O

Triacylglycerol. metabolism

Glycerol

Phosphoenolpyruvate

Pyruvate \

Phosphoenolpyruvate

Lactate

Amino acid intermediates

Amino acid intermediates

FIGURE 4-26

Gluconeogenic pathway by which pyruvate, lactate, glycerol, and various amino acid intermediates can be converted into glucose in the liver. Note the points at which each of these precursors, supplied by the blood, enters the pathway.

Lactate

Amino acid intermediates

Amino acid intermediates

FIGURE 4-26

Gluconeogenic pathway by which pyruvate, lactate, glycerol, and various amino acid intermediates can be converted into glucose in the liver. Note the points at which each of these precursors, supplied by the blood, enters the pathway.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART ONE Basic Cell Functions

I. Basic Cell Functions

4. Protein Activity and Cellular Metabolism and 10 (see Figure 4-19) are irreversible, and additional enzymes are required, therefore, to form glucose from pyruvate. Pyruvate is converted to phospho-enolpyruvate by a series of mitochondrial reactions in which CO2 is added to pyruvate to form the four-carbon Krebs-cycle intermediate oxaloacetate. [In addition to being an important intermediary step in glu-coneogenesis, this reaction (Equation 4-4) provides a pathway for replacing Krebs-cycle intermediates, as described earlier.] An additional series of reactions leads to the transfer of a four-carbon intermediate derived from oxaloacetate out of the mitochondria and its conversion to phosphoenolpyruvate in the cytosol. Phosphoenolpyruvate then reverses the steps of gly-colysis back to the level of reaction 3, in which a different enzyme from that used in glycolysis is required to convert fructose 1,6-bisphosphate to fructose 6-phosphate. From this point on, the reactions are again reversible, leading to glucose 6-phosphate, which can be converted to glucose in the liver and kidneys or stored as glycogen. Since energy is released during the glycolytic breakdown of glucose to pyruvate in the form of heat and ATP generation, energy must be added to reverse this pathway. A total of six ATP are consumed in the reactions of gluconeogenesis per molecule of glucose formed.

Many of the same enzymes are used in glycolysis and gluconeogenesis, so the question arises: What controls the direction of the reactions in these pathways? What conditions determine whether glucose is broken down to pyruvate or whether pyruvate is converted into glucose? The answer lies in the concentrations of glucose or pyruvate in a cell and in the control of the enzymes involved in the irreversible steps in the pathway, a control exerted via various hormones that alter the concentrations and activities of these key enzymes (Chapter 18).

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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