Urease Activation

Abstract

Synthesis of the urease dinuclear nickel active site requires the apoprotein (Apo), nickel ions, bicarbonate/CO 2 , Mg·GTP, and the participation of several accessory proteins beyond those that transport this metal ion into the cell. Bacteria and archaea typically require UreD (alternatively named UreH in some microorganisms), UreE, UreF, and UreG maturation proteins, whereas fungi and plants possess homologues of UreD, UreF, and UreG but lack proteins comparable to UreE. Unique to the bacterium Helicobacter pylori , the hydrogenase activation proteins HypA and HypB are also required for urease maturation. The various components have been shown to form specific protein–protein complexes: Apo·UreD, Apo·UreD·UreF, Apo·UreD·UreF·UreG, UreH·UreF, UreD/H·UreF·UreG, HypA·HypB, HypA·UreE, and UreE·UreG under defined conditions. In the current model for urease activation, (i) a lysine residue at the nascent active site of urease is carbamylated by CO 2 to form a metal‐bridging ligand, (ii) the metallochaperone UreE donates nickel to UreG, (iii) nickel‐bound UreG forms the Apo·UreD·UreF·UreG·Ni preactivation complex, (iv) nickel transfer from UreG to Apo is driven by the GTPase activity of UreG, (v) nickel ions pass through a molecular tunnel within the UreF and UreD/H scaffold proteins, (vi) metallocenter assembly is followed by dissociation of the accessory proteins from the urease holoenzyme. In H. pylori , HypA and HypB are thought to provide nickel to UreE. Here, evidence supporting the current model for urease maturation is summarized and the remaining open questions related to this process are discussed.

Changes in Regioselectivity of H Atom Abstraction during the Hydroxylation and Cyclization Reactions Catalyzed by Hyoscyamine 6β-Hydroxylase

“…Nonheme iron oxidases represent an important class of enzymes that catalyze a wide range of oxidative transformations. 1–4 The prototypical catalytic cycle of these enzymes involves the use of molecular oxygen to oxidize α -ketoglutarate ( α KG) coordinated to an active site iron(II) center ( 4 ) to generate succinate, CO 2 , and an activated iron(IV)-oxo complex (see 5 and 8 in Figure 1). 1,5 The iron(IV)-oxo complex then participates in the oxidation of the nominal substrate with the possible involvement of organic radicals confined to the active site.…”

“…1–4 The prototypical catalytic cycle of these enzymes involves the use of molecular oxygen to oxidize α -ketoglutarate ( α KG) coordinated to an active site iron(II) center ( 4 ) to generate succinate, CO 2 , and an activated iron(IV)-oxo complex (see 5 and 8 in Figure 1). 1,5 The iron(IV)-oxo complex then participates in the oxidation of the nominal substrate with the possible involvement of organic radicals confined to the active site. Despite the reactivity of iron(IV)-oxo complexes and radical intermediates, nonheme iron oxidases are able to exert considerable control over the course of the reaction such that they proceed in a regio- and stereoselective manner.…”