2 THE BIOSYNTHESIS OF PROTEINS 



in the model of myoglobin (Kendrew, 1959) (Fig. 4). This picture also 

 shows the position of the haematin prosthetic group within the molecule. 



The haemoglobin molecule is made of four chains, of two different 

 compositions, each of w^hich very much resemble myoglobin by the general 

 shape (Perutz et al, 1960). They are not very strongly bound with one 

 another and they can be made to separate or reassociate rather easily. 



Sometimes, protein molecules associate into structures of a higher order 

 either with identical molecules or w^th others. For instance, many mole- 



12 3 4 5 6 7 8 9 10 11 12 13 14 



Acetyl-Ser-Tyr-Ser-Ileu-Thr-Pro-Thr-Ser-GluNH2-Phe-Val-Phe-Leu-Ser- 



15 16 17 18 19 20 21 22 23 24 25 26, 27 28 



Ser-Ala-Try-Ala-Asp-Pro-Ileu-Glu-Leu-Ileu-Asp*(CyS03H,Thr,Asp*)- 



29 30 31 32 33 34 35 36 37 38 39 40 



Leu-Ala-Leu-Gly-AspNH,-GluNH2-Phe-Glu*-Thr-Glu-GluNH2-Ala- 



41 42 43 44 45 46 47 48 49 50 51 52 53 

 Arg-Thr-Val-Glu*-Val-Arg-GluNH2-Phe-Ser-GluNH2-Val-Try-Lys- 



54 55 56 57 58 59 60 61 62 63 64 65 66 67 

 Pro-Pro-Ser-GluNH2-VaI-Thr-Val-Arg-Phe-Pro-Asp*-Ser-Asp-Phe- 



68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 

 Lys-Val-Tyr-Arg-Tyr-Asp*-Ala-Val-Asp-Pro-Leu-Val-Thr-AIa-Leu- 



83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 

 Leu-Leu-Gly-AIa-Phe-Asp*-Thr-Arg-AspNH2-Arg-Ileu-Glu*-Val-Glu*-Asp*- 



98, 99, 100 101, 102 103 104 105 106 107 108 109 110 111 

 (Ala,GIu*,Thr,Asp*,Pro)-Thr-Ala-Glu*-Thr-Leu-Asp*-Ala-Thr-Arg- 

 112 113 114 115 116 117 118 119 120 121 122 123 124 125 

 Arg-Val-Asp*-Asp*-AIa-Thr-Val-AIa-Ileu-Arg-Ser-Ala-Asp*-Ileu- 



126 127 128 129 130 131 132 133 134 135 136 137 138 139 

 AspNH2-Leu-Ileu-VaI-Glu-Leu-Ileu-Arg-Gly-Thr-Gly-Ser-Tyr-AspNH2- 

 140 141 142 143 144 145 146 147 148 149 150 151 152 153 

 Arg-Ser-Ser-Phe-Glu*-Ser-Ser-Ser-Gly-Leu-Val-Try-Thr-Ser- 

 154 155 156 157 

 Gly-Pro-Ala-Thr 



Asp* = Asp or AspNH2 ; Glu* = Glu or GluNH2 



Fig. 2. Amino acid sequence of the protein of tobacco mosaic virus 

 (Anderer et al., 1960). 



cules of the tobacco mosaic virus protein unite in a tightly packed hollow 

 rod. 



Several levels of complexity can thus be considered in proteins. The 

 polypeptidic backbone is made of amino acids which are firmly bound 

 together and arranged in a strictly determined sequence (Fig. 2); this is 

 the primary structure. Folding of the chains into a construction — helices 

 or otherwise — held together by hydrogen bonds, creates the secondary 

 structure. The tertiary structure results from the folding of the helices in 

 space as in Fig. 4. Higher orders of organization make the transition 

 between molecular and microscopic structures (Fig. 5). 



