Multisubunit enzyme

ATP synthesis is the final step in the conservation of energy via chemios-mosis. This step is catalyzed by FiFo ATP synthase, a multisubunit enzyme complex found exclusively in cytoplasmic membranes of bacteria (Senior 1988). It uses the chemiosmotic energy (ApH) generated from membrane ET to synthesize ATP from ADP and Pi. The most investigated bacterial... 

Figure 8.14. (a) A schematic multisubunit enzyme. Each subunit contains one catalytic (A) and one regulatory (R) site. The subunits ate arranged cyclically, i.e., them + 1 subunit is the same as the first, (b) A single subunit in the two conformational states L and H. 

The topologically defined region(s) on an enzyme responsible for the binding of substrate(s), coenzymes, metal ions, and protons that directly participate in the chemical transformation catalyzed by an enzyme, ribo-zyme, or catalytic antibody. Active sites need not be part of the same protein subunit, and covalently bound intermediates may interact with several regions on different subunits of a multisubunit enzyme complex. See Lambda (A) Isomers of Metal Ion-Nucleotide Complexes Lock and Key Model of Enzyme Action Low-Barrier Hydrogen Bonds Role in Catalysis Yaga-Ozav /a Plot Yonetani-Theorell Plot Induced-Fit Model Allosteric Interaction... 

As with other multisubunit enzymes (e.g., allosteric enzymes), the structural integrity of a membrane-bound enzyme primarily is maintained by noncovalent interactions such as hydrogen bonding, electrostatics, and hydrophobic interactions. Hydrophobic polypeptides (or hydrophobic portions of polypeptides) apparently are used to anchor the enzymes to the membrane through interactions with phospholipids. Therefore, I would characterize the interaction between the enzyme and membrane as chemical in nature rather than as geometric. ... 

Like bacteria, eukaryotes have several types of DNA polymerases. Some have been linked to particular functions, such as the replication of mitochondrial DNA. The replication of nuclear chromosomes involves DNA polymerase a, in association with DNA polymerase S. DNA polymerase a is typically a multisubunit enzyme with similar structure and properties in all eukaryotic cells. One subunit has a primase activity, and the largest subunit (Afr -180,000) contains the polymerization activity. However, this polymerase has no proofreading 3 —>5 exonuclease activity, making it unsuitable for high-fidelity DNA replication. DNA polymerase a is believed to function only in the synthesis of short primers (containing either RNA or DNA) for Okazaki fragments on the lagging strand. These primers... 

Ribonucleotide reductase ribonucleoside diphosphate reductase) is a multisubunit enzyme (two identical B1 subunits and two identical B2 subunits) that is specific for the reduction of nucleoside diphosphates (ADP, GDP, CDP, and UDP) to their deoxy-forms (dADP, dGDP, dCDP, and dUDP). The immediate donors of the hydrogen atoms needed for the reduction of the 2-hydroxyl group are two sulfhydryl groups on the enzyme itself, which, during the reaction, form a disulfide bond (Figure 22.12). 

In bacteria, one species of RNA polymerase synthesizes all of Ihe RNA except for the short RNA primers needed for DNA replication (RNA primers are synthesized by a specialized enzyme, primase, see p. 400). RNA polymerase is a multisubunit enzyme that recognizes a nucleotide sequence (the promoter region) at the beginning of a length of DNA that is to be transcribed. It next makes a comple-... 

There are at least five classes of eukaryotic DNA polymerases. Pol a is a multisubunit enzyme, one subunit of which performs the primase function. Pol a 5 ->3 polymerase activity adds a short piece of DNA to the RNA primer. Pol 8 completes DNA synthesis on the leading strand and elongates each lagging strand fragment, using 3 ->5 exonuclease activity to proofread the newly synthesized DNA. Pol p and pol e are involved in carrying out DNA "repair," and pol y replicates mitochondrial DNA. 

The process of RNA synthesis is called transcription. The enzyme that synthesizes RNA is RNA polymerase, which is a multisubunit enzyme. The core enzyme has four subunits—2 a, 1 p, and 1 p, and possesses 5 —>3 polymerase activity. The enzyme requires an additional subunit—sigma (a) factor—that recognizes the nucleotide sequence (promoter region) at the beginning of a length of DNA that is to be transcribed. Another protein—rho (p) factor—is required for termination of transcription of some genes. 

Bacterial RNA polymerase, the target for the rifamycin class of antibacterial agents, is the enzyme responsible for transcription of genomic DNA in bacteria [40], Like DNA gyrase, RNA polymerase is a multifunctional, multisubunit enzyme with multiple active conformations. This increases the number of possible mechanisms of inhibition of RNA polymerase. For example, in addition to the 3 subunit, which is the apparent target for rifamycin, bacterial transcription initiation is a unique process in which the o subunit plays a unique role in the recognition of bacterial promoter sequences [41], Alternative c subunits, such... 

The actual replication enzyme in E. coli is DNA polymerase III. Its properties contrast with Pol I and Pol II in several respects. Pol III is much more processive than the other enzymes, making about 500,000 phosphodiester bonds on the average. In other words, it is about 5,000 times more processive than Pol I and 50 times more processive than Pol II. Pol III is a multisubunit enzyme. It lacks a 5 to 3 exonucleolytic activity, although a subunit of the enzyme carries out the editing (3 to 5 ) function during replication. Finally, only about 10 molecules of Pol III reside in each cell. This remains consistent with the function of Pol III in replication, because the chromosome only needs to be copied once per generation. Therefore, the... 

Question Bacterial RNA polymerase is a multisubunit enzyme. Are there particular subunits to which rifampicin and streptolydigin bind ... 

Seabra, M.C., Goldstein, J.L., Sndhof, T.C., et al. (1992). Rab geranylgeranyl transferase A multisubunit enzyme that prenylates GTP-binding proteins terminating in Cys-X-Cys or Cys-Cys. J Biol Chem 267 14497-14503. 

Similar stereochemical studies have also been conducted on the orsellinic acid synthase from Penicillium cyclopium, a multisubunit enzyme composed of a 130 kDa protein [129, 130]. The catalytic cycle of this PKS is identical to the 6-MSAS cycle, except that it lacks any ketoreduction or dehydration reactions. Unlike 6-MSAS, enolizations occurring during orsellinic acid biosynthesis are not stereospecific. 

This paper summarizes briefly the physicochemistry and enzymology of plant copper oxidases with particular emphasis on polyphenol oxidase and laccase. A brief comparative discussion of other naturally occurring copper proteins and artificial copper proteins is appropriate when discussing the physicochemistry of the copper site itself. In the case of the copper proteins listed in Table I, we know a great deal more about the copper site than about the physicochemistry of the rest of the protein molecule. This is primarily a result of the availability of sophisticated spectroscopic techniques such as optical spectroscopy (both absorption and circular dichroism) and electron spin resonance which are applicable to the electronic transitions of the copper ion. On the other hand, protein chemistry has progressed more slowly. Many of the proteins are large and complex multisubunit enzymes, difficult to purify, and often unstable. There are several excellent reviews on this group of proteins (59, 60, 61, 62). 

Yan Q, Lennarz WJ. Oligosaccharyltransferase a complex multisubunit enzyme of the endoplasmic rehculum. Biochem. Bio-phys. Res. Comm. 1999 266(3) 684-689. 

RNA polymerase in h. coli is a multisubunit enzyme. The subunit composition of the ". SOO-kd holoenzyme is u >3 3 a and that of the core enzyme is ajpp. IVanscription is initiated at promoter sites consisting of two sequences, one centered near —10 and the other near — TS that is, 10 and d5 nucleotides away from the start site in the 5 (upstream) direction. The consensus sequence of the —10 region is TATA AT The a subunit enables the holoenzyme to recognize promoter sites. When the growth temperature is raised. E. coli expresses a special ex subunit that selectively binds the distinctive promoter of lieat-shock genes, RNA polymerase must unwind the... 

Bateman, 1994). Coleman and colleagues (1994) have documented rapid exchange between subunits of dimeric ornithine decarboxylase and suggest that this exchange facilitates regulation by antizyme. In contrast, there is no evidence for a rapid equilibrium between subunits of the 14-subunit chaperone GroEL (A. Horwich, personal communication), or in a number of other multisubunit enzymes (Distefano et al., 1990 Perry et al., 1992 Wente and Schachman, 1987). The potential importance of subunit exchange to sHsp function is discussed further below. 

In contrast to the single-subunit RNA polymerase found in bacteriophages, the model prokaryotic RNA polymerase from E. coli is a multisubunit enzyme. This polymerase has a five-subunit core that forms a constricted, tunnel-shaped catalytic site [97]. Prokaryotic RNA polymerases require an additional subunit, a, for promoter-specific initiation of transcription [98, 99]. 

Cytochrome c oxidase is the terminal member of the respiratory chain in all animals and plants, aerobic yeasts, and some bacteria." " This enzyme is always found associated with a membrane the inner mitochondrial membrane in higher organisms or the cell membrane in bacteria. It is a large, complex, multisubunit enzyme whose characterization has been complicated by its size, by the fact that it is membrane-bound, and by the diversity of the four redox metal sites, i.e., two copper ions and two heme iron units, each of which is found in a different type of environment within the protein. Because of the complexity of this system and the absence of detailed structural information, spectroscopic studies of this enzyme and comparisons of spectral properties with 02-binding proteins (see Chapter 4) and with model iron-porphyrin and copper complexes have been invaluable in its characterization. 

Embedded within the inner mitochondrial membrane are the electron carriers of the electron transport system, and ATP synthase, the multisubunit enzyme that makes ATP. 

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