Biosynthesis polymers

R180 I. Gosselin, J.-N. Barbolin and J.-C. Portais, NMR Investigations of Polymer Biosynthesis. The Case of a Multiproductive Bacterium, Sinorhizobium Meliloti , p. 331... 

The topological problem of how polymer biosynthesis that proceeds by the addition of monosaccharide units to the non-reducing end can be initiated... 

In plant tissues Cieo can undergo some other modifications (Figure 3), namely elongation, hydroxylation, oxidation, epoxydation, reduction, oxidative decarboxylation, etc. As a result of these modifications many different lipophilic substances are produced. Among these substanees very long-chain FAs (VLCFAs, C>2o), different unusual FAs (hydroxy-, epoxy-, acetylenic, dicarboxylic), fatty aldehydes and alcohols, hydrocarbons, oxilipins, etc. are formed. Some of them are present in plants in free form (are embedded in the complex cuticular lipid matrix or as a components of epicuticular waxes), the others are used as a substrates for more complex lipids and lipid polymers biosynthesis (see below). 

Bacteria can s mthesize a wide range of biopolymers. The key aspects of the production bacterial biopolymers have been reviewed (75,76). It is expected that a better understanding of polymer biosynthesis and material properties can lead to an increased use of bacterial biopol mers. 

We have systematically studied the effects of DNA damage on poly(ADP-ribose) catabolism in vivo following inhibition of polymer biosynthesis with 3-aminobenzamide (5). The results can be summarized as follows (Fig. 2) i) Stimulation of poly(ADP-ribose) catabolism apparently is an obligatory postincisional event in DNA excision repair ii) TTiis stimulation is dependent on the dose of the DNA damaging agent and may be up to 680-fold. As a consequence, the of constitutive polymers, which... 

The mechanisms triggering polysaccharide synthesis by fungi are well understood but the enzymes involved in polysaccharide secretion remain to be identified. Biosynthetic pathways for the production of specific polysaccharides, such as the capsule synthesis in C. neoformans or pullulan by A. pulllulans, have been studied in more detail because of their proven importance for pharmaceutical or food industries, respectively. However, even in those cases, the mechanisms of polymer biosynthesis are far from being understood. 

The inhibition of [ " Cjglucose incorporation into cellulose in azuki bean epicotyl sections was the first indication that polymer synthesis was a target site for dichlobenil. Concentrations of dichlobenil below 1 fiM caused specific inhibition of [ " Cjglucose incorporation into cellulose. This effect was considered to be a direct action on polymer biosynthesis rather than an indirect one occurring via a disruption of MTs involved in cell wall synthesis. 

The biosynthesis process, which consists essentially of radical coupling reactions, sometimes followed by the addition of water, of primary, secondary, and phenohc hydroxyl groups to quinonemethide intermediates, leads to the formation of a three-dimensional polymer which lacks the regular and ordered repeating units found in other natural polymers such as cellulose and proteins. 

All these polyesters are produced by bacteria in some stressed conditions in which they are deprived of some essential component for thek normal metabohc processes. Under normal conditions of balanced growth the bacteria utilizes any substrate for energy and growth, whereas under stressed conditions bacteria utilize any suitable substrate to produce polyesters as reserve material. When the bacteria can no longer subsist on the organic substrate as a result of depletion, they consume the reserve for energy and food for survival or upon removal of the stress, the reserve is consumed and normal activities resumed. This cycle is utilized to produce the polymers which are harvested at maximum cell yield. This process has been treated in more detail in a paper (71) on the mechanism of biosynthesis of poly(hydroxyaIkanoate)s. 

Much of protein engineering concerns attempts to explore the relationship between protein stmcture and function. Proteins are polymers of amino acids (qv), which have general stmcture +H3N—CHR—COO , where R, the amino acid side chain, determines the unique identity and hence the stmcture and reactivity of the amino acid (Fig. 1, Table 1). Formation of a polypeptide or protein from the constituent amino acids involves the condensation of the amino-nitrogen of one residue to the carboxylate-carbon of another residue to form an amide, also called peptide, bond and water. The linear order in which amino acids are linked in the protein is called the primary stmcture of the protein or, more commonly, the amino acid sequence. Only 20 amino acid stmctures are used commonly in the cellular biosynthesis of proteins (qv). 

The special topics discussed are (i) the biological aspects of heterocyclic compounds, i.e. their biosynthesis, toxicity, metabolism, role in biochemical pathways, and their uses as pharmaceuticals, agrochemicals and veterinary products (ii) the use of heterocyclic compounds in polymers, dyestuffs and pigments, photographic chemicals, semiconductors and additives of various kinds and (iii) the use of heterocyclic compounds as intermediates in the synthesis of non-heterocyclic compounds. 

In one of the early experiments designed to elucidate the genetic code, Marshall Nirenberg of the U.S. National Institutes of Health (Nobel Prize in physiology or medicine, 1968) prepared a synthetic mRNA in which all the bases were uracil. He added this poly(U) to a cell-free system containing all the necessary materials for protein biosynthesis. A polymer of a single amino acid was obtained. What amino acid was polymerized ... 

The carbon atom which carries the methyl group is chiral, but biosynthesis is stereoselective, and gives rise to a natural polymer with the R configuration. The polymer is a partially crystalline thermoplastic which melts at about 80 °C. 

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