MCAT Biochemistry Review
Chapter 9: Carbohydrate Metabolism I: Glycolysis, Glycogen, Gluconeogenesis, and the Pentose Phosphate Pathway
The liver maintains glucose levels in blood during fasting through either glycogenolysis or gluconeogenesis. The kidney can also carry out gluconeogenesis, although its contribution is much smaller. These pathways are promoted by glucagon and epinephrine, which act to raise blood sugar levels, and are inhibited by insulin, which acts to lower blood sugar levels. During fasting, glycogen reserves drop dramatically in the first 12 hours, during which time gluconeogenesis increases. After 24 hours, it represents the sole source of glucose. Important substrates for gluconeogenesis are:
· Glycerol 3-phosphate (from stored fats, or triacylglycerols, in adipose tissue)
· Lactate (from anaerobic glycolysis)
· Glucogenic amino acids (from muscle proteins)
Insulin acts to lower blood sugar levels; the counterregulatory hormones, which include glucagon, epinephrine, cortisol, and growth hormone, act to raise blood sugar levels by stimulating glycogenolysis and gluconeogenesis. The regulation of metabolism is discussed in Chapter 12 of MCAT Biochemistry Review.
The last item of this list merits some explaining. Amino acids can be subclassified as glucogenic, ketogenic, or both. Glucogenic amino acids (all except leucine and lysine) can be converted into intermediates that feed into gluconeogenesis, while ketogenic amino acids can be converted into ketone bodies, which can be used as an alternative fuel, particularly during periods of prolonged starvation. See Chapter 11 of MCAT Biochemistry Review for more information on amino acid and protein metabolism.
Amino acids and proteins are extremely important topics for the MCAT. Check out Chapter 1 of MCAT Biochemistry Review for more information on amino acids, peptides, and proteins.
Dietary fructose and galactose can also be converted to glucose in the liver, as described earlier in this chapter.
In humans, while glucose is converted into acetyl-CoA through glycolysis and pyruvate dehydrogenase, it is not possible to convert acetyl-CoA back to glucose. Because most fatty acids are metabolized solely to acetyl-CoA, they are not a major source of glucose either. One minor exception is fatty acids with an odd number of carbon atoms (for example, fatty acid tails containing 17 carbons), which yield a small amount of propionyl-CoA, which is glucogenic.
The pathway of gluconeogenesis is diagrammed in Figure 9.12. Each of the important gluconeogenic intermediates—lactate, alanine, and glycerol 3-phosphate—have enzymes that convert them into glycolytic intermediates. Lactate is converted to pyruvate by lactate dehydrogenase. Alanine is converted to pyruvate by alanine aminotransferase. Glycerol 3-phosphate is converted to dihydroxyacetone phosphate (DHAP) by glycerol-3-phosphate dehydrogenase.
Figure 9.12. Gluconeogenesis
IMPORTANT ENZYMES OF GLUCONEOGENESIS
Most steps in gluconeogenesis represent a reversal of glycolysis and have thus been omitted from the diagram. However, the four important enzymes to know are those required to catalyze reactions that circumvent the irreversible steps of glycolysis in the liver (those catalyzed by glucokinase, phosphofructokinase-1, and pyruvate kinase).
Pyruvate carboxylase is a mitochondrial enzyme that is activated by acetyl-CoA (from β-oxidation). The product, oxaloacetate (OAA), is a citric acid cycle intermediate and cannot leave the mitochondrion. Rather, it is reduced to malate, which can leave the mitochondria via the malate–aspartate shuttle, which is described in Chapter 10 of MCAT Biochemistry Review. Once in the cytoplasm, malate is oxidized to OAA. The fact that acetyl-CoA activates pyruvate carboxylase is an important point. Acetyl-CoA inhibits pyruvate dehydrogenase because a high level of acetyl-CoA implies that the cell is energetically satisfied and need not run the citric acid cycle in the forward direction; in other words, the cell should stop burning glucose. Rather, pyruvate will be shunted through pyruvate carboxylase to help generate additional glucose through gluconeogenesis. Note that the source of acetyl-CoA is not from glycolysis and pyruvate dehydrogenase in this case, but from fatty acids. Thus, to produce glucose in the liver during gluconeogenesis, fatty acids must be burned to provide this energy, stop the forward flow of the citric acid cycle, and produce massive amounts of OAA that can eventually lead to glucose production for the rest of the body.
Because glycolysis contains three irreversible steps (those catalyzed by hexokinase, phosphofructokinase-1, and pyruvate kinase), different enzymes must exist in gluconeogenesis to allow the body to revert pyruvate to glucose.
Phosphoenolpyruvate Carboxykinase (PEPCK)
Phosphoenolpyruvate carboxykinase (PEPCK) in the cytoplasm is induced by glucagon and cortisol, which generally act to raise blood sugar levels. It converts OAA to phosphoenolpyruvate (PEP) in a reaction that requires GTP. PEP continues in the pathway to fructose 1,6-bisphosphate. Thus, the combination of pyruvate carboxylase and PEPCK are used to circumvent the action of pyruvate kinase by converting pyruvate back into PEP. PEPCK is activated by glucagon and cortisol.
Fructose-1,6-bisphosphatase in the cytoplasm is a key control point of gluconeogenesis and represents the rate-limiting step of the process. It reverses the action of phosphofructokinase-1, the rate-limiting step of glycolysis, by hydrolyzing phosphate from fructose 1,6-bisphosphate to produce fructose 6-phosphate. A common pattern to note is that phosphatases oppose kinases. Fructose-1,6-bisphosphatase is activated by ATP and inhibited by AMP and fructose 2,6-bisphosphate. This should make sense: high levels of ATP imply that a cell is energetically satisfied enough to produce glucose for the rest of the body, whereas high levels of AMP imply that a cell needs energy and cannot afford to produce energy for the rest of the body before satisfying its own requirements. Fructose 2,6-bisphosphate (F2,6-BP) is sometimes thought of as a marker for satisfactory energy levels in liver cells. It helps these cells override the inhibition of phosphofructokinase-1 that occurs when high levels of acetyl-CoA are formed, signaling to the liver cell that it should shift its function from burning to storing fuel. F2,6-BP, produced by PFK-2, controls both gluconeogenesis and glycolysis (in the liver). Recall from the earlier discussion of this enzyme and Figure 9.2 that PFK-2 is activated by insulin and inhibited by glucagon. Thus, glucagon will lower F2,6-BP and stimulate gluconeogenesis, whereas insulin will increase F2,6-BP and inhibit gluconeogenesis.
Glucose-6-phosphatase is found only in the lumen of the endoplasmic reticulum in liver cells. Glucose 6-phosphate is transported into the ER, and free glucose is transported back into the cytoplasm, from where it can diffuse out of the cell using GLUT transporters. The absence of glucose-6-phosphatase in skeletal muscle means that muscle glycogen cannot serve as a source of blood glucose and rather is for use only within the muscle. Glucose-6-phosphatase is used to circumvent glucokinase and hexokinase, which convert glucose to glucose 6-phosphate.
Because gluconeogenesis requires acetyl-CoA to occur (to inhibit pyruvate dehydrogenase and stimulate pyruvate carboxylase), gluconeogenesis is inextricably linked to fatty acid oxidation. The source of acetyl-CoA cannot be glycolysis because this would just burn the glucose that is being generated in gluconeogenesis.
Although alanine is the major glucogenic amino acid, almost all amino acids are also glucogenic. Most of these are converted by individual pathways to citric acid cycle intermediates, then to malate, following the same path from there to glucose.
It is important to note that glucose produced by hepatic (liver-based) gluconeogenesis does not represent an energy source for the liver. Gluconeogenesis requires expenditure of ATP that is provided by β-oxidation of fatty acids. Therefore, as mentioned above, hepatic gluconeogenesis is always dependent on β-oxidation of fatty acids in the liver. During periods of low blood sugar, adipose tissue releases these fatty acids by breaking down triacylglycerols to glycerol (which can also be converted to the gluconeogenic intermediate DHAP) and free fatty acids.
Because red blood cells lack mitochondria, they cannot carry out aerobic metabolism. Rather, pyruvate is converted to lactic acid to regenerate NAD+. However, lactate is acidic; it must be removed from the bloodstream to avoid acidifying the blood. Red blood cells deliver this lactate to the liver, where it can be converted back into pyruvate and, through gluconeogenesis, become glucose for the red blood cells to use. This is known as the Cori cycle: glucose is converted to lactate in red blood cells, and lactate is converted to glucose in liver cells.
Although the acetyl-CoA from fatty acids cannot be converted into glucose, it can be converted into ketone bodies as an alternative fuel for cells, including the brain. Extended periods of low blood sugar are thus usually accompanied by high levels of ketones in the blood. Ketone bodies can be thought of as a transportable form of acetyl-CoA that is primarily utilized in periods of extended starvation.
MCAT Concept Check 9.6:
Before you move on, assess your understanding of the material with these questions.
1. Under what physiological conditions should the body carry out gluconeogenesis?
2. What are the four enzymes unique to gluconeogenesis? Which irreversible glycolytic enzymes do they replace?
3. How does acetyl-CoA shift the metabolism of pyruvate?