1. Activated Carrier Molecules

• Activated carriers couple energy release from spontaneous, energetically favorable reactions to energetically unfavorable reactions.
• ATP is the most widely used activated carrier molecule.. In one day, each one of us consumes half our body weight in ATP!
• Energy is stored in the phosphate-phosphate bonds of ATP through resonance stabilization and electrostatic repulsion
• All nucleoside triphosphates can readily interchange phosphates and transfer energy between them. For example ATP + GDP <-> ADP + GTP occurs which almost no Gibbs energy change.
• ATP +H₂O -> ADP + PO₃ releases about 7.3 kcal/mole under standard conditions and about 10 - 12 kcal/mole under actual conditions in a real cell. This energy is not the highest or the lowest of activated carriers.
• NAD and NADP are other important activated carrier molecules.
• Acetyl-CoA is another activated carrier.

2. Glycolysis is the first state in the breakdown of glucose to produce activated carriers.

• Glycolysis and subsequent processes (fermentation in prokaryotes and in vertebrate muscle and the citric acid cycle and electron transport chain in eukaryotes) operate by stepwise oxidation.
• Overall view of glycolysis through the electron transport chain (IN Eukaryotes). (fig. 12-3, Lodish)
• Glycolysis involves "priming" steps which require energy, cleavage of the 6 carbon sugar into two three carbon molecules, and energy generation.
• The overall pathway of glycolysis includes steps which have standard free energies which are negative (favorable) and some which are positive (unfavorable). (A diagram by Prof. Jim Clegg, shows this clearly.).
• The cleavage of the six carbon fructose 1,6-biphosphate occurs asymmetrically to produce dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. Then, dihydroxyacetone phosphate is isomerized to form glyceraldehyde-3-phosphate.
• The activated, "high energy" carriers NADH and ATP are produced in energy conserving steps.
• Reactions of glycolysis are catalyzed and regulated by key enzymes:
• Glucose -> glucose 6-phosphate is catalyzed by glucokinase in the vertebrate liver but hexokinase elsewhere. and requires Mg⁺⁺. (ATP must be present as MgATP.) Glucokinase has a higher K_M than hexokinase. When glucose concentration is very high, as after a meal, this enzyme functions most effectively. Note that the phosphorylated form of glucose is used within cells - the phosphorylation provides energy to bring glucose into the cell and labels it to keep it there!
• Fructose 6-phosphate -> Fructose 1,6-biphosphate is catalyzed by 6-phosphofructokinase. This enzyme requires Mg⁺⁺ and is an allosteric enzyme. Inhibitors include ATP, citrate, and long-chain fatty acids. Activators are AMP and ADP. (fig. 12-4, Lodish)
• In vertebrates, fructose 1,6-biphosphate -> dihydroxyacetone phosphate + glyceraldehyde-3-phosphate is catalyzed by an aldolase which uses -SH bonds to the substrate. But, the aldolases in bacteria operate by a different mechanism.
• Glyceraldehyde 3-phosphate -> 1,3-biphosphoglycerate is catalyzed by an allosteric enzyme in which the subunits are negatively cooperative - As each unit binds a substate molecule, the other units become less able to bind and catalyze subsequent substrate molecules.
• Glycogen, a polymer of glucose units, is the major storage form of glucose in vertebrates. (A diagram by Prof. Jim Clegg.).
• Free energy changes for sequential reactions are additive - favorable reactions can overcome unfavorable ones!
• Anaerobic breakdown of pyruvate permits prokaryotes, who don't have any mitochondria, to get rid of the NADH produced in glycolysis. Verterbrates can also do fermentation in the muscles to form lactate (fig. 12-5a, Lodish) (fig. 12-5b, Lodish). (Think about what would happen if our muscles had the enzymes to produce ethanol instead of lactate!)

3. Gluconeogenesis lets lactate produced by muscles regenerate glucose.

• Gluconeogenesis regenerates PEP from Pyruvate, thus forcing the reactions of glycolysis backwards. In this backwards glycolysis, the two priming reactions to add phosphate (via kinases) are replaced with reactions that are catalyzed by phosphorylases. Note that this series of reactions requires energy.




All text and images, not attributed to others, including course examinations and sample questions, are Copyright, 2006, Thomas J. Herbert and may not be used for any commercial purpose without the express written permission of Thomas J. Herbert.