ieamreaeoTetayyamahn au-Cwm 2 vee moglobn in the water m ule (de 8ceehee92Pnsptooe6ago22abind 26-9 Biosynthesis of Proteins:Nucleic Acids 5ourchesoraCieneernhe ween the four chains gives 88 The solid phase synthesis has the great advantage that all the reaction intermediates are immobilized on the polymer and can be isolated by simple filtration and washing. The Merrifield synthesis is performed automatically by a machine that requires only a few hours for each cycle. The total synthesis of insulin (51 amino acids) took more than 5,000 separate steps, but only took several days using the Merrifield automated procedure. Polypeptides in Nature: Oxygen Transport by the Proteins Myoglobin and Hemoglobin 26-8 Two proteins carry oxygen in vertebrates, myoglobin in the muscles and hemoglobin in the blood. Both proteins contain a special nonpolypeptide unit called a heme group attached to the protein. Heme is a cyclic organic molecule (poryphrin) made up of four linked, substituted pyrrole units surrounding an iron atom. The fifth coordination position of the heme groups is occupied by a nitrogen atom from a histidine imidazole group. The sixth position is occupied by an oxygen molecule (oxyhemoglobin) or a water molecule (deoxyhemoglobin). A second histidine group is positioned close to the oxygen binding site of the heme, which helps prevent the binding of carbon monoxide. Hemoglobin consists of four polypeptide chains: two α-chains of 141 residues each and 2 β-chains of 146 residues each. Each chain has its own heme group and a tertiary structure similar to that of myoglobin. The arrangement and interaction between the four chains gives hemoglobin its quaternary structure. 26-9 Biosynthesis of Proteins: Nucleic Acids Four heterocycles define the structure of nucleic acids. DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are polymers of phosphoric acid, a sugar (ribose in RNA and deoxyribose in DNA), and four nitrogen bases (ACTG in DNA and ACUG in RNA)