RESUMO
Starch biogenesis in corn endosperm from Flint, Sugary, Waxy, as a function of the grain filling/period was studied. We have differentially identified the initiation from the elongation process. After incubating under unprimed conditions, two glucose radiolabelled protein bands of 39,5 and 36 kDa were obtained. UDP(14C)Glc was the preferred glucosyl donor but also ADP(14C)Glc was. It was additionally found that more than one glucose was transferred to the protein or to the alpha 1,4-glucan linked to protein from UDPGlc. These results were supported by the fact that the glucosylated protein from UDPGlc liberates maltooligosaccharides after alpha- or beta-amylase treatment. The elongation activity in the first steps related to the glucan linked to protein is different from starch synthase. Therefore, we are proposing a model for starch biogenesis where two new transglucosylating enzyme activities are necessary to prepare the primer for starch synthase.
Assuntos
Polissacarídeos/metabolismo , Amido/biossíntese , Zea mays/metabolismo , Enzima Ramificadora de 1,4-alfa-Glucana/metabolismo , Trifosfato de Adenosina/farmacologia , Glucosiltransferases/metabolismo , Glicoproteínas/metabolismo , Modelos Biológicos , Sintase do Amido/metabolismo , Uridina Trifosfato/farmacologia , Zea mays/crescimento & desenvolvimentoRESUMO
A rat brain extract, able to synthesize from UDP-Glc an alpha-1,4-glucan covalently bound to a protein in the absence of added primer is described. The compound formed is precipitable by dilute trichloroacetic acid (TCA). In the presence of glycogen, added as primer, this molecule is enlarged and is not precipitable by TCA. Unprimed and primed activities differ in several aspects, such as the behavior in the presence of some effectors, and the optimum pH. Umprimed and primed activities presented two pHs optima, both sharing only one. The proteoglucans synthesized under the different pHs gave different patterns after analysis under denaturing PAGE and the oligosaccharides synthesized on the protein backbone differ in the glucosyl length. It is concluded that also in rat brain, the initiation process of glycogen biosynthesis is mediated through the formation of a glycoprotein. Our present results showed that the step of the putative "Glycogen Initiator" proposed by use before, requires two enzymes UDPGlc-transglucosylating activities, Glycogen Initiator 1 and Glycogen Initiator 2, before Glycogen Synthase in the alpha-1,4-glucosidic linkages formation.