Domain level organization

H. M. Jespersen, E. A. MacGregor, M. R. Sieiks, and B. Svensson, Comparison of the domain-level organization of starch hydrolases and related enzymes, Biochem. J., 280 (1991) 51-55. 

Figure 8. Schematic domain level organization of starch-hydrolases and related enzymes (see text for further details).

In conclusion, sterol biosynthesis in Bacteria is probably limited to a small number of taxa that either have an incomplete sterol biosynthetic pathway or lack the capacity to alkylate the side chain. Therefore, steranes ((66b)-(66e) and (69b) and (69c)) in bitumens and oils can be reliably attributed to the activity of eukaryotic organisms (contra Cavalier-Smith, 2002). Additionally, some steranes with a diagnostic alkylation pattern ((66d)-(66e), and (70)) have taxonomic value below domain level. 

As previously mentioned, natural fibres present a multi-level organization and consist of several cells formed out of semi-crystalline oriented cellulose micro fibrils. Each microfibril can be considered as a string of cellulose crystallites, linked along the chain axis by amorphous domains (Fig. 19.10) and having a modulus close to the theoretical limit for cellulose. They are biosynthesized by enzymes and deposited in a continuous fashion. A similar structure is reported for chitin, as discussed in Chapter 25. Nanoscale dimensions and impressive mechanical properties make polysaccharide nanocrystals, particularly when occurring as high aspect ratio rod-like nanoparticles, ideal candidates to improve the mechanical properties of the host material. These properties are profitably exploited by Mother Nature. 

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