Lecture: 12/24/2016 CHAPTER 16
Glycolysis
Usain Bolt Chapter 16 Outline
Why is glucose such a prominent fuel in all life forms?
1. Glucose may have been available for primitive biochemical systems because it can form under prebiotic conditions.
2. Glucose is the most stable hexose.
3. Glucose has a low tendency to nonenzymatically
glycosylate proteins.
Glycolysis converts one molecule of glucose to 2 molecules of pyruvate with the generation of 2 molecules of ATP.
Glycolysis can be thought of as occurring in 2 stages:
1. Stage 1 traps glucose in the cell and modifies it so that it can be cleaved into a pair of phosphorylated 3‐carbon compounds.
2. Stage 2 oxidizes the 3‐carbon compounds to pyruvate while generating 2 molecules of ATP.
Hexokinase Traps Glucose in the Cell and Begins Glycolysis
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Upon entering the cell through a specific transport protein, glucose is phosphorylated at the expense of ATP to form glucose 6‐phosphate.
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Hexokinase, which requires Mg2+ or Mn2+ as a cofactor, catalyzes the reaction.
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Hexokinase in muscle (glucokinase in liver), like most kinases, employs substrate‐binding induced fit to minimize hydrolysis of ATP.
The conversion of glucose 6‐phosphate to fructose 6‐phosphate is catalyzed by phosphoglucose isomerase. The reaction is readily reversible.
The carbohydrate is trapped in the fructose form by the addition of a second phosphate to form fructose 1,6‐bisphosphate. This irreversible reaction is catalyzed by the allosteric enzyme phosphofructokinase (PFK).
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The second phase of glycolysis begins with the cleavage of fructose 1,6‐
bisphosphate into dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3‐phosphate (GAP). This readily reversible reaction is catalyzed by the enzyme aldolase.
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GAP can be processed to pyruvate to yield ATP, whereas DHAP cannot. The enzyme triose phosphate isomerase interconverts GAP and DHAP, allowing the DHAP to be further metabolized.
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Triose phosphate isomerase deficiency is lethal and is characterized by severe hemolytic anemia and neurodegeneration.
A compound with high phosphoryl transfer potential, 1,3‐
bisphosphoglycerate, is generated by the oxidation of GAP in a reaction catalyzed by glyceraldehyde 3‐phosphate dehydrogenase.
The formation of glyceraldehyde 1,3‐bisphosphate can be thought of as occurring in two steps: the highly exergonic oxidation of carbon 1 in GAP to an acid, and the highly endergonic formation glyceraldehyde 1, 3‐
bisphosphate from the acid.
These two reaction are linked by the formation of an energy‐rich thioester in the active site of glyceraldehyde 3‐phosphate dehydrogenase.
Free‐energy profiles for glyceraldehyde oxidation followed by acyl‐phosphate formation
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The energy of oxidation of the carbon atom, initially trapped as 1,3‐bisphosphoglucerate, is used to form ATP. •
Glyceraldehyde 1,3‐bisphosphoglycerate has a greater phosphoryl transfer potential than ATP, and thus can be used to power the synthesis of ATP from ADP and Pi in a reaction‐
catalyzed phosphoglycerate kinase.
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3‐Phosphoglycerate is converted into 2‐phosphoglycerate by phosphoglycerate mutase. A dehydration reaction, catalyzed by enolase, results in the production of phosphoenolpyruvate (PEP). •
Phosphoenolpyruvate is a high phosphoryl‐transfer compound because the presence of the phosphate traps the compound in the unstable enol tautomer.
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ADP is phosphorylated at the expense of PEP, generating ATP and pyruvate, in a reaction catalyzed by pyruvate kinase.
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The net reaction for glycolysis is
Diverse fates of pyruvate
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The conversion of glucose into pyruvate generates ATP, but for ATP synthesis to continue, NADH must be reoxidized to NAD+. This vital coenzyme is derived from the vitamin niacin (B3).
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NAD+ can be regenerated by further oxidation of pyruvate to CO2, or by the formation of ethanol or lactate from pyruvate.
Ethanol and lactate can be formed by reactions that include NADH. Alternatively, a two‐carbon unit from pyruvate can be coupled to coenzyme A (see Chapter 18) to form acetyl CoA.
• Fermentations are ATP‐generating pathways in which electrons are removed from one organic compound and passed to another organic compound. • The formation of ethanol from pyruvate regenerates NAD+. Pyruvate carboxylase requires the vitamin thiamine (B1).
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The conversion of glucose into two molecules of ethanol is called alcoholic fermentation.
• The NADH generated by glyceraldehyde 3‐phosphate dehydrogenase is oxidized by alcohol dehydrogenase, regenerating NAD+.
• NADH can also be oxidized by converting pyruvate to lactate in a reaction catalyzed by lactate dehydrogenase.
• The conversion of glucose into 2 molecules of lactate is called lactic acid fermentation.
Maintaining redox balance
in lactic acid fermentation.
• Obligate anaerobes cannot survive in the presence of oxygen. Some obligate anaerobic microorganisms are pathogenic.
• Skeletal muscles in most organisms can function anaerobically for a short period of time, resulting in the build‐up of lactate.
• Fructose from table sugar or high‐fructose corn syrup and galactose from milk sugar are converted into glycolytic intermediates.
• In the liver, fructose is metabolized by the fructose 1‐phosphate pathway.
• In other tissues, such as adipose tissue, fructose is directly phosphorylated by hexokinase.
Excess fructose consumption has been linked to obesity, fatty liver, and the development of type 2 diabetes.
Because of the manner in which fructose is processed in the liver, the key regulatory enzyme of glycolysis, phosphofructokinase, is by‐passed.
Excess acetyl CoA is synthesized and converted to fats.
• Galactose is converted into glucose 6‐phosphate by the galactose‐
glucose conversion pathway, which begins with the phosphorylation of galactose by galactokinase.
• The sum of the reaction of the galactose‐glucose conversion pathway is:
• Glucose 1‐phosphate can be converted into glucose 6‐phosphate by phosphoglucomutase.
• Lactose intolerance or hypolactasia occurs because most adults lack the enzyme to degrade lactose.
• Northern Europeans have a mutation that prevents the decline of lactase activity after weaning.
• In lactase‐deficient individuals, gut bacteria metabolize lactose, generating CH4 and H2, and disrupt water balance in the intestine.
Lactose tolerance is most common in Europe.
• Classic galactosemia results if galactose 1‐phosphate uridyl transferase
activity is deficient.
• Symptoms include failure to thrive, jaundice, and liver enlargement that can lead to cirrhosis. Cataract formation may also occur. • Cataracts, a clouding of the lens due to water accumulation, form because the galactose is converted into galactitol, which is poorly metabolized. Galactitol is osmotically active and causes water to diffuse into the lens.
• Enzymes catalyzing irreversible reaction in metabolic pathways are potential control sites.
The allosteric regulation of phosphofructokinase.
• In glycolysis, these enzymes are hexokinase, phosphofructokinase, and pyruvate kinase.
• Phosphofructokinase is the key regulator of glycolysis in mammals. The enzyme is allosterically inhibited by ATP and allosterically stimulated by AMP.
• When ATP needs are great, adenylate kinase generates ATP from 2 ADP.
• AMP then becomes the signal for the low‐
energy state.
A high level of ATP inhibits the enzyme by decreasing its affinity for fructose 6‐
phosphate. AMP diminishes the inhibitory effect of ATP. The regulation of glycolysis in muscle
• Hexokinase is allosterically inhibited by glucose 6‐phosphate.
• In muscle, glycolysis is regulated to meet the energy needs of contraction.
• Pyruvate kinase is inhibited by the allosteric signals ATP and alanine, and stimulated by fructose 1, 6‐bisphosphate, the product of the phosphofructokinase reaction.
• The key regulators of phosphofructokinase in liver are citrate, which reports on the status of the citric acid cycle, and fructose 2,6‐
bisphosphate.
• Citrate inhibits phosphofructokinase while fructose 2,6‐bisphosphate is a powerful activator.
The regulation of phosphofructokinase by fructose 2,6‐bisphosphate.
The activation of phosphofructokinase by fructose 2,6‐bisphosphate.
(A) The sigmoidal dependence of velocity on substrate concentration becomes hyperbolic in the presence of 1 mM fructose 2,6‐bisphosphate, indicating that more of the enzyme is active at lower substrate concentrations in the presence of fructose 2,6‐
bisphosphate. (B) (B) ATP, acting as a substrate, initially stimulates the reaction. As the concentration of ATP increases, it acts as an allosteric inhibitor. The effect of ATP is reduced by fructose 2,6‐bisphosphate, which renders the enzyme less sensitive to ATP inhibition
• Hexokinase is an allosteric enzyme in the liver as it is in muscle.
• The enzyme primarily responsible for phosphorylating glucose in the liver is glucokinase (hexokinase IV). Glucokinase
is active only after a meal, when blood glucose levels are high.
• Pyruvate kinase in the liver is regulated allosterically as it is in muscle. However, liver pyruvate kinase is also regulated by covalent modification. Low blood glucose leads to the phosphorylation and inhibition of liver pyruvate kinase.
The control of the catalytic activity of pyruvate kinase
Pyruvate kinase is regulated by allosteric effectors and covalent modification. Five glucose transporters, termed GLUT1‐5, facilitate the movement of glucose across the cell membrane.
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Rapidly growing tumors obtain ATP by metabolizing glucose to lactate even in the presence of oxygen, a process termed aerobic glycolysis or the Warburg effect.
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When patients are infused with a non‐metabolizable analog of glucose, tumors are readily visualized by tomography because of their excess metabolism of glucose.
• Aerobic glycolysis generates lactic acid, which acidifies the environment and may allow further tumor growth.
• The rapid accumulation of glucose and glucose 6‐phosphate provide building blocks for a host of other biomolecules.
• Because tumors can grow faster than the blood vessels, aerobic glycolysis allows growth in the absence of oxygen.
The transcription factor hypoxia‐inducible transcription factor 1 (HIF‐1) facilitates aerobic glycolysis.
Exercise training also stimulates HIF‐1, which enhances the ability to generate ATP anaerobically and stimulates new blood vessel growth.
• Insulin is secreted by β cells of the pancreas in response to high blood levels of glucose. This secretion is stimulated by the metabolism of glucose by the β cells.
• Glucose enters β cells through GLUT2 and is metabolized to pyruvate, and the pyruvate is subsequently oxidized to CO2 and H2O.
• The increase in ATP closes a K+
channel, which alters the charge across the cell membrane. This alteration in turn opens Ca2+ channels. The influx of Ca2+ ions stimulates the release of insulin.