31 - Reciprocal Regulation of Glycolysis and Gluconeogenesis

The path of carbon through the glycolytic pathway is shown in the figure. Some of these steps are reversible and catalyzed by the same enzyme acting in either direction, glycolysis or gluconeogenesis. Note that many of these steps are reversible and should be shown with equilibrium arrows. However, because this question asks about reversible and irreversible reactions, the arrows are written in one direction. different enzyme in gluconeogenesis than in glycolysis? 1 2 3 4 5 6 7 8 9 10

1, 3, 10 Three of the steps of glycolysis are essentially irreversible, with negative free energies. Step 1: glucose + ATP →(hexokinase) glucose-6-phosphate + ADP Δ𝐺∘′= −4.0 kcal/mol Step 3: fructose-6-phosphate + ATP → (phosphofructokinase) fructose-1,6-bisphosphate + ADP Δ𝐺∘′=−3.4 kcal/mol Step 10: phosphoenolpyruvate + ADP → (pyruvate kinase) pyruvate + ATP Δ𝐺∘′=−7.5 kcal/mol In gluconeogenesis, steps 1, 3, and 10 are carried out by different enzymes. Step 10 requires two reactions to overcome the energy difference in the reverse direction.

Forming one glucose molecule from two pyruvate molecules via gluconeogenesis requires the use of ____ ATP, _____ GTP, and ______ NADH molecules.

4, 2, 2 In gluconeogenesis, forming one glucose molecule from two pyruvate molecules requires the use of 4ATP, 4GTP, and 2NADH molecules. 2pyruvate + 4ATP + 2GTP + 2NADH + 2H+ + 6H2O ⟶ glucose + 4ADP + 2GDP + 6Pi + 2NAD+ Gluconeogenesis is not a simple reversal of glycolysis. Glycolysis has three steps in which the free‑energy change heavily favors the pathway in the direction of pyruvate formation, and which are thus irreversible in gluconeogenesis. While glycolysis is useful for trapping metabolites into an overall energetically favorable pathway, it means the pathway cannot simply go into reverse when glucose needs to be synthesized. In gluconeogenesis, the conversion of pyruvate to oxaloacetate, and then from oxaloacetate to phosphoenolpyruvate, consumes one ATP and one GTP, respectively. When fructose 1,6‑bisphosphate is hydrolyzed to fructose 6‑phosphate and glucose 6‑phosphate is hydrolyzed to glucose, the ATP molecules originally consumed in glycolysis are not recovered. These three steps use different enzymes than those used in glycolysis so that the irreversible, or energetically unfavorable, steps can be circumvented. The remaining steps in gluconeogenesis are catalyzed by the same enzymes as in glycolysis. Thus, those steps exist at or near equilibrium, with the reaction proceeding in the direction favored by conditions in the cell at that time. The conversion of 3‑phosphoglycerate to 1,3‑bisphosphoglycerate consumes ATP. Next, the conversion of 1,3‑bisphosphoglycerate to glyceraldehyde 3‑phosphate consumes one molecule of NADH. Because forming one glucose molecule requires two pyruvate molecules, each of the reactions requiring an ATP, GTP, or NADH molecule happen twice.

Fructose 2,6‑bisphosphate is a regulator of both glycolysis and gluconeogenesis for the phosphofructokinase reaction of glycolysis and the fructose 1,6‑bisphosphatase reaction of gluconeogenesis. In turn, the concentration of fructose 2,6‑bisphosphate is regulated by many hormones, second messengers, and enzymes. How do these events affect glycolysis and gluconeogenesis? Answer Bank inhibition of PFK‑2 (phosphofructokinase‑2) increased glucagon levels increased levels of cAMP activation of PFK‑2 (phosphofructokinase‑2) increased levels of fructose 2,6‑bisphosphate

Activate glycolysis / Inhibit gluconeogenesis: activation of PFK‑2 (phosphofructokinase‑2) increased levels of fructose 2,6‑bisphosphate Activate gluconeogenesis / Inhibit glycolysis: inhibition of PFK‑2 (phosphofructokinase‑2) increased glucagon levels increased levels of cAMP Explanation: Fructose 2,6‑bisphosphate is an inhibitor of fructose 1,6‑bisphosphatase (FBPase), and therefore inhibits gluconeogenesis. It is also an allosteric activator of PFK‑2, activating glycolysis. PFK‑2 and FBPase make up a bifunctional enzyme with opposing actions. PFK‑2 converts fructose 6‑phosphate to fructose 2,6‑bisposphate. FBPase cleaves a phosphate group from fructose 2,6‑bisphosphate, yielding fructose 6‑phosphate. When PFK‑2 is active, the concentration of fructose 2,6‑bisphosphate increases, and glycolysis is activated (gluconeogenesis is inhibited). When FBPase is active, the concentration of fructose 2,6‑bisphosphate decreases, and gluconeogenesis is activated (the corresponding step of glycolysis is inhibited). Glucagon raises cAMP levels, a second messenger derived from ATP - that is, cAMP amplifies the glucagon signal. Increased cAMP concentration leads to the phosphorylation (and inactivation) of PFK‑2 and to the inactivation of pyruvate kinase. Therefore, increased cAMP concentration leads to the inhibition of glycolysis and the activation of gluconeogenesis.

Complete the statements about the gluconeogenesis pathway in the fasting state. Glucagon levels ______ in the fasting state. _______ FBPase‑2 causes the gluconeogenesis pathway to ________ Pyruvate kinase, in the liver, is ________ and inactive.

Glucagon levels increase in the fasting state. Activate FBPase‑2 causes the gluconeogenesis pathway to speed up Pyruvate kinase, in the liver, is phosphorylated and inactive. Glucagon and insulin signaling are reciprocally regulated. Glucagon levels increase in the fasting state. The pancreas releases glucagon, a peptide hormone. FBPase‑2 is active in the fasting state. Glucagon binding to its receptor leads to protein kinase A activation. Protein kinase A phosphorylates and activates FBPase‑2. Active FBPase‑2 promotes gluconeogenesis. Gluconeogenesis speeds up in the fasting state to maintain blood glucose levels. The liver controls gluconeogenesis to produce free glucose for tissues such as the brain. Pyruvate kinase is phosphorylated in the fasting state. Glucagon activation of protein kinase A causes phosphorylation of pyruvate kinase. Phosphorylation of pyruvate kinase inactivates the enzyme that is responsible for the final step of glycolysis.

Categorize each enzyme based on its specific function in glycolysis, gluconeogenesis, or both pathways. Answer Bank: phosphofructokinase pyruvate kinase phosphoglucose isomerase hexokinase glucose 6‑phosphatase fructose 1,6‑bisphosphatase

Glycolysis: phosphofructokinase pyruvate kinase hexokinase Gluconeogenesis: glucose 6‑phosphatase fructose 1,6‑bisphosphatase Both: phosphoglucose isomerase Explanation: Glycolysis and gluconeogenesis are the opposite of each other in terms of result but not processes. Many steps and enzymes are common to both pathways. Of the ten glycolytic enzymes, three enzymes are specific to glycolysis. Hexokinase, phosphofructokinase, and pyruvate kinase are unique to glycolysis. These reactions are highly exergonic and irreversible, unlike the seven steps common to both pathways. Glucose 6‑phosphatase and fructose 1,6‑bisphosphatase are unique to gluconeogenesis. These enzymes are required to bypass the exergonic reactions of glycolysis. Both pathways contain phosphoglucose isomerase. In all, there are seven shared enzymes between glycolysis and gluconeogenesis. These enzymes catalyze readily reversible reactions. Thus, glucose production or catabolism can be promoted depending on cellular needs.

Match each condition to the appropriate pathway. Answer Bank: increase in glucagon increase in insulin fed fasting increase in fructose 2,6‑bisphosphate increase in ATP increase in AMP increase in acetyl CoA increase in citrate

Glycolysis: conversion of glucose to pyruvate. It's a catabolic process increase in insulin fed increase in fructose 2,6‑bisphosphate (its a potent activator of PFK-1) increase in AMP Gluconeogenesis: production of glucose from non-carbohydrate sources. It's an anabolic process) increase in ATP increase in acetyl CoA increase in citrate increase in glucagon fasting Both the glycolytic and gluconeogenic pathways regulate blood‑glucose levels and are tightly controlled processes. Phosphorylation and allosteric inhibition of enzymes allow the pathways to be turned off or on, depending on cellular requirements. If energy or metabolic intermediates are needed, glycolysis is favored. If there is an excess of energy and metabolic intermediates, gluconeogenesis is favored. After a meal, blood‑glucose and insulin levels rise, activating the glycolytic pathway. Phosphofructokinase regulates a key step of glycolysis, namely the conversion of fructose 6‑phosphate into fructose 1,6‑bisphosphate. Phosphofructokinase is activated when AMP concentrations increase. Fructose 2,6‑bisphosphate regulates phosphofructokinase activity, and fructose 6‑phosphate production increases fructose 2,6‑bisphosphate concentrations. Feedforward stimulation by glucose stimulates glycolysis. When fasting, blood-glucose concentrations drop and glucagon levels rise, leading to the activation of gluconeogenesis. The high concentrations of ATP, citrate, and acetyl CoA signal for glucose production. High levels of acetyl CoA activate pyruvate carboxylase to convert pyruvate to oxaloacetate. Oxaloacetate is ultimately converted into fructose 1,6‑bisphosphate. Fructose 1,6‑bisphosphatase regulates fructose 1,6‑bisphosphate conversion to fructose 6-phosphate. High levels of citrate inhibit phosphofructokinase and activate fructose 1,6‑bisphosphatase, thereby promoting gluconeogenesis.

To answer this question, you may reference the Metabolic Map. Complete the passage about the gluconeogenesis pathway in the fed state. Insulin levels ________ in the fed state. Fructose 1,6‑bisphosphatase is ______ due to ________ levels of fructose 2,6‑bisphosphate. Gluconeogensis __________.

Insulin levels increase in the fed state. Fructose 1,6‑bisphosphatase is inactive due to increasing levels of fructose 2,6‑bisphosphate. Gluconeogensis slows down. Explanation: Insulin levels increase in the fed state. Transport of glucose out of the bloodstream and into a cell requires insulin. After pancreatic beta cells release insulin, glucose can enter cells and metabolic pathways such as glycolysis. Fructose 1,6‑bisphosphatase is inactive in the fed state. High levels of fructose 2,6‑bisphosphate and AMP inhibit fructose 1,6‑bisphosphatase. Inhibition of the enzyme slows down gluconeogenesis. Glycolysis speeds up in the fed state to produce energy and metabolic intermediates such as pyruvate. Increased levels of glucose leads to increased levels of fructose 6‑phosphate, which is converted to fructose 2,6‑bisphosphate. Fructose 2,6‑bisphosphate activates phosphofructosekinase 2 and inhibits gluconeogenesis. Gluconeogenesis slows down in the fed state. Blood glucose levels are already elevated in fed state. The liver does not need to perform gluconeogenesis produce free glucose for the brain and other tissues.

Arrange the steps of gluconeogenesis in order, from pyruvate to glucose. Answer Bank: carboxylation of pyruvate phosphorylation of 3‑phosphoglycerate isomerization of fructose 6‑phosphate hydrolysis of glucose 6‑phosphate hydrolysis of fructose 1,6‑bisphosphate

Pyruvate —> carboxylation of pyruvate —> phosphorylation of 3‑phosphoglycerate —> hydrolysis of fructose 1,6‑bisphosphate —> isomerization of fructose 6‑phosphate —> hydrolysis of glucose 6‑phosphate —> Glucose Explanation:

The path of carbon through glycolysis and gluconeogenesis is shown in the figure. Answer two questions about the regulation of gluconeogenesis. Which step of gluconeogenesis is the main negative control point? Use the numbering system in the image. Select all of the compounds that inhibit the enzyme in this step in the liver. ATP fructose‑2,6‑bisphosphate acetyl‑CoA insulin glucose‑6‑phosphate AMP

Step 8 Enzymes: insulin, fructose-2,6-biphosphate, AMP Explanation: Step 8, catalyzed by the enzyme fructose‑1,6‑bisphosphatase, is the major negative control point of the pathway. The regulator fructose‑2,6‑bisphosphate inhibits fructose‑1,6‑bisphosphate, and therefore gluconeogenesis, and activates phosphofructokinase, driving glycolysis. AMP activates glycogen phosphorylase, and in turn phosphofructokinase, driving glycolysis. Insulin lowers blood glucose levels, in part by suppressing gluconeogenesis. The required main point of control may be found among hexokinase, phosphofructokinase or pyruvate kinase. Among these 3 enzymes, phosphofructokinase is most important for glycolytic pathway. High level of ATP are inhibitor and lower the affinity for fructose 6-phophsate.

During sustained exercise, the ATP/AMP ratio in muscle cells decreases. What effect does this decreased ATP/AMP ratio have on the activity of phosphofructokinase and pyruvate kinase? The activity of both enzymes decreases due to a lack of allosteric binding of ATP. Phosphokinase activity increases and pyruvate kinase activity decreases. The activity of both enzymes remains the same, maintaining normal glucose metabolism. The activity of both enzymes increases due to a lack of allosteric binding of ATP.

The activity of both enzymes increases due to a lack of allosteric binding of ATP. PFK1 is allosterically inhibited by high levels of ATP but AMP reverses the inhibitory action of ATP. Therefore the activity of the enzyme increases when the cellular ATP/AMP ratio is lowered. Due to the allosteric inhibitory effects of ATP on pyruvate kinase, a decrease in ATP results in diminished inhibition and the subsequent stimulation of pyruvate kinase.

In the Cori cycle, when glucose is degraded by glycolysis to lactate in muscle, the lactate is excreted into the blood and returns to the liver. In the liver, lactate is converted back into glucose by gluconeogenesis. For each given enzyme, identify whether it is involved in the glycolysis pathyway, gluconeogenesis pathway, both pathways, or neither pathway. The enzyme triose phosphate isomerase is involved in The enzyme fructose‑1,6‑bisphosphatase is involved in The enzyme transketolase is involved in The enzyme phosphoenolpyruvate carboxykinase is involved in The enzyme phosphofructokinase‑1 is involved in The enzyme enolase is involved in The enzyme hexokinase is involved in The enzyme fumarase is involved in

The enzyme triose phosphate isomerase is involved in both pathways The enzyme fructose‑1,6‑bisphosphatase is involved in gluconeogenesis pathway The enzyme transketolase is involved in neither The enzyme phosphoenolpyruvate carboxykinase is involved in gluconeogenesis pathway The enzyme phosphofructokinase‑1 is involved in glycolysis pathway The enzyme enolase is involved in both pathways The enzyme hexokinase is involved in glycolysis pathway The enzyme fumarase is involved in neither The pathway for glycolysis and gluconeogenesis is shown. Enzymes involved in both pathways are shown in black, enzymes involved only in glycolysis are shown in green, and enzymes involved only in gluconeogenesis are shown in blue. Pyruvate is reduced to lactate or lactic acid in animal tissues under anaerobic conditions by the enzyme lactate dehydrogenase. This uses NADH and regenerates NAD+. Additionally, some cells that lack mitochrondria, such as red blood cells, produce lactate even under aerobic conditions. In the liver, lactate dehydrogenase converts lactate back to pyruvate. Pyruvate is then converted to glucose through gluconeogenesis. Therefore, lactate dehydrogenase functions in both glycolysis and gluconeogenesis. Other enzymes: Glucose 6‑phosphate dehydrogenase, transketolase, and transaldolase are enzymes of the pentose phosphate pathway. Although the pentose phosphate pathway utilizes glucose 6‑phosphate, it is not part of glycolysis or gluconeogenesis. Pyruvate dehydrogenase connects the glycolytic pathway to the citric acid cycle by converting pyruvate to acetyl‑Co A. Fumarase and malate dehydrogenase are enzymes of the citric acid cycle. Pyruvate decarboxylase converts pyruvate to acetaldehyde, and alcohol dehydrogenase converts acetalydehyde to ethanol.

Glycolysis and gluconeogenesis are both pathways that the body utilizes for energy production. Although both pathways employ common enzymes, they are not the reverse of one another. They are also not usually used simultaneously. What pathways are used during intense physical exercise? glycolysis only glycolysis and gluconeogenesis gluconeogenesis only

glycolysis and gluconeogenesis Explanation: The fuel sources for glycolysis are glucose, fructose, and sometimes galactose. The main precursors entering gluconeogenesis are lactate, amino acids, and glycerol. Consider when these compounds are the main sources of energy. Lactate is produced during intense physical exercise. At the same time, glucose is needed to produce energy to maintain the physical exertion. What pathways would be utilized? Both glycolyis and gluconeogenesis are used during intense physical exercise. Glycolysis converts glucose, fructose, and even galactose into energy. This pathway is employed during normal cellular functions as well as during intense physical exercise. Gluconeogenesis is employed when glucose levels are low or exhausted and the body must use other precursors, such as amino acids from protein, lactate, and glycerol for energy. During intense physical exercise there is a high production of lactate, so this precursor is often converted to more glucose through gluconeogenesis.

Phosphofructokinase catalyzes the phosphorylation of fructose 6‑phosphate to fructose 1,6‑bisphosphate in glycolysis. Fructose 1,6‑bisphosphatase catalyzes the hydrolysis of fructose 1,6‑bisphosphate to fructose 6‑phosphate in gluconeogenesis. This is the two step reaction between fructose six phosphate and fructose one six bis phosphate. The reaction from fructose six phosphate to fructose one six bis phosphate is catalyzed by frustose one six bis phosphatase. The reverse reaction is catalyzed by phospho fructo kinase. How does fructose 2,6‑bisphosphate (F26BP) affect the activity of the enzymes phosphofructokinase‑1 (PFK) and fructose 1,6‑bisphosphatase (FBPase)? decreases PFK activity, increases FBPase activity increases PFK activity, decreases FBPase activity decreases PFK activity, decreases FBPase activity increases PFK activity, increases FBPase activity

increases PFK activity, decreases FBPase activity Glycolysis and gluconeogenesis are reciprocally regulated. Fructose 2,6‑bisphosphate promotes glycolysis and slows gluconeogenesis by allosterically regulating the enzymes PFK and FBPase. When F26BP binds to PFK, it increases PFK affinity for its substrate (fructose 6‑phosphate) and decreases its affinity for its inhibitors. When F26BP binds to FBPase, it decreases FBPase affinity for its substrate (fructose 1,6‑bisphosphate).

The processes of gluconeogenesis and glycolysis are said to be reciprocally regulated. Reciprocal regulation means that one cell predominantly uses glycolysis, whereas another predominantly uses gluconeogenesis. molecules that activate or inhibit one process have the opposite effect on the other process. opposing sets of molecules, such as ATP and AMP, have opposite effects on the process. molecules that activate or inhibit one process have the same effect on the other process.

molecules that activate or inhibit one process have the opposite effect on the other process.

Suppose 10 glucose molecules are formed during gluconeogenesis. Calculate the amount of pyruvate, ATP, and NADH molecules required.

pyruvate molecules = 20 ATP molecules = 40 NADH molecules = 20 Explanation: Gluconeogenesis produces glucose from pyruvate in an energy‑intensive process. To answer all three questions, use the chemical equation for gluconeogenesis and multiplication. 2 pyruvate + 2 NADH + 4 ATP + 2 GTP + 6H2O ⟶ glucose + 2 NAD+ + 4 ADP + 2 GDP + 6 Pi + 2H+ One glucose molecule is formed from two pyruvate molecules. To calculate the number of pyruvate molecules required to form 10 glucose molecules, multiply the number of glucose molecules by two. 2 pyruvate x 10 glucose molecules = 20 pyruvate molecules Four ATP molecules are requied to produce one glucose molecule. To calculate the number of ATP molecules required to form 10 glucose molecules, multiply the number of glucose molecules by four. 4 ATP x 10 glucose molecules = 40 ATP molecules Similar to pyruvate, two NADH molecules are required for gluconeogenesis. To calculate the number of NADH molecules required to form 10 glucose molecules, multiply the number of glucose molecules by two. 2 NADH x 10 glucose molecules = 20 NADH molecules