126 BIG MOLECULES 



interference microscopes, the X-ray diffraction camera, and the elertron 

 microscope — the last now in such an advanced stage of development that, in 

 proper hands, it can resolve or "see" small particles just a few atomic 

 diameters apart — researchers have been able to gain new insight into the 

 actual shape of the molecule in the tissue, and even into the positions of 

 atoms and groups of atoms within the molecule. 



Running concurrently with these physical researchers have been chemical 

 studies which have finally solved the puzzle of the complete chemical 

 composition of a few large, biologically important molecules. For example, 

 although the hormone, insulin, has been known and used widely in the treat- 

 ment of diabetes for nearly forty years, it was only in 1955 that Sanger and 

 his colleagues at Cambridge were finally able to write down the complete 

 structural formula. It contains 777 atoms! Since then, ribonuclease 

 (RNAse), an enzyme containing 1876 atoms and which catalyzes the 

 cleavage of ribonucleic acid, has also yielded the secret of its composition to 

 the attack of persistent chemists. This completes the first big step toward 

 knowing how this molecule works as a catalyst, although details of the struc- 

 ture at and around the active site(s) are not yet known. This is the next big 

 task, for if more than one of the chemical groups must exert their chemical 

 effects on a specific part of the molecule whose hydrolysis is to be promoted, 

 then their spatial arrangement must be very important. Not only must they 

 be present, but they must be present at the proper positions in space if the 

 catalytic activity of the site is to exist. In other words, if one of the players 

 is out of position, the game is lost. 



Table 6-1 gives a spectrum of biologically important organic molecules, 

 small and large — some containing a metallic oxidizable and reducible ion 

 which enters the chemical reactions of the molecule. Although some details 

 are given in the following sections, the discussion is just an indication of the 

 scope of the subject. There are excellent reference sources: for example, 

 the recent book of Tanford. 16 



STRUCTURE 



Our purpose, first, will be to outline the structure of two big molecules of 

 critical biological importance, myoglobin and hemoglobin, learned in 

 the recent work of the schools of Kendrew and Perutz, respectively. The 

 method used was X-ray crystallography, and although the chemical com- 

 position has not yet been fully worked out for these two molecules, X-ray 

 crystallographic studies have completely outlined the form of the molecule 

 in the dry crystalline state. 



The second part of this section on structure is concerned with the cross- 

 linked structure of liquid crystals, such as in the aqueous humor of the lens 



