Integration of metabolism

We have been working our way through the metabolism one pathway at a time. In earlier modules, we've discussed glycolysis, PDC, β-oxidation, the citric acid cycle, and oxidative phosphorylation. In this module, we continue with glycogen metabolism, gluconeogenesis, the pentose phosphate pathway, and the urea cycle.

Within a liver cell, which is capable of the full complement, these pathways operate simultaneously. How are the pathways coordinated? Key ideas include the need to prevent a futile cycle. When glycolysis is operating at full tilt, for example, gluconeogenesis will be running at only a minimal basal level. Other key ideas include the recognition of key crossroads between the pathways such as glucose-6-phosphate, acetyl CoA, and oxaloacetate, as well as familiarity with the primary modes of regulation within each pathway.

The exam is not likely to be as sophisticated as you may fear within this domain, or as sophisticated as it can be within other areas of biochemistry like enzyme kinetics. There is only so far you can be expected to have gotten without the specific pathways of amino acid degradation and the biosynthesis of amino acids, lipids, purines and pyrimidines, so try not to worry. A good focus for MCAT level study of metabolic integration is regulation of glucose and glycogen metabolism in the liver including the signaling pathways.