Encoded enzymes

The shell of all picomaviruses is built up from 60 copies each of four polypeptide chains, called VPl to VP4. These are translated from the viral RNA into a single polypeptide, which is posttranslationally processed by stepwise proteolysis involving viraily encoded enzymes. First, the polypeptide chain is cleaved into three proteins VPO (which is the precursor for VP2 and VP4), VPl and VP3. These proteins begin the assembly process. The last step of the processing cascade occurs during completion of the virion assembly the precursor protein VPO is cleaved into VP2 and VP4 by a mechanism that is probably autocatalytic but may also involve the viral RNA. VPl, VP2, and VP3 have molecular masses of around 30,000 daltons, whereas VP4 is small, being 7000 daltons, and is completely buried inside the virion. 

All NRTIs, as exemplified for AZT (Fig. 7), act in a similar fashion following their uptake by the cells, they are phosphorylated successively to their 5 -monophosphate, 5 -diphosphate, and 5 -triphosphate form (De Clercq 2002). Unlike the first phosphorylation step in the metabolic pathway of the acyclic guanosine analogues (see above), which is carried out by a virus-encoded enzyme (thymidine kinase), the first as well as the subsequent phosphorylations of the 2, 3 -dideoxynucleosides are carried out by cellular enzymes, that is, a 2 -deoxynucleoside (e.g., dThd) kinase, a 2 -deoxynucleotide (e.g., dTMP) kinase, and a (2 -deoxy)nucleoside 5 -diphosphate (NDP) kinase. 

M. S. Chen, J. Walker, and W. H. PrusolF, J. Biol. Chem., 254 (1979) 10,747-10,753 related references on virus-encoded enzymes are cited therein. 

Type I disorders are due to mutations in genes encoding enzymes (eg, phosphomannomutase-2 [PMM-2], causing CDG la) involved in the synthesis of dolichol-P-P-oligo-saccharide... 

Three enzymes in Azotobacter vinelandii (Riittimann-Johnson et al. 2003) and Rho-dopseudomonas palustris (Oda et al. 2005) are capable of reducing dinitrogen the nif-encoded enzyme containing molybdenum and iron, the vn/that encodes a vanadium and iron enzyme, and the anf that is an iron-only enzyme. 

In the degradation of toluene, the plasmid-borne genes encode enzymes for the oxidation to benzoate, whereas those that are chromosomally borne encode ring 2,3-dioxygenation. 

Krum JG, SA Ensign (2001) Evidence that a linear megaplasmid encodes enzymes of aliphatic alkene and epoxide metabolism and coenzyme M (2-mercaptoethanesulfonate) biosynthesis in Xanthobacter strain Py2. J Bacterial 183 2172-2177. 

Burkholderia sp. strain NK8 can be grown with benzoate, and both 3- and 4-chloro-benzoate. The genes encoding enzymes for the dioxygenation of benzoates (cbeABCD) and catechols (catA, catBC) have been cloned and analyzed (Francisco et al. 2001). Both... 

Hughes MA, PA Williams (2001) Cloning and characterization of the pnb genes, encoding enzymes for 4-nitrobenzoate catabolism in Pseudomonas putida TW3. J Bacteriol 183 1225-1232. 

Co-expression of the human a- and p-subunits in the yeast Pichea pastoralis produces only trace amounts of active tetramer, with the majority being present in an unassembled form. Co-expression with human type III collagens, however, increases this assembly level tenfold. This indicates that collagen synthesis and the formation of an active prolyl 4-hydroxylase complex are mutually dependent processes (Vuorela et al, 1997). A similar observation has been noted for baculovirus encoded enzymes in insect cells (Lamberg et al, 1996). These findings support the hypothesis that this unusual control mechanism may be a common feature of collagen synthesis in all cell types. 

The presence of a PHA synthase alone is not sufficient to allow synthesis of PHAs. PHA biosynthesis will not occur if genes encoding enzymes required for the synthesis of hydroxyacyl-coenzyme A thioesters are absent or if the pathways constituted by these enzymes are for whatever reason not functionally active. This becomes evident, for example, when a PHA synthase gene is expressed in wild type or normal laboratory strains of E. coir, even if a functionally active PHA synthase is expressed, no or only traces of PHAs are accumulated. 

Blanc, V. Lagneaux, D. Dider, P., et al., Cloning and analysis of structural genes from streptomyces pristinaspirales encoding enzymes involved in the conversion of pristinamycin PUB to PIIA PIIA synthase and NADH riboflavin 5 phosphate oxidoreductase. 1995, 177, 5206-5214. 

Genes involved in drug metabolism encode enzymes or receptor proteins that dictate drug response. Variations in gene structure, primarily single nucleotide... 

Fig. 2.2 A. Chemical structure of the three types of lignin monomer units H, p-hydroxyphenyl- G, guaiacyl and S, syringyl. Note the methyl groups (boxed) in the methoxy moieties of the G and S monomers. B. Pie chart showing the proportions of pine xylem ESTs that putatively encode enzymes of Ci metabolism, glycolysis, and the TCA cycle.

Fig. 10.8 Selected cDNAs isolated in recent years that encode enzymes involved in the biosynthesis of various classes of isoquinoline alkaloids. 6-OMT, norcoclaurine 6-0-methyltransferase 23 CYP80A1, berbamunine synthase 19 CYP80B1, (S)-A-methylcoclaurine 3 -hydroxylase 20 CPR, cytochrome P-450 reductase 29 4 -OMT, (5)-3 -hydroxy-A-methylcoclaurine 4 -0-methyltransferase 30 BBE, berberine bridge enzyme 12 SalAT, salutaridinol 7-O-acetyltransferase 28 COR, codeinone reductase.25...

In summary, the de novo isolation of the cDNAs encoding enzymes of alkaloid biosynthesis is still achieved by using a variety of classical techniques, such as protein purification followed by partial amino acid sequence determination, and by newer techniques such as proteomics coupled to functional heterologous expression. The current status of cloned cDNAs specifically related to isoquinoline alkaloid biosynthesis is schematically presented in Figure 10.8. New additions to this list will certainly be made in the future as a result of a combination of approaches both new and old. 

It has proven extraordinarily difficult to obtain an inhibitor of sufficient potency (15), bioavailability, and safety to be used in the clinic. The ability of HIV to mutate, and thus to develop resistance to many inhibitors, is an additional hurdle. In spite of the difficulties, chemists persisted, motivated by the knowledge that inactivating this HIV-encoded enzyme inactives the virus. Their efforts have provided HTV protease inhibitors that are being evaluated for clinical efficacy (16). 

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