Bioenergetics of Glycolysis
The defective enzyme is triose-phosphate isomerase, which catalyzes the following reaction.
Edward Senkbeil Salisbury State University Salisbury, MD 21801
dihydroxyacetonephosphate Z? D-glyceraldehyde-3-phosphate
Understanding metabolic pathways is an important wncept in the study of biochemistry. Memorizing the enzymatic steps and the properties of each individual reaction (such as in glycolysis), does not necessarily insure that the student understands the overall significance and interrelationship of the steps in the pathway. The following question involves the bioenergetic aspects of -~lvcolvsis. " " It has oroved useful in challeneins! the students and testing their comprehension. It is suitable for nndereraduate biochemistrv students. and it assumes that they hive a wpy of the pa&way handy.
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The Question Normal erythrocytes use only anaerobic glywlysis for the net production of ATP, thus forming two molecules of ATP for every molecule of glucose. There exists a mutant strain of erythrocytes that is defective in only one enzyme in the glycolytic pathway. When i t is grown on glucose as the sole energy source, it produces a net of 0 molecules of ATP for every molecule of glucose. Nonetheless, lactate is produced as a n end product of its anaerobic glycolysis.
Name the defective glycolytic enzyme and describe the complete reaction that it catalyzes. Explain how the lack of this enzyme's activity causes the above results.
412
Journal of Chemical Education
The Acceptable Answer
The lack of this enzyme would allow only one molecule of the three-carbon sugar, D-glyceraldehyde-3-phosphate,to continue through the glywlytic pathway for every molecule of glucose that entered it. It would also allow the production of one molecule of lactate for every molecule of glucose in an anaerobic system. Nevertheless, no net energy (i.e., no ATP) would be produced. Before the step that is catalyzed by triose-phosphate isomerase, the glycolysis cycle expends two molecules of ATP and converts one molecule of hexose sugar to two molecules of triose phosphates. Due to the defective enzyme, only one triose phosphate would be metabolized through glycolysis to give the high-energy compounds that drive the synthesis of two ATP molecules. Thus, no net ATP is produced, but lactate is still formed. An Additional Challenge Anv other defective dvcolvtic enzvme would com~letelv blwkthe pathway a n d i t ailow lactate production..An additional challen~cwould be to ask thc students for the following information.
Suggest alternative energy sources or pathways that would allow the mutant strain to survive.
