Energy and Metabolism

ID #2200

What is the difference between rate limiting step and first committed step?

The rate limiting step is the slowest step in a pathway, which determines how fast the whole pathway can be carried out. (Prof. Mehrtens likes to use the metaphor of a slow car on a small country road with several sports cars stuck behind it and unable to pass, to illustrate this idea.) An example is reaction #3 in glycolysis, where fructose-6-phosphate is converted to fructose-1,6-bisphosphate by the enzyme phosphofructokinase. On the other hand, the first committed step is the first step that locks you into a set biochemical pathway, in other words, the point of no turning back in a pathway (as in you now cannot go back to the starting material). An example of a first committed step is pyruvate oxidation during aerobic respiration. The pyruvate from glycolysis will be processed into acetyl-CoA. From then on, that cell is committed to going through the rest of the Krebs cycle and ETC. The cell can no longer use that molecule of pyruvate for fermentation. The rate limiting step and the first committed step are not necessarily the same thing, meaning it could be found at the same step in pathway or at different steps in a pathway. 

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