Flashcards in Glycolysis Ebook Deck (88):
Two stages of glycolysis
(1) energy inverstment stage
(2) energy generation stage
What occurs during the energy investment stage?
Phosphorylated intermediates are made at the expense of ATP
What occurs during the energy generation stage?
A net 2 molecules of ATP are formed by substrate level phosphorylation per glucose metabolized
What is produced at the end of glycolysis?
(1) 2 NADH (has 2 different fates depending upon whether they are metabolized under aerobic or anaerobic conditions)
(2) Net 2 molecules of ATP
In the first stage of glucose metabolism (energy investment phase), why are two high-energy ATP molecules consumed?
Production of fructose 1,6-bisphosphate
Can sugar passively diffuse across the cytoplasmic membrane?
Sugars are highly hydrophilic compounds, unable to diffuse passively across the hydrophobic barrier of the cytoplasmic membrane
Function of the GLUT1/GLUT5 transporters?
Function to move glucose down its concentration gradient into the interior of the cell
How are glucose transporters variable?
These transporers differ in their tissue distributions, their Kms for different sugars, and their responsiveness to insulin
When does the conformation of glucose transporters change?
Upon binding of glucose, conformation of transporters changes allowing glucose to be transported across the membrane
What do GLUT-1, GLUT-3, and GLUT-4 have in common?
Primarily involved in glucose uptake from the blood
What is unique about GLUT-2?
Found in the liver, kidney and pancreas can transport glucose into and out of the cell
Which GLUT transporter has insulin sensitivity?
GLUT-4; allows for insulin regulated uptake; allows for insulin regulated uptake and storage of glucose in fat and muscle during tims of sufficient blood glucose
Where is GLUT-4 located?
Muscle and fat cells
What is the relation between km and affinity?
low Km value implies high receptor affinity for a given substrate
Which GLUT transporters have low Km value?
GLUT 1&3 have low Km values (also have no insulin senstivity); insure a constant basal uptake of glucose
Role of GLUT-5?
GLUT-5 - primary transporter for fructose inthe small intestine and the testes
Which GLUT transporter has the highest Km?
When will GLUT-2 take in glucose?
GLUT-2 with its higher Km will more easily take in glucose during periods of high blood glucose
GLUT-4, with a lower Km than GLUT-2 will insure that skeletal and adipose tissue extract glucose from the blood faster than the liver
Importance of phosphorylation of intrancellular glucose?
Phosphorylation of intracellular glucose (1) helps maintain a 'downhill' gradient of sugar from outside to inside the cell and (2) traps phosphorylated sugar inside (the transporters do not recognize the phosphorylated forms
What are isoenzymes?
Distinct proteins that catalyze the same chemical reaction
What is the first covalent modification to intracllular glucose?
The addition of a phosphate onto carbon 6, making glucose 6-phosphate (reaction catalyzed by one of two enzymes - hexokinase or glucokinase, depending upon the tissue in question)
Description of hexokinase
- functions in most tissues
- low Km and a low Vmax
- broad specificity for six carbon sugars
Description of glucokinase
- located in the liver and in the pancreas
- high Km and a low Vmax
Why should glucokinase have a high Km?
The high Km of glucokinase in liver, the first organ to receive the sugar rich blood that drains the digestive tract after a meal, permits it to respond to high concentrations of blood sugar that obtains following a meal
Why should hexokinase have a low Km?
The low Km of hexokinase 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
What does not effect hexokinase? What inhibits hexokinase?
Not effected by insulin. High levels of glucose 6-phosphate inhibit hexokinase
What does not inhibit glucokinase? What does inhibit glucokinase?
Glucokinase is not inhibited by glucose 6-phosphate. Glucokinase is inhibited by fructose 6-phosphate in a mechanism involing transport into the nucleus and binding to the glucokinase regulatory protein (this inhibition is reversed under conditions either of high intracellular glucose or fructose 1-phosphate)
Positive regulation of glukinase?
- Glucokinase expression is positively influenced by insulin
Discuss how glucokinase reacts to excess glucose
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 Kms.
Discuss how hexokinase reacts to excess glucose
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 Glucose-6-P inhibition insuring that the cells do not take more glucose then metabolically necessary.
Synthesis of glucose 6-phosphate from glucose and ATP ...
is an irreversible reaction
Fate of glucose 6-phosphate
Isomerization to fructose 6-phosphate
After production of fructose 6-phosphate, what occurs?
A second, tightly regulated, phosphorylation taes place, catalyzed by the enzyme phosphofructokinase-1 (PFK-1), to produce fructose 1,6-bisphosphate
Significance of reaction involving PFK-1
This is the most influential and rate-limiting regulatory step in glycolysis; it is also an irreversible reaction
How many enzymes regulate PFK-1 allosterically?
Four compounds, either activating or inhibiting the enzyme, regulate PFK-1 allosterically.
Which compounds negatively regulate PFK-1?
ATP and citrate (an intermediate in the TCA cycle) negatively regulate PFK-1
Why does it make sense that ATP and citrate negatively regulate PFK-1?
ATP and citrate, when present in the cell at high concentrations, are indicative of an energy rich state and the appropriateness of throttling down glycolytic activity
What else inhibits PFK-1?
Whats compounds positively regulate PFK-1 activity?
AMP and fructose 2,6-bisphosphate positively regulate PFK-1 activity (AMP is an indicator of energy need)
What is AMP?
Describe frctose 2,6-bisphosphate
Fructose 2,6-bisphosphate is an allosteric activator of PFK-1.
Function of PFK-2/FBP-2?
alters intracellular levels of fructose 2,6-bisphosphate
Function of PFK-2?
(a kinase) responsible for the synthesis of fructose 2,6-biphosphate, from fructose 6-phosphate and ATP
Function of FBP-2?
(a phosphatase) catalyzes the production of fructose 6-phosphate and Pi from fructose 2,6-bisphosphate
What regulates the pair of acivities of PFK-2/FBP-2?
Phosphorylation or dephosphorylation of the PFK-2/FBP-2 protein itself
What directs the phosphorylation or dephosphorylation of PFK-2/FBP-2?
Signal transduction cascades beginning with insulin and gluagon, on cellular membrane receptors, and ending with protein kinase and phosphatase activities
Increased levels of insulin have what effect on PFK-2?
dephosphorylation of PFK-2, which leads to active PFK-2 and hence increased fructose 2,6-bisphosphate. Elevated levels of fructose 2,6-bisphosphate acts as an intracellular signal, indicating that glucose is abundant
Increased levels of insulin, have what effect on PFK-2?
Lead to phosphorylation of PFK-2 through glucagon signaling cascade, which leads to inactive PFK-2 and hence decreased fructose 2,6-bisphosphate. Reduced levels of fructose 2,6-bisphosphate inhibit the activity of PFK-1, thus the rate of glycolysis in liver
Fate of fructose 1,6-bisphosphate?
split into glyceraldehyde 3-phosphate and dihydroxyacetone phosphate (catalyzed by aldolase A)
What does triose phosphate isomerase do?
Catalyze the interconversion of glyceraldehyde 3-phosphate and dihydroxyacetone phosphate
Fate of glyceraldehyde 3-phosphate?
Recuited by glyceraldehyde 3-phosphate dehydrogenase to produce 1,3-bisphosphogycerate
(leads to the first production of ATP, actually a pair of ATPs)
Requirements for glyceraldehyde 3-phosphate dehydrogenase
Requires NAD+ for oxidation of glyceraldehyde 3-phosphate, and produces the reduced NADH + H+
How is 3-phosphoglycerate produced?
The phosphate on carbon#1 of 1,3 bisphosphoglycerate is transferred to ADP, by phosphoglycerate kinase, to produce ATP and 3-phosphoglycerate
Function of phosphoglycerate mutase?
Phosphoglycerate mutase converts 3-phosphoglycerate into 2-phosphoglycerate
Function of enolase?
Enolase converts 2-phosphoglycerate into phosphenolpyruvate, with the release of one molecule of water
Importance of phosphoenolpyruvate
Phosphoenolpyruvate participates in the second substrate level phosphorylation in glycolysis, and the production of a second ATP molecule
Function of enzyme pyruvate kinase?
Pyruvate kinase dephosphorylates phosphoenolpyruvate, producing ATP and pyruvate, in an irreversible reaction
Stimulation of pyruvate kinase
(1) Fructose 1,6-bisphosphae
(2) direct phosphorylation (causing inactivation) and dephosphorylation (causing re-activation) mediated by glucagon (glucagon-induced gluconeogenesis) and insulin
What happens without oxygen?
In the absence of a mechanism to re-oxidize the NADH that is produced in the glyceraldehyde 3-phosphate dehydrogenase reaction, glycolysis will come to a halt.
Two means by which cells can replenish NAD levels:
(1) oxidative phosphorylation
(2) conversion of pyruvate to lactate
Without oxidative phosphorylation, what is the cells option?
Without oxidative phosphorylation (that is without O2 and/or mitochondria), the cell uses lactate dehydrogenase to convert pyruvate to lactate and re-oxidize NADH to NAD+
Why do RBCs not have the option for oxidative phosphorylation?
No mitochondria; rely on anaerobic glycolysis to recycle NADH
What is the risk with buildup of lactate in tissue
transporters allow lactate and H+ to leave cells, to be carried away by the circulation; high levels of lactate can drop the local pH, and in turn the pH of the blood
What is the cori cycl?
refers to the metabolic pathway in which lactate produced by anaerobic glycolysis in the muscles moves to the liver and is converted to glucose, which then returns to the muscles and is metabolized back to lactate
Final bookkeeping on anaerobic glycolysis
Glucose + 2 Pi + 2 ADP -> 2 lactate + 2 ATP + 2 H20
Final bookkeeping on aerobic glycolysis
Glucose + 2Pi + 2 ADP + 2 NAD+ -> 2 pyruvate + 2 NADH + 2 H+ + 2H2O
Which produces more ATP, anaerobic or aerobic glycolysis?
Three irreversible steps during glycolysis
(1) hexo- and glucokinase
(3) pyruvate kinase
PFK-1 allosteric feedback
fructose 2,6 bisphosphate, signal transduction systems of insulin and glucagon
Pyruvate kinase allosteric feedback
fructose 1,6-bisphosphate, regulation by phosphorylation/dephosphorylation as part of the signal transduction systems of insulin and glucagon - covalent form of allosteric regulation, bifunctional enzyme PKF-2/FBF-2
Function of glucagon?
Glucagon signaling typically stimulates protein kinases to phosphorylate target enzymes
Function of insulin?
Insulin signaling typically stimulates protein phosphatases to remove phosphates
(1) mutation of M (muscle) subunit of PFK-1 resulting in decreased glycolysis in muscle tissue and RBCs
Signs/symptoms of PFK-1 deficiency
muscle weaness, increased serum creatine from muscle breadown, hemolytic anemia (mild) from RBC loss, hyperuricemia secondary to increased purine metabolism
Effectt of pyruvate kinase deficiency?
decrease in pyruvate kinase leads to disruption of glycolysis and the build up of glycolytic intermediates
Signs/symptos of pyruvate kinase deficiency?
(1) chronic hemolysis
(2) ATPase activity
(3) Alterations in RBCs (from swelling -> echinocytes (spiey RBCs, think echidna))
(4) Increased ,3-BPG
Treatment of pyruvate kinase deficiency?
(1) transfusion to treat hemolytic anemia
(2) hematopoietic stem cell transplantation
Purpose of GLUT-1?
GLUT-1 is responsible for basal glucose uptake in the brain
decreased GLUT-1 will result in less glucose reaching neural tissue. This results in several developmental defects and neurological manifestations
Signs and symptoms of GLUT-1 deficiency?
Developmental delay in the newborn, microcephaly. Neurologically these infants will present with repeated seizures and may also exhibit muscle spasticity and hypotonia
Treatment of GLUT-1 deficiency?
Results of glucokinase deficiency?
increased blood glucose, needed to trigger insulin secretion (analogous to type 1 diabetes, but much milder)
Glucokinase deficiency fun facts
- autosomal dominant
- condition termed "maturity onset diabetes of the young"
Signs and symptoms of glucokinase deficiency?
Heterozygotes: typically exhibit increased blood glucose levels
Homozygotes: can present with neonatal diabetes
treatment for glucokinase deficiency?
Fate of pyruvate?
(1) Pyruvate may be converted to oxaloacetate (as part of the process of gluconeogenesis)
(2) Converted to acetyl CoA