enzyme, an antibody, and a structural protein like keratin all do their specific jobs because of the specific shape they fold into. Heat, extreme pH, or certain chemicals can denature a protein: break the bonds holding its secondary, tertiary, or quaternary structure and destroy its shape (and therefore its function) without necessarily breaking the primary sequence of amino acids itself -- cooking an egg white permanently denatures its proteins, which is why a cooked egg white never turns transparent and runny again. Nucleic acids are the other major class of macromolecule, built from monomers called nucleotides, each with a sugar, a phosphate group, and one of several nitrogen-containing bases. DNA and RNA, the two nucleic acids, store and transmit the genetic instructions that, among many other things, specify the exact amino acid sequence of every protein a cell can build -- the subject of Unit 7.