Starch biosynthesis: the primer nonreducing-end mechanism versus the nonprimer reducing-end two-site insertion mechanism

Arabidopsis irregular xylem8andirregular xylem9: Implications for the Complexity of Glucuronoxylan Biosynthesis

“…However, chain termination may still occur at relatively low frequency in the absence of glycosyl sequence 1 (e.g., by transfer of the nascent GX chain to water). Regardless of the actual mechanism operating in vivo, obtaining unambiguous evidence for the direction of xylan backbone elongation will be technically challenging, although a general approach to address this question has been proposed (Mukerjea and Robyt, 2005). Several explicit models that include specific roles for the IRX8, IRX9, and FRA8 gene products in GX biosynthesis are consistent with the available data.…”

“…The second mechanism requires that GX is synthesized by adding monosaccharides to the reducing end and that transfer of glycosyl sequence 1 (either one residue at a time or en bloc) to the reducing end terminates chain elongation. The synthesis of hyaluronan (Bodevin-Authelet et al, 2005;Tlapak-Simmons et al, 2005) and starch (Mukerjea and Robyt, 2005), which have been reported to occur by the addition of monosaccharides to the reducing end, serve as precedents for this mechanism. Furthermore, the presence of GX chains that do not have glycosyl sequence 1 at the reducing end suggests either that glycosyl sequence 1 is cleaved from some of the GX chains during wall biogenesis or that GX chains can be synthesized without involving glycosyl sequence 1 as a primer.…”

Starch biosynthesis: experiments on how starch granules grow in vivo

“…Because of these problems and the lack of definitive experiments supporting the nonreducing end primer elongation of starch chains, Mukerjea and Robyt [12][13][14] carried out a number of experiments to test the nonreducing end primer mechanism. The first set of experiments was a pulse with ADP-[ 14 C]Glc and a chase with nonlabeled ADPGlc of eight different kinds of starch granules.…”

Xylan Biosynthesis: News from the Grass

“…However, in their study of AX biosynthesis in wheat, Porchia et al (2002) have reported reversible arabinosylation of a 41-kD protein (possibly an RGP). York and O'Neill (2008) proposed a mechanistic model for GX elongation in Arabidopsis in which the xylan backbone could be elongated from its reducing end, but then a mechanism to terminate the elongation of the backbone is needed (Bodevin-Authelet et al, 2005;Mukerjea and Robyt, 2005;Tlapak-Simmons et al, 2005). The identification of sequence 1 at the reducing ends of GXs from dicots and gymnosperms was proposed to act as a signal to terminate the xylan backbone (Pena et al, 2007;York and O'Neill, 2008) and may suggest an elongation mechanism from the reducing end.…”