severe degenerative brain diseases of unknown 

 etiology may be caused by kuru-type slow virus- 

 es. One of these disorders now knowt. to be 

 caused by a similar agent is Creutzfeldt-Jakob 

 disease, a progressive presenile dementia, which 

 is more widespread than kuru. The same basic 

 cellular lesions occur in both kuru and Creutz- 

 feldt-Jakob, but the molecular and immunological 

 structure of both viruses remains hidden, eluding 

 attempts to classify them through the usual labo- 

 ratory techniques. 



The viruses involved in kuru and a few other 

 diseases appear to be totally atypical and uncon- 

 ventional. Today only four of these viruses are 

 known; those causing kuru and Creutzfeldt-Jakob 

 disease in man; one causing "scrapie," a fatal ill- 

 ness of sheep and goats; and the causative agent 

 in a disease of mink called transmissible mink 

 encephalopathy. 



All these findings have led to new concepts of 

 the etiology of brain degeneration. 



Dr. Gajdusek is now coordinating a worldwide 

 collaborative effort to determine the role of kuru- 

 like agents in human disease. What has been dis- 

 covered thus far may be only the tip of the ice- 

 berg; similar slow or latent viruses may be impli- 

 cated in many of the more common chronic and 

 degenerative diseases of the nervous system. 



Protein Structure and Function 



Striking advances have been made in the past 

 ten years in understanding the relationship be- 

 tween the structure of enzymes and how they 

 function. Much of this progress has been made 

 through information gained from application of 

 the techniques of amino acid sequencing and x- 

 ray crystallography. 



Enzymes are complicated protein molecules 

 made up of strings of amino acids. In order to 

 understand the structure of a particular enzyme, 

 two kinds of information are necessary. The pri- 

 mary structure must be determined; this is the se- 

 quence of amino acids as they are joined together 

 to form a polypeptide chain. Also, the secondary 

 and tertiary structures must be known; this is the 

 stable three-dimensional conformation of the mole- 

 cule as determined by the folding and intertwining 

 of its polypeptide chains; it is unique for each en- 

 zyme and maintained by interactive forces within 

 the molecule, such as disulfide bridges and hydro- 

 gen bonding between amino acid side chains. 



Chemical techniques are used for determining 

 the primary sequence of a polypeptide chain. 

 They generally involve the use of specific en- 

 zymes and chemical reagents to split off and iden- 

 tify one amino acid at a time, working from one 

 end of the chain to the other. 



The determination of the amino acid sequences 

 of proteins has been proven to be indispensable 

 for biochemists working in a variety of areas. The 

 sequence structure is needed for the ultimate 

 analysis of x-ray data, for studies on molecular 

 evolution, and for identification of active site re- 

 gions of enzymes (the part of the molecule actual- 

 ly involved in triggering the chemical reaction that 

 takes place), to give but a few examples. The 

 work itself is time consuming and expensive, and 

 requires a high degree of expertise. One of the 

 most distinguished investigators supported by 

 NIH in this field is Dr. Hans Neurath of the Uni- 

 versity of Washington. Dr. Neurath has spent most 

 of his scientific career analyzing the structure and 

 function of a specific group of proteolytic (pro- 

 tein-digesting) enzymes, the serine proteases. 

 Most recently, his sequence studies of the factors 

 involved in blood coagulation, performed in col- 

 laboration with Dr. Carl W. Davie, also of the 

 University of Washington, have shown that many 

 of these factors are proteolytic enzymes that are 

 homologous with each other and with serine pro- 

 teases in general. Furthermore, the mechanism by 

 which the blood coagulation process operates in- 

 volves a sequential activation of inactive precur- 

 sors, similar to the activation of other known ser- 

 ine proteases. 



X-ray crystallography is a technique used in 

 determining the three-dimensional structure of an 

 enzyme. It is based on interpretation of the dif- 

 fraction pattern produced on a photographic plate 

 by an x-ray beam passing through a protein crys- 

 tal. The characteristics of the diffraction pattern 

 are controlled by the positions of scattering cen- 

 ters — the atoms — within the molecules making up 

 the crystal. Crystals of protein are used so that all 

 the molecules will be lined up the same way; oth- 

 erwise, each differently oriented molecule would 

 produce a different diffraction pattern, and all 

 these patterns superimposed on a photographic 

 plate would be uninterpretable. Positions of the 

 atoms can be calculated by Fourier analysis of the 

 characteristics of the diffraction pattern, and this 

 information together with the primary sequence 

 defines the complete three-dimensional structure 

 of the enzyme. 



An important study in the area was performed 

 by Dr. William N. Lipscomb and his colleagues at 

 Harvard University. They obtained an x-ray 

 structure of carboxypeptidase A, a proteolytic 

 enzyme produced by the pancreas. Their work, 

 together with the primary sequence provided by 

 Dr. Neurath and his collaborators, completely 

 defined the structure of the enzyme. A question 

 remained, however: Did the enzyme have the 

 same shape when bound to its substrate (the 

 substance it acts upon) as it had in a purified state? 



1 02 HEALTH, EDUCATION AND WELFARE 



