Abstract：
:The role of Tyr-14 of 3-oxo-delta 5-steroid isomerase (KSI) was probed by analysis of the spectra of 3-amino-1,3,5(10)-estratrien-17 beta-ol (4) and equilenin (5) bound to the active site of KSI. The ultraviolet spectrum of 4 bound to KSI is identical to that for 4 in neutral solution. This observation indicates that Tyr-14 does not protonate the amine group of 4 at the active site. By analogy, it is argued that the 3-oxo group of steroid substrates for KSI is not protonated during the reaction. In contrast, the fluorescence excitation spectra of 5 bound to KSI show characteristics of an ionized phenol, even at pH values as low as 3.8. It is concluded that the pKa of equilenin is perturbed from its value in solution of 9 to less than or equal to 3.5 at the active site of KSI. Similarly, the pKa of the intermediate dienol in the KSI reaction should be lowered to less than or equal to 4.5 when it is bound to KSI. Thus, the function of Tyr-14 as an electrophilic catalyst is likely the stabilization of the anion of the dienol by hydrogen bonding rather than by proton transfer.
journal_name
Biochemistryjournal_title
Biochemistryauthors
Zeng BF,Bounds PL,Steiner RF,Pollack RMdoi
10.1021/bi00120a032subject
Has Abstractpub_date
1992-02-11 00:00:00pages
1521-8issue
5eissn
0006-2960issn
1520-4995journal_volume
31pub_type
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