Biosynthesis and biodegradation of pectin

Little is known about the L-arabinofuranosyl transfer machinery. A gene in the genome of A. thaliana has been identified as a membrane-bound arabino-furanosyl transferase by its membrane-insertion sequence and the fact that mutants in the gene led to expression of mRNA, but phenotypes that were nonetheless defective in arabinan side-chains of their pectin. Attempts to use UDP-a-L-arabinopyranose as arabinofuranose donor in crude extracts, presumably in an attempt to uncover any mutase similar to UDP galactopyranose mutase (see Section 5.12.4), merely resulted in the incorporation of pyranose units into the arabinan. However, an enzyme using this donor has been shown to add the terminal arabinopyranosyl residue to the 3-position of the terminal arabinose of the 1,5-arabininan of rhamnogalacturonan 1. [By contrast, the o-arabinofuranosyl transfer machinery that produces the 

D-arabinogalactan of Mycobacterium tuberculosis is known to involve conversion of 5-phosphoribosyl-ot-pyrophosphate to 5-phospho-(3-ribosyldecaprenyl phosphate by a membrane-bound enzyme, which has been cloned, sequenced and functionally expressed. The sugar attached to the decaprenyl glycosyl donor (not dolichol, since the sugar-proximal prenyl unit is not reduced) is then 

The degradation of the pectin main chain is catalysed not only by pectinases, which hydrolyse the glycosidic link, but also by lyases, which carry out and iilcB-like elimination across C4 and C5, leaving a 4-deoxy-4,5-dehydro-galacturonic acid at the new non-reducing end (see Section 6.7.4). 

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