Holoenzyme

The two subunits of CODH/ACS have been dissociated to offer a clearer picture of the ACS active site 135). The holoenzyme contains 2 Ni, 12 Fe, and 14 S 120) that are organized into 3 discrete clusters, whereas the isolated a subunit contains only 1 Ni and 4 Fe and has spectroscopic properties similar to those of Cluster A in the native enzyme 186). Based on EXAFS spectroscopy of the a. subunit, the Ni site in Cluster A has been proposed to be coordinated to 2 sulfur ligands at 2.19 A and 2 nitrogen or oxygen ligands at 1.89 A in a distorted square plane 186). 

The rpoH gene is expressed from four different promoters. Under normal physiological conditions, the PI promoter is responsible for most of the rpoH transcription, while P2 and P4 promoters contribute varying minor amounts. The P3 promoter is under the control of the Eo holoenzyme (see below) and becomes induced at temperatures above 45 °C. The rpoH gene is expressed at all temperatures, and after a heat shock its transcription is increased by a factor of 1.5 only, but there is a large transient increase in intracellular levels. Two factors contribute significantly to this increase an enhanced rate of translation of the rpoH mRNA, and a transient stabilization in the half-life of... 

The polymerase III holoenzyme (the dnoE gene ptoduct in E colt) binds to template DNA as patt of a multiptotein complex that consists of sevetal polymetase accessoty factors ((3, y, 8, S ", and x). DNA polymerases only synthesize DNA in the j to 3 ditection. 

Template binding RNA polymerase (RNAP) binds to DNA and locates a promoter (P) melts the two DNA strands to form a preinitiation complex (PIQ. (2) Chain initiation RNAP holoenzyme (core + one of multiple sigma factors) catalyzes the coupling of the first base (usually ATP or GTP) to a second ribonucleoside triphosphate to form a dinucleotide. (3) Chain elongation Successive residues are added to the 3 -OH terminus of the nascent RNA molecule. (4) Chain termination and release The completed RNA chain and RNAP are released from the template. The RNAP holoenzyme re-forms, finds a promoter, and the cycle is repeated. 

Pol II associates with other proteins to form a holoenzyme complex. In yeast, at least nine gene products—called Srb (for suppressor of RNA polymerase B)—bind to the CTD. The Srb proteins—or mediators, as they are also called—are essential for pol II transcription, though their exact role in this process has not been defined. Related proteins comprising even more complex forms of RNA polymerase II have been described in human cells. 

Recent evidence suggests that there is another possible mechanism of PIC formation and transcription regulation. First, large preassembled complexes of GTFs and pol II are found in cell extracts, and this complex can associate with a promoter in a single step. Second, the rate of transcription achieved when activators are added to limiting concentrations of pol II holoenzyme can be matched by increasing the concentration of the pol II holoenzyme in the absence of activators. Thus,... 

GTFs and RNA polymerase with promoters—or in one step by the recognition of the promoter by a preformed GTF-RNA polymerase holoenzyme complex. 

Biotin functions to transfer carbon dioxide in a small number of carboxylation reactions. A holocarboxylase synthetase acts on a lysine residue of the apoenzymes of acetyl-CoA carboxylase, pymvate carboxylase, propi-onyl-CoA carboxylase, or methylcrotonyl-CoA carboxylase to react with free biotin to form the biocytin residue of the holoenzyme. The reactive intermediate is 1-7V-carboxybiocytin, formed from bicarbonate in an ATP-dependent reaction. The carboxyl group is then transferred to the substrate for carboxylation (Figure 21—1). 

WUson CJ, Chao DM, Imbalzano AN, Schnitzler GR, Kingston RE, Young RA (1996) RNA polymerase II holoenzyme contains SWI/SNF regulators involved in chromatin remodeling. Cell 84(2) 235-244... 

GGPPS functions as part of a complex metabolon. In the plastid, as shown in Capsicum chromoplasts," GGPPS is a homodimer and associated but not integral to the plastid envelope. GGPPS is also associated with the next enzyme in the pathway as part of a holoenzyme complex." " ... 

A homogeneous electrochemical enzyme immunoassay for 2,4-dinitrophenol-aminocaproic acid (DNP-ACA), has been developed based on antibody inhibition of enzyme conversion from the apo- to the holo- form Apoglucose oxidase was used as the enzyme label. This enzyme is inactive until binding of flavin adenine dinucleotide (FAD) to form the holoenzyme which is active. Hydrogen peroxide is the enzymatic product which is detected electrochemically. Because antibody bound apoenzyme cannot bind FAD, the production of HjOj is a measure of the concentration of free DNP-ACA in the sample. 

Type III-PHA synthase is represented by the enzyme of Chromatium vino-sum and is encoded by phaECCv. It consists of the two different subunits PhaCCv and PhaECv exhibiting molecular weights of 39,730 and 40,525 Da, respectively [23]. The native PHA synthase, as isolated from recombinant strains of E. coli, exhibited a molecular weight of approximately 390 and 400 + 20 kDa as revealed in our laboratory [54] or 360 + 50 kDa but also 520 + 50 kDa as revealed in another laboratory [55]. The lower molecular weights for the holoenzyme are... 

Alley, S.C., Ishmael, F.T., Jones, A.D., and Benkovic, S.J. (2000) Mapping protein-protein interactions in the bacteriophage T4 DNA polymerase holoenzyme using a novel trifunctional photo-cross-linking and affinity reagent./. Am. Chem. Soc. 122, 6126-6127. 

Marr, M.T., Datwyler, S.A., Meares, C.F., and Roberts, J.W. (2001) Restructuring of an RNA polymerase holoenzyme elongation complex by lambdoid phage Q proteins. PNAS 98, 8972-8978. 

Enzymes may not function well or at all unless some other species known as a cofactor is present. An enzyme alone is referred to as the apoenzyme and the combination of enzyme and cofactor is known as the holoenzyme. Among the species that function as cofactors are organic compounds that interact with the enzyme. If the organic moiety is strongly attached to the enzyme, it is called a prosthetic group, but if it is loosely bound to the enzyme, it is referred to as a coenzyme. For the purposes of this discussion, the most interesting cofactors are metal ions. Depending on the type of enzyme, the appropriate metal ion cofactor may be Mg2+, Ca2+, K+, Fe2+, or Cu2+. A sizeable number of enzymes are sometimes called metalloenzymes because they have active sites that contain a metal. 

Kinesins mediate anterograde transport in a variety of organisms and tissues. Since its discovery, much has been learned about the biochemical, pharmacological and molecular properties of kinesin [44, 45], Kinesin is the most abundant member of the kinesin family in vertebrates and is widely distributed in neuronal and nonneuronal cells. The holoenzyme is a heterotetramer comprising two heavy chains (115-130 kDa) and two light... 

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