Concept explainers
What would happen if a microorganism that depended on the glyoxylate cycle no longer produced the enzyme isocitrate lyase? Could this defect be corrected by nutrients in the media? If so, which nutrients?
The glyoxylate cycle is a pathway in which the acetyl-CoA is converted to succinate, for the synthesis of carbohydrates. This cycle is similar to the tricarboxylic (TCA) cycle, but certain steps in the TCA cycle are bypassed. Initial and final steps are similar for both cycles. The final result of the glyoxylate cycle is glucose production from fatty acids.
Explanation of Solution
Isocitrate lyase is an enzyme involved in the glyoxylate cycle. Isocitrate lyase (ICL) enzyme cleaves the isocitrate into glyoxylate and succinate. The micro-organisms that undergo glyoxylate cycle use isocitrate lyase and malate synthase in order to synthesize glucose, and also its metabolic intermediates. When the ICL gene was deleted, then the microorganism cannot survive under conditions where the glyoxylate cycle was important, as they cannot synthesize glucose. This is called gene knockout, where the genes of organisms are made inoperative.
This can be corrected by providing succinate and glyoxylate in the media and this is known as nutritional complementation of the knockout, where the absence of this gene is compensated by providing the products produced by the gene in the media. The presence of succinate and glyoxylate in the media can bypass the requirement of this enzyme (ICL). In this case, glucose can also be provided in the media. So, the micro-organisms that cannot undergo the glyoxylate cycle and synthesize glucose, can utilize the glucose available in the media.
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Chapter 12 Solutions
Prescott's Microbiology
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