Beta-amylase degradation by serine endoproteinases from green barley malt

Authors: Schmitt, Mark; Marinac, Laurie

Submitted to: Journal of Cereal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/30/2007
Publication Date: 7/15/2007
Citation: Schmitt, M., Marinac, L.A. 2008. Beta-amylase degradation by serine endoproteinases from green barley malt. Journal of Cereal Science. 47:480-488.

Interpretive Summary: Protein degradation in germinating seeds is generally viewed as being necessary to convert stored forms of proteins into amino acids that can either be used for growth by the developing seed or as nutrition by yeast during fermentation to produce ethyl alcohol. Such degradation of storage proteins is not thought to have a direct negative effect on mobilization of the primary carbohydrate reserve in the seed (starch). However, several different types of evidence presented in this publication show that one class of enzyme in germinating barley seeds is capable of degrading a protein that is critical for production of useable sugars from starch. Although uncontrolled degradation of this critical enzyme could be disastrous for the seed, it appears that the seed has mechanisms in place to regulate this process preventing inappropriate loss of the sugar-producing capacity, although what the specific nature of those regulatory mechanisms are is currently unclear. The impact of the research reported here is that it shows that proteinase activity during seed germination may not only degrade storage proteins lacking a metabolic function, but may also degrade enzymes that are important for converting starch into useable sugars. Therefore, identifying malting barley varieties with the best mix of properties may require a detailed knowledge of their protein degradation machinery.

Technical Abstract: Proteolytic degradation of barley proteins is examined in green malt from Hordeum vulgare L. cv. Harrington. Zymographic analysis of the Harrington green malt extracts using commercial preparations of barley beta-amylase incorporated as a proteolytic substrate in 2-D SDS gels shows at least three distinct proteolytic activities. Zymogram gels treated with class-specific proteinase inhibitors show that serine-class proteinase activities appear to be responsible for barley beta-amylase degradation. Western blot analysis also shows that proteolytic enzymes eluted from 1-D electrophoretic gels degrade barley beta-amylase, and that the degradation is blocked by PMSF. However, serine proteinase activities in extracts which have not been treated first with mild denaturing conditions appear to be less able to degrade intact, active barley beta-amylase, as judged by beta-amylase activity assays, electrophoresis and Western blot analysis. Mention of trade names or commercial products in this presentation is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.