PROBLEMS 159 



There may be other modifications out farther in the protein, but this is 

 not yet known. Likewise there may be many more modifications of hemo- 

 globin than those listed. The work is really quite new. Unfortunately, prac- 

 tically nothing is known of the shapes of these molecules — and won't be 

 until more is known of their structure. Sufficient familiarity with the physio- 

 logical reactions has been estimated to be about ten years away. 



The sickling of erythrocytes occurs when the hemoglobin-.^ is in an at- 

 mosphere low in oxygen, and is a remarkable example of what "bad" mole- 

 cules can do. It is now fairly well established that these bad molecules are 

 so shaped that they can fit into each other and be piled up like a stack of 

 saucers. In so piling, their strength is sufficient to push out the sides of the 

 erythrocyte and cause it to buckle in the middle, i.e., to become sickle- 

 shaped. On oxygenation, the stack collapses, presumably because the mo- 

 lecular shapes are no longer so nicely complementary. Apparently the 

 process resembles the growth of a crystal. The reader is asked to meditate 

 on the known structure of myoglobin (Figure 6-3), and to study the pictures 

 of Perutz et al. 24 on hemoglobin, before pressing further into this subject via 

 Reference 25. 



Others. There are well over 20 diseases for which a "bad" molecule has 

 been postulated as the cause. One other which is receiving considerable at- 

 tention now is phenylketonuria. This is associated with mental deficiencies, 

 and has been traced to the fact that one of the enzymes which catalyze the 

 oxidation of phenylalanine through various steps toward pyruvic acid is not 

 doing its job fast enough. Whether the offending enzyme molecule is not 

 being synthesized, or has some physical deformity which renders it only 

 partially active; or whether it or the substrate is not being transported fast 

 enough to the place of catalysis, is not yet known. However, the result is ac- 

 cumulation of phenylalanine in the blood stream, and interference with syn- 

 thesis of nerve tissue. 



PROBLEMS 



6-1: Erythrocyte DNA has a molecular weight of above five million. Calculate the 

 diameter of the smallest sphere into which one molecule could be compressed. 

 (Assume an average atomic weight of 12: it has Cs, N's, O's, H's, and a few 

 P's and S's. Assume also that each atom occupies a cube 1 .2 A on each side.) 



If it were stretched out, the atoms end to end, what would he the total length 

 of the chain? 



6-2: Have you figured out how the two helical strands of DNA can unwind: for repli- 

 cation, or for coding transfer- 1<\ A.' 



6-3: Describe the four possibilities open to a big molecule in an electronically ex< ited 

 state. 



