Room-temperature insertion of elemental tellurium into the Cspł-Br and -I bonds of α-bromo-and α-iodopinacolone

Pinacolyltellurium(IV) dihalides, (t-BuCOCH₂)₂TeX₂ (X ) Br (1b), I (1c)) and Ar(t-BuCOCH₂)TeCl₂ (Ar == 1-C₁₀H₇ (Np) (2a), 2,4,6-Me₃C₆H₂ (Mes) (3a)), are readily prepared at room temperature by the oxidative insertion of elemental tellurium into the C_sp³-Br or -I bond of the α-halopinacolone and by the reaction of ArTeCl₃ with the pinacolone t-BuCOCH₃. The bromides Np(t-BuCOCH₂)TeBr₂ (2b) and Mes(t-BuCOCH₂)TeBr₂ (3b) can be prepared by the addition of bromine to the telluride Ar(t-BuCOCH₂)-Te or of α-bromopinacolone to ArTeBr. Variable-temperature ¹H and ¹³C NMR of the separate signals for the o-Me groups in 3a,b indicate a very high barrier to rotation about the Te-C(aryl) bond. Crystal diffraction data for 1c, 2a-c, and 3b show that intramolecular 1,4-Te …O(C) secondary bonding interactions (SBIs) are retained even in the presence of bulky aryl groups and intermolecular Te …X SBIs are subject to electronic population and steric congestion around the Te(IV) center in the solid state.

Angiotensin I-converting enzyme transition state stabilization by HIS1089 : evidence for a catalytic mechanism distinct from other gluzincin metalloproteinases

Angiotensin (Ang) I-converting enzyme (ACE) is a member of the gluzincin family of zinc metalloproteinases that contains two homologous catalytic domains. Both the N- and C-terminal domains are peptidyl-dipeptidases that catalyze Ang II formation and bradykinin degradation. Multiple sequence alignment was used to predict His¹⁰⁸⁹ as the catalytic residue in human ACE C-domain that, by analogy with the prototypical gluzincin, thermolysin, stabilizes the scissile carbonyl bond through a hydrogen bond during transition state binding. Site-directed mutagenesis was used to change His¹⁰⁸⁹ to Ala or Leu. At pH 7.5, with Ang I as substrate, k_cat/K_m values for these Ala and Leu mutants were 430 and 4,000-fold lower, respectively, compared with wild-type enzyme and were mainly due to a decrease in catalytic rate (k_cat) with minor effects on ground state substrate binding (K_m). A 120,000-fold decrease in the binding of lisinopril, a proposed transition state mimic, was also observed with the His¹⁰⁸⁹ --> Ala mutation. ACE C-domain-dependent cleavage of AcAFAA showed a pH optimum of 8.2. H1089A has a pH optimum of 5.5 with no pH dependence of its catalytic activity in the range 6.5-10.5, indicating that the His¹⁰⁸⁹ side chain allows ACE to function as an alkaline peptidyl-dipeptidase. Since transition state mutants of other gluzincins show pH optima shifts toward the alkaline, this effect of His¹⁰⁸⁹ on the ACE pH optimum and its ability to influence transition state binding of the sulfhydryl inhibitor captopril indicate that the catalytic mechanism of ACE is distinct from that of other gluzincins.