Glycolysis Flashcards
Where does glycolysis occur?
Cytoplasm
What are the major carbohydrates in the human diet?
Starch, sucrose, lactose, fructose, glucose
What enzymes catalyze the breakdown of starch?
Salivary α-amylase and pancreatic α-amylase
What are the products of starch breakdown?
α-maltose, α-isomaltose, and trisaccharides
What enzyme breaks down sucrose?
Sucrase
What is the structure of sucrose?
Disaccharide of glucose linked to fructose in an α(1-2) linkage
Sucrase-isomaltase complex deficiency
Results in an intolerance of ingested sucrose.
Highly prevalent in Inuit people.
Treatment: dietary restriction of sucrose and enzyme replacement therapy
What enzyme catalyzes breakdown of lactose?
Lactase
Structure of lactose
Disaccharide of galactose and glucose linked through β(1-4)
What causes lactose intolerance?
Absence of intestinal lactase
Sodium-Glucose Linked Transporter 1 (SGLT1)
Symporter that transports glucose and galactose against a concentration gradient by using electrochemical Na+ gradient.
Present on apical border of intestinal epithelial cells
GLUT2
Facilitated glucose transporter.
Low Vmax (high capacity) but high Km (low affinity) for glucose.
Present on basolateral side.
GLUT5
Facilitated fructose transporter.
Present on apical border of intestinal epithelial cells.
Why is glucose the substrate for glycolysis?
Most stable hexose.
Low potential for non-enzymatic protein glycosylation.
Readily polymerized for storage.
Enzymes can open the ring system when open form is needed.
Overall glycolysis reaction
Glucose + 2 NAD+ + 2 ADP + 2 Pi –> 2 Pyruvate + 2 NADH + 2 ATP + 2 H+ + 2 H2O
What is the first half of glycolysis called?
Preparatory phase
Glycolysis Step 1
Glucose –> Glucose 6-Phosphate
Catlyzed by hexokinase
Exergonic
Irreversible
1st ATP is used here
Hexokinase reaction
Phosphoryl group transfer.
Glucose does nucleophilic attack on γ-phosphate in ATP.
Yields ADP (good leaving group) and glucose 6-phosphate
What type of molecule is glucose 6-phosphate?
Phosphate ester
Which divalent cation does ATP/ADP form complexes with?
Mg2+
Effect of Mg2+ coordination on phosphorous
Makes phosphorous more electrophilic
What molecule binds to hexokinase first?
Glucose
Conformation change of hexokinase
Cleft closing dehydrates the active site, preventing nucleophilic attack by water and nonproductive ATPase action
Effect of phosphorylation on glucose
Traps glucose inside the cell.
Aids in enzyme recognition and provides increased binding free energy.
Acts as a progenitor for capturing high-energy intermediates.
What is the physiological plasma glucose concentration?
5-7 mM
What is hexokinase I inhibited by?
Feedback inhibited by glucose 6-phosphate.
Prevents hexokinase I from typing up all intracellular Pi as G-6P
What type of enzyme is glucokinase?
Hexokinase IV.
High Km and high Vmax enzyme.
Where is glucokinase expressed?
Hepatocytes and pancreatic beta cells
Glycolysis Step 2
Glucose-6P <–> Aldehyde <–> Ketone <–> Fructose-6P
Conversion of aldose to ketose sugar.
Catalyzed by phosphoglucose isomerase (aldehyde to ketone) or glucose-6PO4 isomerase (ketone to aldehyde).
Why do we have to convert glucose-6P to fructose-6P?
Glucose-6P is energetically difficult to split into 3-carbon products, fructose-6P is easier to convert
Glycolysis Step 3
Fructose-6P –> Fructose 1,6-bisphosphate
Catalyzed by PFK-1.
2nd ATP is used.
First committed step.
Irreversible.
Rate-limiting.
How is PFK-1 regulated?
Inhibited allosterically by ATP.
AMP reverses this inhibition.
Adenylate kinase reaction
ADP + ADP <–> ATP + AMP
Occurs in muscles during contraction.
Regulation of PFK-1 by AMP
Increase in AMP concentration drives flux through glycolytic pathway
Regulation of PFK-1 by citrate
Allosterically inhibited by citrate.
When citrate levels are high, fructose-6P converts back to glucose-6P.
This inhibits hexokinase and shunts glucose-6P towards glycogen synthesis.
PFK-1 regulation by fructose 2,6-bisphosphate
Fructose 2,6-bisphosphate activates PFK-1, leads to increase in fructose-6P affinity
Synthesis of fructose 2,6-bisphosphate
Fructose-6P –> fructose 2,6-bisphosphate
Catalyzed by PFK-2.
Glycolysis Step 4
Fructose 1,6-bisphosphate <–> dihydroxyacetone phosphate + glyceraldehyde-3P
Catalyzed by aldolase
Triose phosphate products
Reversible
Endergonic but products are rapidly depleted which pulls reaction forward
Glycolysis Step 5
Dihydroxyacetone phosphate <–> glyceraldehyde-3P
Catalyzed by triose phosphate isomerase.
Reversible.
End of energy investment phase.
2 glyceraldehyde-3P produced from one glucose.
Why is triose phosphate isomerase the “perfect” enzyme?
Diffusion controlled, rate-limiting step is binding of the substrate
What is congenital hemolytic anemia caused by?
Deficiency of triose phosphate isomerase
What is the second half of glycolysis called?
Payoff phase
Glycolysis Step 6
Glyceraldehyde-3P + Pi + NAD+ <–> 1,3-Bisphosphoglycerate (1,3-BPG) + NADH + H+
Catalyzed by glyceraldehyde-3P dehydrogenase (GAPDH).
What type of molecule is 1,3-BPG?
Acyl phosphate: mixed anhydride of phosphoric acid and carboxylic acid
What intermediate is present in GAPDH reaction mechanism?
Thioester
Glycolysis Step 7
1,3-BPG + ADP + H+ <–> 3-phosphoglycerate + ATP
Catalyzed by phosphoglycerate kinase.
Reversible.
Substrate level phosphorylation - substrate is phosphate donor.
First set of 2 ATPs is generated here.
Glycolysis Step 8
3-phosphoglycerate <–> 2-phosphoglycerate
Catalyzed by phosphoglycerate mutate.
Reversible.
Converting into a higher energy phosphate.
Glycolysis Step 9
2-phosphoglycerate <–> phosphoenolpyruvate
Catalyzed by enolase.
Reversible.
Produces higher energy molecule.
What does enolase deficiency cause?
Hemolytic anemia
Glycolysis Step 10
Phosphoenolpyruvate + ADP –> pyruvate + ATP
Catalyzed by pyruvate kinase, aided by Mg2+ and K+.
Irreversible.
Regulatory reaction.
Substrate level phosphorylation.
Pyruvate product first appears as enol form.
Which form of pyruvate is most unstable?
Enol form
Most common causes of chronic hemolytic anemia
Glucose-6P dehydrogenase deficiency, then pyruvate kinase deficiency
T/F: erythrocytes are completely dependent on glycolysis for ATP production
True
How much ATP is produced in aerobic conditions?
30-32 ATP per glucose
How much ATP is produced in anaerobic conditions?
2 ATP per glucose
Function of lactate dehydrogenase (LDH)
Convert pyruvate to lactate in order to regenerate NAD+ from NADH.
Lactic acid fermentation reaction
Pyruvate <–> L-lactate
Catalyzed by lactate dehydrogenase.
Regenerates NAD+.
What causes lactic acidosis?
Excessive lactate production in vivo
Ethanol fermentation reaction
Pyruvate <–> acetaldehyde <–> ethanol + CO2
First step catalyzed by pyruvate decarboxylase.
Second step catalyzed by alcohol dehydrogenase.
Regenerates NAD+.
Warburg effect
Tumors of nearly all types carry out glycolysis at a much higher rate than normal tissue, even when oxygen is available
How are PET scans carried out?
Fluorodeoxyglucose is administered intravenously to patients to look for potentially cancerous, high glucose metabolizing tissues
What are the effects of low glucose in the blood?
Metabolism slows, decreased ATP levels in cell, K+ leaks out of cell, voltage-gated Ca2+ channel closes, no insulin is secreted
What are the effects of high glucose in blood?
Increased rates of glycolysis and citric acid cycle, increased ATP levels in cell, less K+ leaves cell, Ca2+ channel opens, insulin is secreted
Function of glycogen phosphorylase
Catalyzes phosphorolysis of glucose units from glycogen.
Acts at non-reducing ends of glycogen, releasing glucose-1P.
Aided by debranching enzyme.
Doesn’t use ATP.
Function of phosphoglucomutase
Convert glucose-1P to glucose-6P before entering glycolysis.
Doesn’t use ATP.
How many ATP per glucose are generated when starting glycolysis with glycogen?
3 ATP per glucose
