Mode of glucan degradation by purified phosphorylase ec 22.214.171.124 forms from spinach spinacia oleracea leaves
Steup, M.; Schaechtele, C.
Planta (Heidelberg) 153(4): 351-361
The glucan specificity of the purified chloroplast and non-chloroplast forms of .alpha.-1,4-glucan phosphorylase (EC 126.96.36.199) from spinach [S. oleracea] leaves (Steup and E. Latzko 1979 was investigated. Phosphorolysis by the 2 enzymes was studied using a series of linear maltodextrins (degree of polymerization .ltoreq. 11), amylose, amylopectin, starch and glycogen as substrates. For all unbranched glucans (amylose and maltodextrins G5-G11), the chloroplast phosphorylase had a 7- to 10-fold higher apparent affinity (determined by initial velocity measurements) than the non-chloroplast phosphorylase form. For both enzyme forms, the minimum chain length required for a significant rate of phosphorolysis was 5 glucose units. Likewise, phosphorolysis ceased when the maltodextrin was converted to maltotetraose. With the chloroplast phosphorylase, maltotetraose was a linear competitive inhibitor with respect to amylose or starch (Ki = 0.1 mmol l-1); the inhibition by maltotetraose was less pronounced with the non-chloroplast enzyme. In contrast to unbranched glucans, the non-chloroplast phosphorylase exhibited a 40-, 50- and 300-fold higher apparent affinity for amylopectin, starch and glycogen, respectively, than the chloroplast enzyme. With respect to these kinetic properties the chloroplast phosphorylase resembled the type of maltodextrin phosphorylase.