Comments on the structure of a-chymotrypsin 



B. S. HARTLEY 



School of Biochemistry, University of Cambridge 



I must apologize for presenting somewhat preliminary results on the struc- 

 ture of a-chymotrypsin, but I feel I might be able to supplement some of 

 the information derived from Dr Sorm's peptides. 



I have been studying the B-chain of performic acid-oxidized a-chymotrypsin 

 which was first isolated by Meedom (Acta. Chem. Scand., 10, 881 (1956)). 

 It contains about 160 residues — approximately two-thirds of the enzyme — 

 and has a C-terminal tyrosine and an A^-terminal Ileu.Val-sequence. It is 

 especially interesting since the active centre of chymotrypsin probably in- 

 volves a histidine and a unique serine residue, and both these residues are 

 present in this chain. 



Both tryptic and chymotryptic digests of the B-chain have been studied. 

 Some of the more profitable results were obtained by following the chymo- 

 tryptic digestion in a pH-stat and stopping the reaction after 12-15 bonds 

 were spht. The fraction of the digest soluble in 50% ethanol at —5°, pH 

 3-6, consisted almost entirely of 16 neutral and basic peptides, which were 

 purified by paper chromatography and electrophoresis. The composition 

 and sequence in these peptides, which account for about half of the residues 

 in the B-chain, is at present being studied, but some preliminary results 

 may be relevant to this discussion. 



The oxidized B-chain contains two arginine residues which appear in the 

 following three peptides after chymotryptic digestion : 



1. Ala. Arg. (Thr, Ala, Val). Leu 



2. Ala. Asp. Thr. (Asp, Ala, Pro). Arg. lieu. (Glug, Sera, Ala, Pro). Leu 



3. Ala. Asp. (Asp, Thr, Ala, Pro). Arg. lieu 



It seems probable that peptide 3 arises by further chymotryptic digestion 

 of 2. A tryptic digest of the B-chain yielded Ala. Arg and Ala. (Asp2, Thr, 

 Ala, Pro). Arg as the only peptides containing arginine, confirming that 

 peptides 1. and 2. above represent the environment of the two arginine 

 residues in this chain. 



A further peptide which may be of interest is Ala. (Ala, Asp-NHg) — 

 isolated in good yield from the chymotryptic digest. This may be another 

 example of an asparaginyl split by chymotrypsin, such as was mentioned 

 by Professor Desnuelle, though one has to be wary of peptides from this 

 oxidized protein because of the unknown fate of the tryptophan residues. 



