Chapter 11 (part 2) Regulation of Glycolysis
Pyruvate can go in three major directions after glycolysis • Under aerobic conditions pyruvate is oxidized to Acetyl-CoA which can enter Citric acid (TCA) cycle. • Under anaerobic conditions pyruvate can be reduced to ethanol (fermentation) or lactate • Under anaerobic conditions formation of ethanol and lactate is important in the oxidization NADH back to NAD+ • Under aerobic conditions NADH is oxidized to NAD+ by the respiratory electron transport chain.
Need to recycle NAD+ from NADH if gylcolysis is to continue under anaerobic conditions
Lactate formation • In animals under anaerobic conditions pyruvate is converted to lactate by the enzyme lactate dehydrogenase • Impt for the regeneration of NAD+ under anaerobic conditions.
The circulatory systems of large animals are not efficient enough O2 transport to sustain long periods of muscular activity. • Anaerobic conditions lead to lactacte accumulation and depletion of glycogen stores • Short period of intense activity must be followed by recovery period • Lactic acidosis causes blood pH to drop Cori Cycle
Alcohol Fermentation • Important for the regeneration of NAD+ under anaerobic conditions • Process common to microorganisms like yeast • Yields neutral end products (CO2 and ethanol) • CO2 generated impt in baking where it makes dough rise and brewing where it carbonates beer.
Free Energy Change in Glycolysis Hexokinase Phosphofructokinase-1 Pyruvate kinase
Regulation of Hexose Transporters • Intra-cellular [glucose] are much lower than blood [glucose]. • Glucose imported into cells through a passive glucose transporter. • Elevated blood glucose and insulin levels leads to increased number of glucose transporters in muscle and adipose cell plasma membranes.
Regulation of Hexokinase • Glucose-6-phosphate is an allosteric inhibitor of hexokinase. • Levels of glucose-6-phosphate increase when down stream steps are inhibited. • This coordinates the regulation of hexokinase with other regulatory enzymes in glycolysis. • Hexokinase is not necessary the first regulatory step inhibited.
Regulation of PhosphoFructokinase (PFK-1) • PKF-1 has quaternary structure • Inhibited by ATP and Citrate • Activated by AMP and Fructose-2,6-bisphosphate • Regulation related to energy status of cell.
PFK-1 regulation by adenosine nucleotides • ATP is substrate and inhibitor. Binds to active site and allosteric site on PFK. Binding of ATP to allosteric site increase Km for ATP • AMP and ADP are allosteric activators of PFK. • AMP relieves inhibition by ATP. • ADP decreases Km for ATP • Glucagon (a pancreatic hormone) produced in response to low blood glucose triggers cAMP signaling pathway that ultimately results in decreased glycolysis.
Regulation of PFK by Fructose-2,6-bisphosphate • Fructose-2,6-bisphosphate is an allosteric activator of PFK in eukaryotes, but not prokaryotes • Formed from fructose-6-phosphate by PFK-2 • Degraded to fructose-6-phosphate by fructrose 2,6-bisphosphatase. • In mammals the 2 activities are on the same enzyme • PFK-2 inhibited by Pi and stimulated by citrate
Glucagon Regulation of PFK-1 in Liver • G-Protein mediated cAMP signaling pathway • Induces protein kinase A that activates phosphatase activity and inhibits kinase activity • Results in lower F-2,6-P levels decrease PFK-1 activity (less glycolysis)
Regulation of Pyruvate Kinase • Allosteric enzyme • Activated by Fructose-1,6-bisphosphate (example of feed-forward regulation) • Inhibited by ATP • When high fructose 1,6-bisphosphate present plot of [S] vs Vo goes from sigmoidal to hyperbolic. • Increasing ATP concentration increases Km for PEP. • In liver, PK also regulated by glucagon. Protein kinase A phosphorylates PK and decreases PK acitivty.
Deregulation of Glycolysis in Cancer Cells • Glucose uptake and glycolysis is ten times faster in solid tumors than in non-cancerous tissues. • Tumor cells initally lack connection to blood supply so limited oxygen supply • Tumor cells have fewer mitochondrial, depend more on glycolysis for ATP • Increase levels of glycolytic enzymes in tumors (oncogene Ras and tumor suppressor gene p53 involved)
Pasteur Effect • Under anaerobic conditions glycoysis proceeds at hire rates than during aerobic conditions • Slowing of glycolysis in presence of oxygen is the Pasteur Effect. • Cells sense changes in ATP supply and demand and modulate glycolysis
How other sugars enter glycolysis • Mannose can be phosphorylated to mannose-6-phosphate by hexokinase and then converted to fructose-6-phosphate by phosphomannose isomerase. • Fructose can be phosphorylated by fructokinase to form fructose-1 phosphate (F-1-P). F-1-P can then be converted to glyceraldehyde and DHAP by F-1-P aldolase. Triose kinase then converts glyceraldehyde to G-3-P.
Galactosemia • Deficiency of galactose-1- phosphate uridylytransferase. • galactose-1-phosphate accumulates • Leads to liver damage • Untreated infants fail to trive often have mental reatrdation. • Can be treated with galactose free diet.
Lactose Intolerance • Humans undergo reduction in lactase at 5 to 7 years of age. • In lactase deficient individuals, lactose is metabolized by bacteria in the large intestine. • Produce CO2, H2 and short chain acids. • Short chain acids cause ionic imbalance in intestine (diarrhea)