Regulation of Glycoysis

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.

H3C C C

OO

O

NADH NAD

NADH NAD

H3C CH

C

OHO

O

• 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

HexokinasePhosphofructokinase-1

Pyruvate kinase

Control Points in Glycolysis

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.

Insulin Induced Exocytosis of Glucose Transporter

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.

Effect of ATP on PFK-1 Activity

Effect of ADP and AMP on PFK-1 Activity

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.

Pyruvate Kinase Regulation

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

Other Sugars can

enter 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)