Hydrogen bonding between every fourth amino acid holds spiral shape of an a helix. The sheet was the second pattern discovered. Tertiary structure results when proteins of secondary structure are folded, due to various interactions between the R groups of their constituent amino acids. Quaternary structure results when two or more polypeptides combine. Hemoglobin is globular protein with a quaternary structure of four polypeptides.
Most enzymes have a quaternary structure.
Nucleotides have metabolic functions in cells. Coenzymes are molecules which facilitate enzymatic reactions. Nucleotides also serve as nucleic acid monomers. Nucleic acids are huge polymers of nucleotides with very specific functions in cells. Complementary base pairing occurs where two strands of DNA are held together by hydrogen bonds between purine and pyrimidine bases.