5.4.8.1 Apiogalacturonan-galacturonosyltransferase (AP:GalAT)

It is not known whether apiogalacturonan is synthesized on preexisting HG that is synthe­sized by GAUT1 or related GalATs, or whether a unique GalAT is responsible for apiogalac­turonan synthesis. There have been no reports of efforts specifically targeted at identifying the apiogalacturonan:GalAT.

5.4.8.2 Apiogalacturonan-apiosyltransferase (AP:ApiT)

Apiogalacturonan is a substituted galacturonan that is produced in some aquatic mono — cotyledonous plants (188,189) and that consists of HG substituted at O-2 or O-3 with apiose or apiobiose (D-Api/-p-1,3-D-apiose) (188,189). The anomeric configuration of the linkage of apiose to HG may be in the p configuration (189). It is not known whether the same apiogalacturonan:ApiTs synthesize RG-II (see below) and apiogalacturonan. For example, RG-IIhastwo of its four side branches attached to an HG backbone by a p-Api/ linked to the O-2 of HG (158), and thus, the possibility exists that the p 1,2-apiosyltransferase involved in RG-II synthesis may also synthesize apiogalacturonan. In vivo synthesis of apiogalacturonan has been studied in vegetative fronds of Spirodelapolyrrhiza (343) and D-apiosyltransferase activity has been characterized in cell-free particulate preparations from duckweed (Lemna minor) (341). The apiosyltransferase in particulate membrane preparations from Lemna transfers [14C]-apiose fromUDP-[14C]-apiose onto endogenous acceptors. The enzyme has an apparent Km for UDP-apiose of 4.9 ^M and a pH optimum of 5.7 (341). Since, the rate of apiosyltransferase activity increased twofold when UDP-GalA was added to the reaction (341) and the product synthesized in the presence of UDP-GalA bound anion exchange resin more tightly than the product synthesized without UDP-GalA (342), it is likely that the apiosyltransferase transfers apiose onto a growing HG chain. The ApiT has not been purified and the gene has not been identified.