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Two glycogen synthase activities associated with proteoglycogen in retina
1Centro de Investigaciones en Química Biológica de Córdoba (CIQUIBIC), UNC-CONICET, Departamento de Química Biológica Dr. Ranwel Caputto, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Argentina. jcurtino@dqbfcq.uncor.edu
Neurochemical Research
|February 24, 2000
Summary
Bovine retinal glycogen synthase exists in two forms, linked to different proteoglycogen fractions. One form, crucial for building larger glycogen structures, has a lower affinity for UDP-glucose.
Area of Science:
- Biochemistry
- Cell Biology
- Glycogen Metabolism
Background:
- Proteoglycogen, a complex of glycogen and protein, plays a role in cellular energy storage.
- Glycogen synthase is the key enzyme in glycogen biosynthesis.
- Understanding the localization and properties of glycogen synthase is crucial for elucidating glycogen metabolism.
Purpose of the Study:
- To investigate the association of bovine retinal glycogen synthase with different proteoglycogen fractions.
- To characterize the kinetic properties of glycogen synthase in relation to proteoglycogen structure.
- To elucidate the role of glycogen synthase in proteoglycogen biosynthesis and maturation.
Main Methods:
- Enzymatic assays to determine kinetic parameters (Km) of glycogen synthase.
- Fractionation of proteoglycogen into acid-insoluble and acid-soluble components.
- Proteolytic digestion (pronase) to assess structural integrity and interconversion of fractions.
Main Results:
- Bovine retinal glycogen synthase was found associated with both acid-insoluble and acid-soluble proteoglycogen fractions.
- The synthase linked to the acid-insoluble precursor exhibited an 8-fold lower Km for UDP-glucose compared to the synthase in the acid-soluble fraction.
- Detergent inhibited the synthase associated with the acid-insoluble fraction, and pronase digestion converted this fraction to an acid-soluble form.
Conclusions:
- The acid-insolubility of the proteoglycogen fraction is attributed to the strong association of glycogen synthase with its growing polysaccharide chain.
- A low Km glycogen synthase likely facilitates the enlargement of the polysaccharide moiety during early proteoglycogen biosynthesis.
- A high Km glycogen synthase is proposed to be involved in the conversion of the proteoglycogen precursor to the mature acid-soluble form.