Submitted to: Cereal Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/21/1998
Publication Date: N/A
Citation: Interpretive Summary: When barley is converted into malt, the starting material for brewing beer, the seeds are germinated under controlled conditions to activate enzymes that can degrade portions of the seed into small molecules that the yeasts can use for growth and to make alcohol. The purpose of this study was to learn more about the enzymes that degrade the proteins; these are called proteinases. We need to define the characteristics of these enzymes so that researchers can use plant breeding and/or genetic engineering methods to alter them to develop better malting barleys. We isolated and characterized a group of proteinases from resting and germinated barley seeds. The germinated seeds contained six enzyme forms, while the resting ones had only four, indicating that at least two new enzymes are formed during the germination process. Most of the biochemical characteristics of the various enzymes were similar, but some had different structures, indicating that there are two distinct types of proteinases in germinated barley. These proteinases did not hydrolyze the seed proteins normally believed to be used for plantlet growth, but selectively destroyed a group of proteins that were previously postulated to protect the seed against microbial or insect attack. It appears likely that these enzymes are not located in the germinating seed to furnish food to the growing plantlet, but are there to control some aspect of its basic metabolism. Determination of the exact metabolic event(s) being controlled may lead to knowledge that will allow the development of barley lines that are more perfectly suited for malting and brewing.
Technical Abstract: Both resting and germinated barley seeds (Hordeum vulgare L. 'Morex') contain aspartic endopeptidase activities, and the activities increase during germination. These enzymes may be essential to barley germination. We have extracted and partially purified aspartic endopeptidases from both resting seeds and green malt (4-day germinated barley). Six aspartic proteinase activities were found in resting barley seeds while only four activities were detected in green malt. All of the aspartic proteinases had similar pH activity optima (pH 3.5 to 4.5) and pI values (around 4.5). The purified green malt aspartic proteinases selectively digested a group of barley seed proteins, postulated to serve as defensive proteins, that are coded by the 'amylase/trypsin inhibitor' super gene family. The aspartic proteinases that bound to a pepstatin A affinity column at pH 4.5 cross-reacted with antiserum raised against aspartic proteinases purified from barley seed. However, those that did not bind the affinity column also did not cross-react with the antiserum, indicating that there are two distinct groups of aspartic proteases in germinating barley.