Human placental estradiol 17.beta.-dehydrogenase: evidence for inverted substrate orientation ("wrong-way" binding) at the active site

Human placental estradiol 17 beta-dehydrogenase (EC 1.1.1.62) was affinity labeled with 17 alpha-estradiol 17-(bromo[2-14C]acetate) (10 microM) or 17 beta-estradiol 17-(bromo[2-14C]acetate) (10 microM). The steroid bromoacetates competitively inhibit the enzyme (against 17 beta-estradiol) with Ki values of 90 microM (17 alpha bromoacetate) and 134 microM (17 beta bromoacetate). Inactivation of the enzyme followed pseudo-first-order kinetics with a t1/2 = 110 min (17 alpha bromoacetate) and t1/2 = 220 min (17 beta bromoacetate). Amino acid analysis of the affinity radioalkylated enzyme samples from the two bromoacetates revealed that N pi-(carboxy[14C]methyl)histidine was the modified amino acid labeled in each case. Digestion with trypsin produced peptides that were isolated by reverse-phase high-performance liquid chromatography and found to contain N pi-(carboxy[14C]methyl)histidine. Both the 17 alpha bromoacetate and also the 17 beta bromoacetate modified the same histidine in the peptide Phe-Tyr-Gln-Tyr-Leu-Ala-His(pi-CM)-Ser-Lys. Previously, the same histidine had been exclusively labeled by estrone 3-(bromoacetate) and shown not to be directly involved in catalytic hydrogen transfer at the D-ring of estradiol. Therefore, this histidine was presumed to proximate the A-ring of the bound steroid substrate. The present results suggest that the 17 alpha bromoacetate and 17 beta bromoacetate D-ring analogues of estradiol react with the same active site histidine residue as estrone 3-(bromoacetate), the A-ring analogue of estrone.(ABSTRACT TRUNCATED AT 250 WORDS)