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Starches ......... Amylopectin ......... Amylose .......... Structure ............ Pathway .......... Mutants ............ Enzymes .......... Clones ........... Modifications ............ Functionality .......... EnCapsulation ........... Processing
Biotechnological developments in starch synthesis hold great promise for modifying the basic structural composition of starch. Much progress has already been made using naturally occurring mutants.
Opportunities for structural modifications of starch can be broken-down into a few main classes:
These structural changes may be achieved by an empirical approach in which individual enzymes in the pathway are under and over expressed. However, there is evidence that this kind of approach has its limits and even with the conventional mutants we can see that there are reasons to doubt that we adequately understand the mechanism of starch assembly. In our research we are focussing on what we believe are the major technical challenges facing this new science today. In particular we are faced with a poor understanding of how the enzymes of starch biosynthesis control and contribute to starch structure. Furthermore we are also faced with trying to develop a better understanding of how starch structure and functionality are interrelated. Ultimately, it is only starch functionality that matters to the consumer and end-user/processor. Thus, we have two major technical challenges, first to improve our understanding of the links between functionality and starch fine structure, and second to establish a linkage between starch fine structure and enzymology and genetics.
All of the known enzymes involved in starch synthesis have been cloned from a variety of species (e.g. corn, rice, potato, barley, pea). All such enzymes appear to be present in the nuclear genome and none are known from the plastidial DNA. In bacteria and animals similar enzymes exist, making bacterial and animal glycogen. Glycogen is also composed of a1,4- and a1,6-glucan but differs from starch by being more highly branched and is not stored in water-insoluble granules. The bacterial genes were the first to have been cloned and are collectively known as the "glg" genes (e.g. glgA, glgB, glgC and glgX). Following the early 70's work on glg genes, 10-year-long waves of research has led to clones for all of the known starch pathway genes. Thus from the mid-70's to the mid-80's the waxy or granule-bound starch synthases were cloned, first from corn and then all the major species. From the mid-80's until the present the ADPG pyrophosphorylase genes were cloned, first from spinach. From the late 80's until the present the BE genes were cloned, first from rice. In the early-90's the first soluble starch synthase enzymes were cloned, first from peas and rice. Very recently, debranching enzymes from corn and rice and disproportionating enzymes from potato were cloned. Sequence similarities have been established within specific enzyme family groups, although specific isoforms appear to exist. Work is continuing at a rapid pace by a few key groups in the world.
