Prokaryotic enzymes

The inhibitors of RNA polymerase, which generates RNA from DNA, inhibit a crucial step in gene expression. Inhibition of the eukaryotic form of RNA polymerase is used in cancer chemotherapy and is also an important experimental tool. For example, actinomy-cin D binds to the guanine residues in DNA and blocks the movement of the eukaryotic RNA polymerase. Specific inhibitors of bacterial RNA polymerase can be used as antibacterial agents. Most of these inhibitors like rifamycin bind to the prokaryotic enzyme. 

Restriction enzymes or Prokaryotic enzymes with exquisite sequence recognition of target... 

The process of eukaryotic DNA replication closely follows that of prokaryotic DNA synthesis. Some differences, such as the multiple origins of replication in eukaryotic cells versus single origins of replication in prokaryotes, have already been discussed. Eukaryotic single-stranded DNA-binding proteins and ATP-dependent DNA helicases have been identified, whose functions are analogous to those of the prokaryotic enzymes previously discussed. In contrast, RNA primers are removed by RNase H. 

In eukaryotes, considerable progress has been made in studying the in vitro replication of animal viruses, such as SV40. The importance ascribed to the enzymes that have been characterized is largely based on a comparison of their properties with similar prokaryotic enzymes whose functions are better understood. 

Selectivity of inhibitors Presence or absence of the target in eukaryotes can be assessed through bioinformatics however, certain prokaryotic enzymes with clear orthologs in eukaryotes are the targets of successful antibiotics trimethoprim [102] fluoroquinolones [59],... 

The basic mechanism of RNA synthesis by these eukaryotic RNA polymerases is the same as for the prokaryotic enzyme (see Topic G2), that is ... 

Multidomain proteins tend to occur more frequently in eukaryotes than in prokaryotes. Often the eukaryotic counterpart to a set of individual prokaryotic enzymes that catalyze successive reactions is a single, multidomain protein. The theoretical advantages proposed for such an arrangement include (1) a geometry for the direct transfer of substrates from one active site to another, in a process known as substrate channeling, in order to increase the overall flux of the pathway, (2) the protection of intermediates that may be unstable in aqueous environments or may be acted on inappropriately by other enzymes, (3) the facilitation of interactions between domains for purposes of allosteric regulatory functions, and (4) the establishment of a fixed stoichiometric ratio of the... 

Pola has a cryptic 3 —> 5 -exonuclease that is revealed only upon dissociation of its four subunits. This proofreading ability probably contributes to the fidelity of DNA replication. PolS also has proofreading 3 — 5 -exonuclease activity and can carry out editing functions similar to that of the prokaryotic enzymes. 

Eukaryotic cells contain at least four different DNA-dependent RNA polymerases. Their localization, cellular transcripts, and susceptibility to the cyclic octapeptide a-amanitin (derived from poisonous mushrooms) are shown in Table 11.3. a-Amanitin blocks the elongation phase of RNA synthesis. Although the structures of these enzymes are much more complex than that of the prokaryotic RNA polymerase, the basic mechanism is very similar to that of the prokaryotic enzyme. 

McIntire,W. S., Wemmer, D. E., Christoserdov, A. Y., and Lindstrom, M. E., 1991, A new cofactor in a prokaryotic enzyme Tryptophan tryptophylquinone as the redox prosthetic group in methylamine dehydrogenase, Science 252 817n824. 

The GT-B fold family includes most prokaryotic enzymes that produce secondary metabolites, like the antibiotics streptomycin, oleandomycin (Fig. 1) and vancomycin, and important bacterial cell wall precursors. It is also predicted to contain the vitally important 0-GlcNAc transferase that modifies many nuclear and cytoplasmic proteins and influences gene transcription. The first glycosyltransferase structure reported in 1994 was for the GT-B fold enzyme, P-glucosyltransferase (BGT) from bacteriophage T4 (22). This enzyme attaches glucose to modified... 

The amino acid composition of B. subtilis and E. coli enzymes, calculated for a minimum subunit of 33,500, show a greater divergence. Whereas the E. coli enzyme is not unsimilar to both the mitochondrial and cytoplasmic isozymes from eukaryotic sources, the B. subtilis protein is decidedly different. Interestingly, the E. coli enzyme possesses a molecular weight of about 67,000 as compared to the 117,000 of the B. subtilis protein (S7). Primary structure analyses will be required to ascertain the extent to which either of these prokaryotic enzymes are sequentially related to either of the eukaryotic forms of the enzyme. 

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