BCH 5045 Graduate Survey of Biochemistry Instructor: Charles Guy Producer: Ron Thomas Director: Marsha Durosier Lecture 44 Slide sets available at: http://hort.ifas.ufl.edu/teach/guyweb/bch5045/index.html
David L. Nelson and Michael M. Cox LEHNINGER PRINCIPLES OF BIOCHEMISTRY Fifth Edition CHAPTER 14 Glycolysis, Gluconeogenesis, and the Pentose Phosphate Pathway 2008 W. H. Freeman and Company
Glycolysis has yielded 4 ATPs and 2 NADH, but 2 ATPs were invested, thus the net ATP gain is 2. Still only a fraction of the free energy in glucose has been captured in ATP and NADH produced by glycolysis. Note the different free energy changes depending on which phosphoanhydride bond is hydrolyzed.
The three phosphates of ATP have a degree of electostatic repulsion which when relieved by a hydrolysis event and given the resonance stabilization of the free phosphate group results in a release of significant amounts of free energy that can be used to couple with a range of reactions to make them thermodynamically favorable under physiological conditions.
At standard conditions, the free energy content of PEP, 1,3-bisphosphoglycerate, phosphocreatine and ATP (AMP to PPi) are listed in order of most to least. Keep in mind that under physiological conditions and concentrations, the free energy content would be different. Find out what energy charge is and how it is calculated.
Glucose flux through glycolysis is tightly regulated to maintain ATP levels and metabolic intermediates used by other pathways at as constant concentration as possible. Regulation of the pathway includes allosteric effectors of hexokinase, PFK, and pyruvate kinase. The pathway is also regulated by glucagon, epinephrine and insulin through changes in gene expression. Warburg was the first to show that tumors operate glycolysis at a much higher rate than the related normal tissue. The reduction of oxidative phosphorylation with increased rate of aerobic glycolysis is known as the Warburg effect.
A 2004 survey of public databases EST revealed that the genes of the glycolysis pathway were overexpressed in 24 cancers accounting for more than 70% of human cancer cases. Genes most overexpressed, encoded glyceraldehyde-3-phosphate dehydrogenase, enolase 1, and also pyruvate kinase. Cancers with overexpression of the majority of the glycolysis genes included lymph node, prostate, and brain cancer while those with only some overexpressed genes included cancers of the cartilage and bone marrow (Altenberg and Greulich, 2004). In general, cancer cells produce large amounts of lactate resulting from aerobic glycolysis, but its role in cancer remains controversial. Some argue that rapid cell proliferation requires aerobic glycolysis. Pyruvate produced by glycolysis is converted to lactate by lactate dehydrogenase. Most of this lactate and also alanine is excreted from the cell as waste, and one byproduct of their generation is a robust production of NADPH. Glutamine is a source of alanine and can be converted into lactate which produces NADPH via the activity of NADP + -specific malate dehydrogenase (malic enzyme). Altenberg and Greulich (2004) Genomics 84, 1014-1020.
Fig. 3 Metabolic pathways active in proliferating cells are directly controlled by signaling pathways involving known oncogenes and tumor suppressor genes. This schematic shows our current understanding of how glycolysis, oxidative phosphorylation, the pentose phosphate pathway, and glutamine metabolism are interconnected in proliferating cells. This metabolic wiring allows for both NADPH production and acetyl-coa flux to the cytosol for lipid synthesis. Key steps in these metabolic pathways can be influenced by signaling pathways known to be important for cell proliferation. Activation of growth factor receptors leads to both tyrosine kinase signaling and PI3K activation. Via AKT, PI3K activation stimulates glucose uptake and flux through the early part of glycolysis. Tyrosine kinase signaling negatively regulates flux through the late steps of glycolysis, making glycolytic intermediates available for macromolecular synthesis as well as supporting NADPH production. Myc drives glutamine metabolism, which also supports NADPH production. LKB1/AMPK signaling and p53 decrease metabolic flux through glycolysis in response to cell stress. Decreased glycolytic flux in response to LKB/AMPK or p53 may be an adaptive response to shut off proliferative metabolism during periods of low energy availability or oxidative stress. Tumor suppressors are shown in red, and oncogenes are in green. Key metabolic pathways are labeled in purple with white boxes, and the enzymes controlling critical steps in these pathways are shown in blue. Some of these enzymes are candidates as novel therapeutic targets in cancer. Malic enzyme refers to NADP + -specific malate dehydrogenase [systematic name (S)-malate:NADP + oxidoreductase (oxaloacetate-decarboxylating)]. From: Vander Heiden, Cantley and Thompson (2009) Science. 324: 1029 1033.
Detection of cancerous tissue is possible with positron emission tomography (PET). 18 F- Labeled 2-fluoro-2- deoxyglucose (FdG) is used to identify where glucose is actively metabolized at dark spots which indicate regions of high glucose utilization.
When aerobic respiration is reduced by low oxygen levels, cells with active glycolysis and energy needs can reduce the level of pyruvate and at the same time regenerate NAD + to insure glycolysis is not NAD + limited.
Seven of the ten reactions of glycolysis are physiologically reversible and three are not under physiological conditions. What controls which direction a reaction will go? What about reactions 1, 3 and 10, can they be reversed?