Glycogen-synthesizing enzymes

The exact way of amylopectin biosynthesis in plants is still not known today. In our current research we are using a tandem reaction of two enzymes to synthesize artificial amylopectin or rather (hyper)branched amylose in vitro. One enzyme is responsible for building the linear (amylose) part while the other enzyme introduces die branches, phosphorylase and glycogen branching enzyme respectively. 

Besides application to the analysis of polysaccharides of unknown structure, enzymes are also often used for routine measurements on polysaccharides of essentially established structure, such as the starch components and the glycogens. Enzymes provide extremely quick and accurate information for the characterization of polysaccharides of this type, including the products synthesized in vitro diuring studies on their mechanism of biosynthesis. There is also increasing application to the characterization of other types of polysaccharide, synthesized enzymically in vitro. A further important use is for investigation of the nature of the products formed by the action of other catabolic enzymes on polysaccharides such applications may give information on either the structure of the polysaccharide or the specificity of the catabolic enzyme under investigation. 

In contrast to inhibitor 1, DARPP-32 and NIPP1, which regulate signal transduction, the function of inhibitor 2 appears to be different. There is evidence that inhibitor 2 associates with PP1, as the phosphatase is newly synthesized and contributes to the proper folding of the enzyme [40]. Inhibitor 2 can thus be considered a chaperone protein. The inactive PP 1-inhibitor 2 complex can then be activated upon phosphorylation of inhibitor 2 by glycogen synthase kinase-3. Whether this process is regulated in neurons in association with synaptic activity remains unknown. 

In vivo linear a-l,4-glucans are synthesized from ADP-glucose by the enzyme glycogen synthase [94-97]. The enzyme, as well as the monomer, are quite sensitive and therefore most researchers (at least in the field of polymer science) prefer to use phosphorylase for the synthesis of amylose. 

Response of enzyme to phosphorylation Depending on the specific enzyme, the phosphorylated form may be more or less active than the unphosphorylated enzyme. For example, phosphorylation of glycogen phosphorylase (an enzyme that degrades glycogen) increases activity, whereas the addition of phosphate to glycogen synthase (an enzyme that synthesizes glycogen) decreases activity (see p. 132). 

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