Vivo Biosynthesis

As we have already seen in earlier sections, the biosynthesis of the lipoprotein has a few unique features. Its biosynthesis is extremely resistant to puromycin, and its mRNA is also extraordinarily stable. It is clear that the lipoprotein is synthesized on ribosomes, since its biosynthesis is as sensitive to chloramphenicol, tetracycline, sparsomycin, and kasugamycin as the biosynthesis of the other outer membrane proteins. 

One interesting approach for probing the assembly mechanism of the lipoprotein is to study the exclusive biosynthesis of the lipoprotein in vivo. Since this protein has no histidine, it is the only envelope protein synthesized in the absence of histidine in an E. coli histidine auxotroph as discussed earlier (3.1). As shown in Fig. 11, in the absence of histidine, 

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It is interesting to note that serine peptidases can, under special conditions in vitro, catalyze the reverse reaction, namely the formation of a peptide bond (Fig. 3.4). The overall mechanism of peptide-bond synthesis by peptidases is represented by the reverse sequence f-a in Fig. 3.3. The nucleophilic amino group of an amino acid residue competes with H20 and reacts with the acyl-enzyme intermediate to form a new peptide bond (Steps d-c in Fig. 3.3). This mechanism is not relevant to the in vivo biosynthesis of proteins but has proved useful for preparative peptide synthesis in vitro [17]. An interesting application of the peptidase-catalyzed peptide synthesis is the enzymatic conversion of porcine insulin to human insulin [18][19]. 

Goltermann NR, Rehfeld JF, Roigaard-Petersen H In vivo biosynthesis of cholecystokinin in rat cerebral cortex. J Biol Chem 255 6181-6185, 1980 Goncalves N, Stoll KD Carbamazepine in manic syndromes a controlled double-blind study. Nervenarzt 56 43-47, 1985 Gonella G, Baignoh G, Ecari U Fluvoxamine and imipramine in the treatment of depressive patients a double-blind placebo-controlled study. Curr Med Res Opin 12 177-184, 1990... 

By analogy to the terms co- and posttranslational modifications of peptides and proteins to define these transformations in the in vivo biosynthesis, chemical manipulations at least theoretically can be carried out in a co- or postsynthetic manner. While nature exploits the sequence- and even conformation-dependent regioselectivity of enzymes to expand the molecular and functional diversity of peptides and proteins beyond the genetic code,P l synthetic chemical reactions are insufficient for the required selectivity even with the most advanced conjugation techniques. Therefore, the tactics usually employed involves a cosynthetic approach, i.e. synthesis of polypeptide chains with annino acid derivatives or... 

Figure 5. In vivo biosynthesis ofWJ35 by the extended oxy minimal PKS with OxyD and OxyJ (KR).

Given the present imperfection of one-pot syntheses [6], this definition of the ideal synthesis is utopian, at least for in vitro chemical synthesis. One has to admit, though, that the in vivo biosynthesis of natural products in a living cell by and large fulfills the criterion of a so-defined ideal synthesis. 

X Hebuteme, XD Wang, EJ Johnson, NI Krinsky, RM Russell. Intestinal absorption and metabolism of 9-cw-beta-carotene in vivo biosynthesis of 9-ci5-retinoic acid. 

The Cj5 famesyl pyrophosphate shown in Figure 4.18 is believed to be the initiator of the in vivo biosynthesis of Hevea mbber. This initiator has successfully been used to make Hevea mbber in a culture tube in vitro biosynthesis). Other oligomeric allylic pyrophosphates (geranyl-, famesyl- and geranyl-geranyl-pyrophosphate) can serve as initiators in vitro. The chemical stmctures are shown in Figure 4.18. 

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