Reactivity of singlet oxygen toward proteins: the effect of structure in basic pancreatic trypsin inhibitor and in ribonuclease A.
The reactions of singlet oxygen, 1O2, with large peptides have been described previously. It was found that even in these systems, which in their native form are generally not supposed to possess a stable structure in solution, the polypeptide does impede the access of 1O2 to the amino acids that react readily with 1O2. Here we describe the 1O2 reaction with two proteins of well-defined structure. The quenching of 1O2 by bovine pancreatic trypsin inhibitor (BPTI) and by ribonuclease A (RNase A) was compared to that of a solution at the same concentration as those of its constituent amino acids that react readily with 1O2. The proteins were studied in their native form, when partly denatured by splitting their S-S bonds and when fully denatured. It was found that while in the native form the quenching rate constant was seven times lower in BPTI (2.2 vs 15.2 x 10(7) M-1 s-1) and three times lower in RNase A (11.0 vs 32 x 10(7) M-1 s-1) than in a mixture of its constituent amino acid residues, it increased upon denaturation reaching in the fully denatured state the value of the corresponding amino acid mixture. More striking is the effect of the protein structure when comparing the fraction of the encounters between 1O2 and protein, which cause damage to the protein, as reflected in the decrease of its biological activity. This decrease is assumed to be due to the chemical (oxidative) reactions of 1O2 in the protein. In the exceptionally stable BPTI the fraction of such encounters was 0.05 and in RNase A it was 0.2, whereas for the amino acid tryptophan in solution, 0.7 of the collisions with 1O2 led to a chemical reaction.
Michaeli
A
A
Department of Physical Chemistry, Hebrew University of Jerusalem, Israel.
Feitelson
J
J
D000818:Animals; D007611:Aprotinin / Q000737:chemistry*; D002417:Cattle; D011487:Protein Conformation; D017382:Reactive Oxygen Species; D012259:Ribonuclease, Pancreatic / Q000737:chemistry*; D012996:Solutions
D017382:Reactive Oxygen Species; D012996:Solutions; D007611:Aprotinin; D012259:Ribonuclease, Pancreatic