Amylose biosynthesis

These reactions do not have to occur in perfect sequence, and the phases may have some overlap (e.g., phases 2, 3, and 4 may overlap, and possibly even 5 and 6). However, the present evidence, such as intermediate products formed by starch mutants of C. reinhardtii and of higher plants, supports the sequence of reactions shown in Fig. 1 for amylopectin and amylose biosynthesis. Further experiments are required to test this hypothetic scheme, and attempts to purify and characterize the debranching enzyme, crucial to this hypothesis, are under way. 

Chlamydomonas reinhardtii monocellular algal mutants defective in amylose biosynthesis and granule-bound starch synthase activity accumulate a structurally modified amylopectin. J. Bac-teriol. 1992 174 3612-3620. 

Figure 4. Glycoalkaloid content in genetically modified anti-GBSS potatoes (inhibition of amylose biosynthesis) and the corresponding untransformed control lines upper (a), lower (b).

Extrapolation of the molecular structure of an a-maltohexaose duplex com-plexed with triiodide in single crystals leads to a left-handed, 8-fold, antiparallel double-helix for amylose.90 The pitch of this idealized helix is 18.6 A, so h is only 2.33 A. Although this model is no contender to the fiber data, in terms of biosynthesis, it is doubtful that the native amylose helix favors antiparallel chains. 

Erlander, S. R. (1998a). Starch biosynthesis. I. The size distributions of amylose and amylo-pectin and their relationships to the biosynthesis of starch. Starch/Stdrke. 50,227-240. 

Progress in understanding the biosynthesis of amylose. Trends Plant Sci. 3, 462-467. 

Comment on unresolved questions about the biosynthesis of cellulose, amylose, and amylopec-tin. What glycosyl carrier groups are required ... 

All chapters/subjects that were also in the previous edition have been updated. Chapters have been added on the biochemistry and molecular biology of starch biosynthesis, structural transitions and related physical properties of starch, and cyclo-dextrins. There are two chapters on the structural features of starch granules that present not only advances in understanding the organization of starch granules, but also advances in understanding the fine structures of amylose and amylopectin, both of which are based on techniques that have been developed since 1984. 

What roles do the starch synthase isoforms play in the formation of the crystalline starch granule and amylopectin structures How is amylose formed Why are starch granules from different species different in size and in the number per cell New methodology and much effort have resulted in major advances in the understanding of starch biosynthesis, but many questions remain unanswered. Here we discuss some of these open questions and possible answers. 

In Volume 33 of this Series, we presented1 a review of the crystalline structures of polysaccharides published during the period 1967-1974. Detailed accounts of progress in structural studies on specific types of polysaccharides were presented in the Proceedings of the Twenty-sixth Symposium of the Colston Research Society and were subsequently published as a book.2 Precise methods for X-ray diffraction analysis of biopolymer structures were discussed by Hukins.3 The aspects of the structures of cellulose, mannan, and xylan, their organization in the cell wall, and the biosynthesis of cell-wall polysaccharides were described by Mackie.4 Work on the structures of the connective-tissue polysaccharides, O-acetylcellulose, and the various forms of amylose was reviewed by Atkins,5 Chanzy,6 and Sarko,7... 

The exact way of amylopectin biosynthesis in plants is still not known today. In our current research we are using a tandem reaction of two enzymes to synthesize artificial amylopectin or rather (hyper)branched amylose in vitro. One enzyme is responsible for building the linear (amylose) part while the other enzyme introduces die branches, phosphorylase and glycogen branching enzyme respectively. 

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