Glycolysis

• Glucose used for storage becomes glycogen, starch and sucrose.
• Glucose is oxidated via glycolysis for form pyruvate.
• Glucose oxidated via the pentose phosphate pathway forms ribose 5-phosphate
• Glucose used for the synthesis of structural polymers forms the extracellular matrix and cell wall polysaccharides.
• Metabolites like glucose are often activated with a high energy group before their catabolism.
• Glycolysis is a nearly universal 10 step metabolic pathway for producing ATP by the oxidation of glucose. In this process, two molecules of ATP are invested to activate glucose, but the products of the pathway include four ATP, as well as NADH (a form of reducing power) and the triose pyruvate, which can be metabolized further in other pathways.
• Glucose and other hexoses and hexose phosphates obtained from stored polysaccharides or dietary carbohydrates feed into the glycolytic pathway. By using a common pathway for a number of starting materials, the cell economizes on the number of enzymes that must be synthesized and simplifies the regulation of the common pathway.
• Pyruvate formed under anaerobic conditions is reduced to lactate with electrons from NADH, recycling NADH to NAD+ and allowing continued glycolysis in the processes of lactate or alcohol fermentation.
• Manipulation of the fermentable material and the microorganisms present allows the synthesis of a variety of industrial products and foods.
• Gluconeogenesis is the synthesis of glucose from simpler precursors like pyruvate and lactate. Although it uses seven of the ten enzymes that also act in glycolysis, gluconeogenesis must bypass three of the most exergonic steps in glycolysis with energetically favorable reactions unique to gluconeogenesis.
• Glycolysis and gluconeogenesis are reciprocally regulated so that both processes do not occur simultaneously in a futile cycle. Most regulatory mechanisms act on reactions that are unique to each pathway.
• The pentose phosphate pathway is an alternative pathway for glucose oxidation. It yields pentoses for nucleotide synthesis and reduced cofactors for biosynthesis of fatty acids, sterols, and many other compounds.
• Glycolysis is an almost universal central pathway of glucose catabolism.
• Glycolysis is the process by which a molecule of glucose is degraded in a series of enzyme catalyzed reactions to yield two molecules of the three carbon compound pyruvate. Some free energy is conserved as ATP and NADH.
• In the preparatory phase of glycolysis, ATP is consumed, delta G of the intermediates increases, and hexose carbon chains are converted to glyceraldehyde 3-phosphate.
• In the preparatory phase of glycolysis, glucose is converted to glucose 6-phosphate by hexokinase coupled with ATP. Glucose 6-phosphate is converted to fructose 6-phosphate by phosphohexose isomerase. Fructose 6-phosphate is converted to fructose 1,6-bisphosphate by phosphofructokinase-1 coupled with ATP. Fructose 1,6-bisphosphate is converted to glyceraldehyde 3-phosphate and dihydroxyacetone phosphate by aldolase. Last, the produced dihydroxyacetone is converted to glyceraldehyde 3-phosphate by triose phosphate isomerase.
• The payoff phase of glycolysis is the energy conserved as 2 ATP and 2 NADH and 2 pyruvate.
• In the payoff phase of glycolysis, the 2 glyceraldehyde 3-phosphate is converted into 2 1,3-bisphosphoglycerate by glyceraldehyde 3-phosphate dehydrogenase and produces 2 NADH and 2 H+. Then the 2 1,3-bisphosphoglycerate is converted into 2 3-phosphoglycerate by phosphoglycerate kinase, producing 2 ATP. The 2 3-phosphoglycerate is converted to 2 2-phosphoglycerate by phosphoglycerate mutase. The 2 2-phosphoglycerate is converted into 2 phosphoenolpyruvate by enolase, producing water. Last, the 2 phosphoenolpyruvate is converted into 2 pyruvate by pyruvate kinase, producing 2 ATP.
• What product of the preparatory phase of glycolysis is required, but at twice the concentration, as a reactant in the payoff phase?

  • Dihydroxyacetone phosphate
  • Pyruvate
  • NAD+
  • ADP
  • Phosphate

  ADP. The preparatory phase converts two ATP to two ADP, and the payoff phase converts four ADP to four ATP. Thus, the net ATP yield from glycolysis is two ATP per molecule of glucose.
• There are three noteworthy chemical transformations

  • Degradation of the carbon skeleton of glucose to yield pyruvate
  • Phosphorylation of ADP to ATP by compounds with high phosphoryl group transfer potential, formed during glycolysis
  • Transfer of a hydride ion to NAD+, forming NADH
• The chemical logic of glycolysis, part one

  1. Phosphorylation occurs on C-6 of glucose, as C-1 is a carbonyl group and cannot be phosphorylated.
  2. Isomerization moves the carbonyl to C-2, a prerequisite for steps three and four
  3. C-1, now bearing a hydroxyl group, can be phosphorylated. This ensures that both products of C-C bond cleavage are phosphorylated, and thus interconvertible.
  4. The carbonyl group at C-2 facilitates C-C bond cleavage at the right location to yield two 3-carbon products by the reverse of an aldol condensation.
• The chemical logic of glycolysis, part two

  5. There is the interconversion of dihydroxyacetone phosphate and glyceraldehyde 3-phosphate funnels both products into a single pathway.

  6. Oxidative phosphorylation of glyceraldehyde 3-phosphate, with one NADH produced, is a prerequisite for ATP production in step seven.

  7. ATP production

  8. The remaining phosphoryl group moves from C-3 to C-2, setting up the final steps of the pathway

  9. Dehydration activates the phosphoryl for transfer to ADP in step ten.

  10. ATP production.
• The overall equation for glycolysis is glucose + 2 NAD+ + 2ADP + 2 Pi --> 2 pyruvate + 2 NADH + 2 H+ + 2 ATP + 2 H2O.
• The reduction of NAD+ in glycolysis proceeds by enzymatic transfer of a hydride ion from the aldehyde group of glyceraldehyde 3-phosphate to the nicotinamide ring of NAD+, yielding NADH.
• The conversion of glucose to pyruvate is exergonic:

  • Glucose + 2 NAD+ ---> 2 pyruvate + 2 NADH + 2 H+
  • delta G'degree =-146 kJ/mol
• The formation of ATP from ADP and Pi is endergonic

  • 2 ADP + 2 Pi --> 2 ATP + 2 H2O
  • delta G'degree = 61.0 kJ/mol
• The sum of the conversion of glucose to pyruvate and the formation of ATP from ADP and Pi gives the overall standard free energy change of glycolysis, delta G'degree sum.

  • delta G'degree sum = -146kJ/mol + 61.0 kJ/mol
  • delta G'degree sum = -85 kJ/mol
• Under standard and cellular conditions, glycolysis is essentially irreversible.
• Which enzyme catalyzes the most exergonic reaction of glycolysis?

  • Hexokinase
  • Triose phosphate isomerase
  • Aldolase
  • Phosphofructokinase-1
  • Pyruvate kinase

  Phosphofructokinase-1 catalyzes the transfer of a phosphoryl group from ATP to fructose 6-phosphate to yield fructose 1,6-bisphosphate. The standard free energy change for this reaction is -14.2 kJ/mol.
• Energy stored in pyruvate can be extracted by:

  • Aerobic processes
  • Oxidative reactions in the citric acid cycle
  • Oxidative phosphorylation
  • Anaerobic processes
  • Reduction to lactate
  • Reduction to ethanol
• Pyruvate can provide the carbon skeleton for alanine synthesis or fatty acid synthesis.
• All nine intermediates in glycolysis are phosphorylated.
• The functions of the phosphoryl groups

  • Prevent glycolytic intermediates from leaving the cell
  • Serve as essential components in the enzymatic conservation of metabolic energy
  • Lower the activation energy and increase the specificity of the enzymatic reactions.
• In the preparatory phase of glycolysis

  • Two molecules of ATP are invested to activate glucose to fructose 1,6-bisphosphate
  • The bond between C-3 and C-4 of fructose 1,6-bisphosphate is then broken to yield two molecules of triose phosphate
• Hexokinase activates glucose by phosphorylating at C-6 to yield glucose 6-phosphate

  • ATP serves as the phosphoryl donor
  • Hexokinase requires Mg2- for its activity
  • Irreversible under intracellular conditions
• The conversion of glucose to glucose 6-phosphate with hexokinase has a delta G'degree = -16.7 kJ/mol
• Hexokinase is present in nearly all organisms.
• Humans encode 4 hexokinases (I to IV) that catalyze the same reaction.
• Isozymes are two or more enzymes that catalyze the same reaction but are encoded by different genes.
• How is glucose kept inside the cell, against a concentration gradient?

  • Glucose is kept inside the cell by active transport pumps.
  • Glucose is kept inside the cell by conversion to glucose 6-phosphate
  • Glucose is kept inside the cell through rapid conversion to pyruvate
  • There are no glucose transporters to pump glucose out of the cell.
  • Glucose is converted to fructose and there are no fructose transporters

  Glucose is kept inside the cell by conversion to glucose 6-phosphate. Phosphorylation traps glucose inside the cell because the phosphate group has a negative charge at physiological pH.