Catalytic HAT

HAT indicates the catalytic HAT subunit complex indicates the purified HAT complex. The substrates refer to the in vitro specificity of the HAT subunit. Known functions of each modification in transcription are listed in the last colimm. ND not determined... 

Two key issues are essential for the success of the catalytic HAT (CHAT) reaction. First, the two catalytic systems must be compatible. Because early and late transition metals usually bind ligands with opposing donor properties, they should remain mutually unaffected. Also, the Wilkinson catalyst should be stable under the slightly acidic conditions of the titanocene regeneration system (Zn, 2,4,6-collidine hydrochloride). Since it has been reported that Rh(PPh3)3Cl is even stable towards BF3 Et20, this was expected to be the case [44]. 

HATs do not appear as sole catalytic entities and are always embedded in large multi-protein complexes, which can comprise up to 20 different proteins and are more than 2 MDa in size [1,3]. 

Tip60 Tat-interacting protein of 60kDa. A MYST family HAT, catalytic subunit of the NuA4 complex. Performs key functions in repair of double-stranded DNA breaks. 

Histone acetyltransferases (HATs) are enzymes that acetylate specific lysine residues in histones through the transfer of an acetyl group from an acetyl-coenzymeA (AcCoA) molecule, causing profound effects on chromatin structure and assembly as well as gene transcription. HATs are found in most, if not all, eukaryotic organisms as multiprotein complexes, some HAT catalytic subunits even being shared between various complexes that display different substrate specificities based on their subunit composition [12]. Despite their name, HATs do not restrict themselves to the acetylation of histones, since these enzymes have also been shown to act on nonhistone proteins, broadening their scope of action [13]. 

Inhibition mechanisms by A/-cyclopropyl MPTP analogues are also discussed in terms of two catalytic pathways, one of which is based on an initial SET step from the nitrogen lone pair, as proposed by Silverman, and the second is based on an initial a-carbon hydrogen atom transfer (HAT) step, as proposed by Edmondson, leading to a radical and dihydropyridinium product formation. The observation that MAO B catalyzes the efficient oxidation of certain 1-cyclopropyl-4-substituted-1,2,3,6-tetrahydropyridines to the corresponding dihydropyridinium metabolites suggests that the catalytic pathway for these cyclic tertiary allylamines may not proceed via the putative SET-generated aminyl radical cations [122], Further studies will be necessary to clarify all the facets of the mechanism of inhibition of MAO by cyclopropylamines. 

Nitroxyl radicals have also been used extensively in mechanistic studies of hydrogen atom transfer (HAT), development of catalytic oxidation reactions,109 and for pharmacological purposes.110 Some of the recent examples include HAT from... 

His1 0 and orients it for its catalytic function [54], The putative catalytic thiolate-imldazollum pair at the active site of bromelain is thus, by comparison of the amino acid sequence of bromelain with other cysteine protein ases, likely to have a different conformation from that in (he cysteine proteinases (hat aie tightly inhibited by cystatin [45]. Bromelain also distinguishes itself from other cysteine protein ases by its slow inhibition by the irreversible inhibitor of cysteine oroteinases E-64 rW-ft 3-fraw-cari)OKVOxiran-2 BrboiwlVL-leucvn-amido 4-... 

Organometallic HAT Reagents in Catalytic and Stoichiometric Metal Mediated... 

Instead, it is the goal of this chapter to highlight some conceptually novel developments in the field of HAT reactions. We will concentrate on complexes of borane with N-heterocyclic carbenes (NHCs), on organometallic hydrogen atom donors in stoichiometric and catalytic reactions, and on the activation of water and alcohols for HAT. 

It should be noted that a related reaction has been developed by the Norton group, with HV(CO)4dppe, a compound with an unusually weak M-H bond (about 57 kcal mol-1), serving as a stoichiometric HAT reagent. The organometallic complex cannot be employed catalytically because V(CO)4dppe that is formed after HAT is unable to activate H2 [29, 30],... 

The use of hydrogen as terminal reductant has been accomplished by its activation with transition metal complexes. The resulting weak M-H bonds can be used in both radical generation and reduction through HAT. In this manner, conceptually novel radical chain reactions, such as hydrogen mediated cyclizations, or metal catalyzed processes with coupled catalytic cycles for radical generation and reduction, have been realized. The latter transformations are especially attractive for enantioselective synthesis. 

In all the processes just described, a definite interpretation of the modifying effect of CO2 on the catalytic system cannot be given and further research work must be carried out in this part of CO2 chemistry. Despite the fact (hat the study of CO3 as a co[Pg.210]

The catalytic partial pressures and temperatures used were thus similar to those employed by Boudart and were therefore expected to result in structure insensitivity- However, the hydrocarbon overlayer on the catalyst surfaces was also expected to exceed monolayer coverage In the light of previous results and it was recognised (.hat this might be important ... 

Ad.sorplion-induced restructuring can occur on the chemisorption time scale (isf 10 see for charge transfer or — lO" see for vibrational times). There is evidence, however, (hat adsorbate-induced restmeturing can occur on the lime scale of catalytic reactions (seconds). CO oxidation to CO or ammonia reacting with NO to produce nnd H O show osciliaiiiry behavior under certain cireumsiance.s of temperature and renctant partial pressures. The rcuction rate alternates periodically bc-... 

The novel class of preceramic paper might offer a versatile and economic approach to process light-weight ceramics with tailored macro- and microscopic porosities for a broad field of applications in transportation (particle filtration, friction sheets in clutches), energy (porous burner, hat exchangers, solar radiation receivers, photovoltaic substrates), and environment (water cleaning, exhaust gas purification, catalytic reactor inserts). 

HAT group HAT complex Catalytic subunit Organism Histone acetylation substrates Function... 

---