Biosynthesis structural modification

Subsequent cyclizations, dehydrogenations, oxidations, etc., lead to the individual naturally occurring carotenoids, but little is known about the biochemistry of the many interesting final structural modifications that give rise to the hundreds of diverse natural carotenoids. The carotenoids are isoprenoid compounds and are biosynthesised by a branch of the great isoprenoid pathway from the basic C5-terpenoid precursor, isopentenyl diphosphate (IPP). The entire biosynthesis takes place in the chloroplasts (in green tissues) or chromoplasts (in yellow to red tissues). 

Many structure-function relationships of food colorants are stressed in this book, helping readers understand the effects of their biosynthesis, structures, and function modifications during food storage and processing conditions, and their influences on food quality and safety. This knowledge is necessary to control the rate of... 

During the biosynthesis of nonribosomal peptides, there are two ways to incorporate the nonprotein amino acids. They can be incorporated either as a single unit or as an L-a-amino acid, which then undergoes structural modifications, while attached to the carrier protein. In the case of coronamic acid, L-rr//o-isoleucine is loaded onto the carrier protein and a unique biosynthetic pathway produces a cyclopropyl group containing a nonprotein amino acid. Specific examples of the biosynthesis of nonprotein amino acids will be discussed in the following sections. 

C-Methylation also features in the biosynthesis of usnic acid (Figure 3.41), an antibacterial metabolite found in many lichens, e.g. Usnea and Clado-nia species, which are symbiotic combinations of alga and fungus. However, the principal structural modification encountered involves phenolic oxidative coupling (see page 28). Two molecules of methylphloracetophenone are incorporated, and these are known to derive from a pre-aromatization methylation reaction and not by... 

As mentioned inO Sect. 3.1.1, the lipid A and the core oligosaccharides can undergo structural modifications in response to environmental signals [7]. Modifications of lipid A usually entail removal or modification of the phosphates and fatty acyl chains (O Fig. 15). Modifications occur at both early and late stages of OM biosynthesis. In E. coli grown at low tem-... 

Studies on structure-activity relationships in this family of alkaloids were feasible as a large number of lycorine derivatives had been prepared from 37 and several naturally occurring Amarylllidaceae alkaloids were available. In an initial experiment, twenty-three lycorine derivatives and naturally occurring alkaloids, structurally related to 37, were tested for their ability to inhibit AA biosynthesis in potato tubers. The following relationships between the structure modification and activity were... 

Roth BD, Ortwine DF, Hoefle ML, et al. Inhibitors of cholesterol biosynthesis. 1. trans-6-(2-Pyrrol-1-ylethyl)-4-hydroxypyran-2-ones, a novel series of HMG-CoA reductase inhibitors. 1. Effects of structural modifications at the 2-and 5-positions of the pyrrole nucleus. J Med Chem 1990 33 21-31. 

Introduction - Valuable contributions continue to be made in unfolding the intimate mechanisms of sterol biosynthesis. While the steroid molecule continues to be a useful model for organic synthesis, structural modification of its nucleus has not yielded mammalian hormones with novel or markedly enhanced biological properties. The discovery of new hormones and biologically active substances in the plant and lower animal kingdom has sustained the chemist s interest in steroids. As the chemist adds to the understanding of the role of the steroid hormone in the life process, the biochemist quickens his search for the mechanism of its action. ... 

Battersby AR, McDonald E, Stachulski AB (1983) Biosynthesis, part 26. Synthetic studies on structural modification of late biosynthetic precursors for colchicine. J Chem Soc Perkin Trans 1 3053-3063... 

Brochmann-Hanssen E, Chen CH, Chen CR, Chiang HC (1975) Opium alkaloids, part XVI. The biosynthesis of 1-benzylisoquinolines in Papaver somniferum. Preferred and secondary pathways stereochemical aspects. J Chem Soc Perkin Trans 1 1931-1937 Brochmann-Hanssen E, Cheng CY, Chiang HC (1982) Biosynthesis of opium alkaloids The effects of structural modification of reticuline on racemization and bio transformation. J Nat Prod 45 629-634... 

The repertoire of a-amino acids used by nature in the biosynthesis of proteins is limited to about 20 structures. Since the diversity of synthetically accessible a-amino acids is enormous, initial studies have been undertaken to investigate the possibilities offered by incorporation of noncoded amino acids into en2ymes. Some selected examples of successful modifications are presented below. 

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