BIOL 458 Lecture Notes - Lecture 8: Glycoside Hydrolase, Triose, Hydrolase
31 Mar 2020
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Liberated triose-p is used for sucrose export to other organs: triggered by low triose-p, high [pi], low atp/adp ratio. In assimilatory starch this occurs at night - photosynthesis stopped, overflow starch can be used up. Low atp because no photosynthesis at night] (glucose)n polymer is converted to glucose monomers. A-amylase = a(1,4), an endo-amylase: a hydrolase, uses h2o to cleave glycolytic linkages, cleaves randomly within starch polymer (endo-amylase) B-amylase: exo-amylase - cleaves at end, releasing maltose units (a1,4 linked glucose disaccharide) Starch phosphorylase: sequentially removes glucose from non-reducing end, releasing glucose- 1-p: glucose(n) glucose (n+1) + glucose-1-p. Debranching enzyme ( d-enzyme ) = transglycosylase: cleaves a(1,6) branch point. Glycosidase ( r-enzyme ) removes a(1,6) linkages [different mechanism on same target] Further breakdown and export from plastids: glucose-1-p glu-6-p fru-6-p fru-1,6-bisp triose-p, reverse of glucose-1-p synthesis (except phosphofructokinase step, export to cytoplasm as trioses via the tpt [export also as glucose-6-p and maltose] Reserve starch (long-term in starchy seeds and tubers):
