Glycolysis Flashcards
Represent the three most abundant six-carbon sugars in most people’s diets
Glucose, Fructose, and Galactose
Because of their structural similarity, fructose and galactose metabolism are able to integrate into
-only requires a few enzymatic steps
Glucose Metabolism
The bulk of glucose’s chemical energy remains untapped as glycolysis ends with the formation of
Pyruvate
Fully oxidized to CO2 and H2O in the citric acid cycle
Pyruvate
The conversion of glucose to pyruvate takes place in which two stages?
- ) Energy investment stage
2. ) Energy generation stage
While in the energy investment stage phosphorylated intermediates are made at the expense of
ATP
In the energy generation phase, substrate level phosphorylation generates how many ATP per glucose molecule?
2
At the end of glycolysis, we also see the production of
2 NADH
In the first stage of glucose metabolism, two high-energy ATP molecules are consumed in the production of
Fructose 1,6-bisphosphate
This ‘energy investment’ phase includes two of the 3 enzymatic steps critical to regulation of flux through glycolysis, both of which are
Phosphorylation reactions
Highly hydrophilic compounds, unable to diffuse passively across the hydrophobic barrier of the cytoplasmic membrane
Sugars
Function to move glucose down its concentration gradient into the interior of the cell
GLUT1-GLUT5
Primarily involved in glucose uptake from the blood
GLUT1, GLUT3, and GLUT4
Found in the liver, kidney and pancreas, and can transport glucose into and out of cells
GLUT2
Allows for insulin regulated uptake and storage of glucose in fat and muscle during times of sufficient blood glucose
GLUT4 insulin sensitivity
Recall that a low Km value implies
High receptor affinity for a substrate
Have low Km values and no insulin sensitivity to insure a constant basal uptake of glucose
GLUT1 and GLUT3
Has a higher Km, and will more easily take in glucose during periods of high blood glucose
GLUT2
In Pancreatic B cells this GLUT-2 mediated uptake results in insulin secretion which allows the activation of
GLUT4
Has a lower Km then GLUT-2, and will insure that skeletal and adipose tissue extract glucose from the blood faster than the liver
GLUT4
Unusual in that it is the primary transporter for fructose (instead of glucose) in the small intestine and the testes
GLUT5
Phosphorylation of intracellular glucose helps to maintain a ‘downhill’ gradient of sugar from outside to inside the cell, and it traps the phosphorylated sugar
Inside of the cell
The first covalent modification to intracellular glucose is the addition of a phosphate onto the #6 carbon, making
Glucose-6-phosphate (G-6-P)
This reaction is catalyzed by one of two enzymes, depending upon the tissue in question. These are referred to as
Isoenzymes
Functions in most tissues, and has a low Km and a low Vmax, but a broad specificity for six carbon sugars
Hexokinase
Located in the liver and in the pancreas, and has a high Km and a high Vmax
Glucokinase
Permits the liver to respond to high concentrations of blood sugar that are obtained following a meal
The high Km of glucokinase in the liver
Plays a central role in the regulation of blood glucose, and this capacity of glucokinase helps it to diminish the hyperglycemia that follows a meal
Liver
Serves peripheral tissues by permitting them to metabolize only the quantity of glucose that meets their immediate needs, but also to be able to do so efficiently, even at lower blood glucose concentrations
Low Km of Hexokinase
Consistent with this role hexokinase expression is not effected by
Insulin
Importantly, high levels of G6P inhibit
Hexokinase
Glucokinase is not inhibited by its product, but it is inhibited by
-Binds to the glucokinase regulatory protein
Fructose 6-phosphate
This inhibition is reversed under conditions either of high intracellular glucose or
Fructose-1-phosphate
Glucokinase expression is positively influenced by
Insulin
GLUT-1 & 3 expressing cells insure a steady input of glucose into the cytosol where it can be “secured” by phosphorylation via
Hexokinase (which also has a low Km)
This process is inhibited in times of glucose excess by
-Ensures that cells do not make more glucose than metabolically necessary
G6P
A similar rationale can be seen with the GLUT-2 expressing liver. During glucose excess insulin will trigger increased
Glucokinase Expression
Because glucokinase does not have feedback inhibition it will be able to take in excess glucose even though its transporter and glucokinase have higher
Km’s
Regardless of which kinase is used, synthesis of glucose 6 phosphate from glucose and ATP is an
Irreversible Reaction
Following the isomerization of glucose 6-phosphate to fructose 6-phosphate, a second, tightly regulated, phosphorylation takes place, catalyzed by the enzyme
Phosphofructokinase-1 (PFK-1)
This Reaction catalyzed by PFK-1 produces
Fructose 1,6-bisphosphate
The most influential and regulatory step in glycolysis
The PFK-1 production of F-1,6-BP
The PFK-1 production of F-1,6-BP is an
Irreversible reaction
PFK-1 is negatively regulated by
ATP and Citrate
This is logical as both of these compounds, when present in the cell at high concentrations, are indicative of an
Energy Rich State
PFK-1 is also inhibited by
Low pH
PFK-1 is POSITIVELY regulated by
AMP and Fructose-2,6-BP
An allosteric activator of PFK-1
Fructose-2,6-BP
Intracellular levels of fructose 2,6-bisphosphate are altered by the variable action of a dual function enzyme called
PFK-2/FBP-2
PFK-2/FBP-2 has two different
Catalytic activities
Responsible for the synthesis of fructose 2,6-bisphosphate, from fructose 6-phosphate and ATP
PFK-2 (a kinase)
Catalyzes essentially the reverse of this reaction, production of fructose 6 phosphate and Pi from fructose 2,6-bisphosphate
FBP-2 (a phosphatase)
The PFK-2/FBP-2 itself is regulated by
Phosphorylation or Dephosphorylation
Signal transduction cascades beginning with insulin and glucagon, on cellular membrane receptors, and ending with protein kinase and phosphatase activities regulate
Phosphorylation and dephosphorylation of PFK-2/FBP-2
During the well-fed state, glucagon levels decrease and insulin levels rise. These changes lead to
Dephosphorylation of PFK-2/FBP-2
Dephosphorylation of PFK-2/FBP-2 leads to active
-hence more Fructose-2,6-BP
PFK-2
Elevated levels of fructose 2,6-bisphosphate activate
PFK-1
Thus the rate of glycolysis in the liver is
Increased
Fructose 2,6-bisphosphate acts as an intracellular signal, indicating that glucose is
Abundant
During fasting and starvation, glucagon levels rise and insulin levels decrease. These changes lead to
Phosphorylation of PFK-2
This leads to an inactivation of
PFK-2
Reduced levels of fructose 2,6-bisphosphate inhibit the activity of PFK-1, thus the rate of
Liver glycolysis
Subject to regulation on a local, intracellular level via ATP, AMP, H+, and citrate, small molecule indicators of the local and immediate metabolic/physiologic state
PFK-1
Also subject to regulation on a larger, organ scale via insulin and glucagon, with fructose 2,6-bisphosphate as an intermediary
PFK-1
Splits fructose 1,6 bisphosphate into glyceraldehyde 3-phosphate and dihydroxyacetone phosphate
Aldolase A
Catalyzes the interconversion of these three-carbon compounds
Triose Phosphate Isomerase
Glyceraldehyde 3-phosphate is then recruited by glyceraldehyde 3-phosphate dehydrogenase to produce
1,3-BPG
This intermediate leads to the first production of
ATP
Action of the isomerase permits us to start with one molecule of glucose, and proceed on with two molecules of
1,3-BPG
Take note that in order to oxidize glyceraldehyde 3-phosphate, glyceraldehyde 3-phosphate dehydrogenase requires
NAD+
The phosphate on carbon #1 of 1,3 bisphosphoglycerate is transferred to
ADP
The phosphate on carbon #1 of 1,3 bisphosphoglycerate is transferred to ADP by
-produces ATP and phosphoglycerate
Phosphoglycerate Kinase
This reaction is
Reversible
3-phosphoglycerate is ocnverted into 2-phosphoglycerate by
-Reversible and unregulated
Phosphoglycerate Mutase
Converts 2-phosphoglycerate into phosphoenolpyruvate, with the release of one molecule of water
Enolase
Participates in the second substrate level phosphorylation in glycolysis, and the production of a second ATP molecule
Phosphoenolpyruvate
Dephosphorylates phosphoenolpyruvate, producing ATP and pyruvate
-An IRREVERSIBLE reaction
Pyruvate Kinase
Regulation of this step takes a somewhat unusual form. Fructose 1,6-bisphosphate acts in a feed-forward manner to stimulate
Pyruvate Kinase
Liver pyruvate kinase is itself subject to direct phosphorylation, causing
Inactivation
The phosphorylation (inctivation) and Dephosphorylation (activation) of pyruvate kinase is mediated by
Glucagon and Insulin respectively
The decreased breakdown of PEP is also helpful during glucagon-induced
Gluconeogenesis
Subject to local, intracellular regulation by fructose 1,6 bisphosphate and it’s also subject to higher order regulation via protein phosphorylation/dephosphorylation and the hormones insulin and glucagon
Pyruvate Kinase
Glycolysis will come to a halt without the ability to recycle
NADH to NAD+
What are the two mechanisms by which cells can replenish NAD levels?
- ) Oxidative phosphorylation
2. ) Conversion of Pyruvate to Lactate
Without oxidative phosphorylation, the cell converts pyruvate to lactate using
Lactate dehydrogenase
The conversion of pyruvate to lactate by lactate dehydrogenase also does what?
Oxidizes NADH to NAD+
If rates of production exceed blood-mediate clearance, high levels of lactate can drop the
Local pH
The cramping that occurs during heavy exercise is the consequence of rapid local production of
Lactate
Lactate is eventually taken up by liver, via the circulatory system, and re-converted to pyruvate via the reversible lactate dehydrogenase reaction. This muscle/blood/liver handoff has been designated the
Cori Cycle
The process of oxidative phosphorylation is able to take each NADH produced in aerobic glycolysis and generate
3 ATP
Which produces more ATP per glucose molecule, aerobic or anaerobic glycolysis?
Aerobic
What are the three enzymes that catalyze the three irreversible reactions in glycolysis?
Gluco/Hexokinase, PFK-1, and pyruvate kinase
We see feedback inhibition of hexokinase by
G6P
Neither insulin nor glucagon actually enters the cells that are responsive to
Them
Glucagon signaling typically stimulates
Phosphorylation
Insulin signaling typically stimulates
Dephorphorylation
Leads to disruption of glycolysis and the build up of glycolytic intermediates
Pyruvate Kinase Deficiency
Cell dehydration, contraction, and crenation (echinocytes), leading to premature destruction of the erythrocyte are a sign/symptom of
Pyruvate Kinase deficiency
An Increased offloading of O2 into tissues due to increased 2,3-BPG is a sign/symptom of
-results in improved exercise performance in these patients relative to others with similarly low Hgb
Pyruvate kinase deficiency
How can we treat the hemolytic anemia seen with pyruvate kinase deficiency?
Transfusions, supplemental folic acid, splenectomy
