Speaker: Alan Myers, professor in the Iowa State University Roy J. Carver Department of Biochemistry, Biophysics, and Molecular Biology (BBMB) 

Title: Evolution of starch biosynthesis from primordial glycogen systems

Abstract: Successful primary endosymbiosis that introduced photosynthesis to the eukaryotic kingdom required metabolic adaptation to allow storage of the massively increased carbohydrate supply accumulating during the light phase of the diurnal cycle. The primordial carbohydrate storage system of the eukaryotic host or the cyanobacterial endosymbiont would not have served this purpose owing to osmotic stress limitations. 

These enzyme systems evolved to reach a common ancestor of the plastid-containing species that produced a semi-crystalline polymer, amylopectin, rather than the related hydrosoluble polymer glycogen. This biochemical modification enabled diurnal metabolism in green algae and subsequently the appearance and spread of land plants on earth, eventually providing the foundation of human agriculture. 

Research described in this presentation identified a plausible mechanism for the transition from soluble to insoluble carbohydrate storage polymers that collected enzymes originating in both the eukaryotic host genome and that of the photosynthetic endosymbiont. The results support a model in which an enzyme originally functioning to degrade glycogen in the endosymbiont was adapted into a glucan chaperone that physically coordinates linear glucan chains and stimulates their spontaneous transition into a double helical, crystalline state. 

The presentation will also describe a genetic screen in a chromerid protist that identified a second mechanism that converted soluble glycogen into insoluble amylopectin, independent of endosymbiosis and photosynthesis.

Host: Josh Strable, GDCB assistant professor