286 



CELL HEREDITY 



.AU-fc AU-»L>-**Phe*Glu ♦ArK -^S^r-^Thr -►S^r-'-S^r-^Asp -►His -*-Mtt — Glu -»AU-»AU 



AU 



f 



Ser 

 » 

 Glu 



^Val 'Ala •C>s-Glu-<;iy -Asp-Pro <-T\r-Val-Pro-Val-^is-Phe -Asp -Ala -Ser -Val 



Mil 



VII 



"*~Lys-Glu-^Ala-^Asp-.-Thr--Thr-L.vs-T>r-Cys-«-Ala--Asp--Pro-»-T\T--Lys-Ser-' >/ 



Lys 



His-^Val-^Phe— Thr*- Asp — Val— Pro — Lys -^ C> s^ Arg-^ Asp -^ Lys-^Thr-^Leu-^ Asp-^ Arg — Ser, 



II 



FIGURE 10.6. The amino acid sequence of ribonuclease. A two-dimensional sche- 

 matic diagram of the structure of bovine ribonuclease showing the arrangement of the 

 disulphide bonds and the sequence of the amino acid residues. The arrows indicate 

 the direction of the peptide chain starting from the amino end i after Spackman, Stein, 

 and Moore, 1960, J. Biol. Chem., 235:648 . 



difficulties increase tremendousK with molecular size. The "beautiful 

 string of pearls, as Linderstrom-Lang dubbed the amino acid sequence 

 of ribonuclease, is shown in Figure 10.6. 



Knowledge of the primary structure of ribonuclease has not made ob- 

 vious the nature of its folded configuration, but recent experiments have 

 provided some clues. One experiment posed this problem: If the folded 

 configuration is destro\ed bv breaking all the S-S bridges, can the mole- 

 cule regain activit\ if ph\sical conditions are proxided for rejoining of 

 the cvsteine residues by oxidation of the -SH groups":^ Ribonuclease was 

 denatured bv breaking the S-S bridges by reduction, giving rise to com- 

 pletelv inactixe molecules. Subsequentlv, by careful oxidation, the mole- 

 cules returned to a form exhibiting full enzvmatic acti\ ity, with the 

 S-S bridges restored. This experiment indicates a kind of molecular 

 autonom\" in which the folded configuration seems to be determined bv 

 the primary structure. Whether or not the same is true of larger pro- 

 teins remains an outstanding problem in the field of protein chemistry. 



An experiment of particular interest in relation to enzvme actix ation 

 is diagrammed in Figure 10.7. Ribonuclease can be treated with the 

 enzvme subtilisin under conditions in w hich only a single peptide bond, 

 located between serine and alanine, is cleaxed, giving two fractions — 

 one consisting of most of the molecule, and the other, a short peptide of 

 20 amino acid residues. Under suitable conditions, the fractions do not 



