Glycolysis
What occurs during a phosphoryl transfer reaction?
A phosphate group is transferred from an ATP molecule to an intermediate or from a high energy intermediate to ADP (substrate level phosphorylation) via a kinase.
What effect does high ATP concentrations have on PFK? What reverses this?
ATP reduces the affinity of the enzyme for F-6-P. This effect is reverse by AMP or F-2,6-BP
What effect does high citrate concentrations have on PFK?
It inhibits PFK, given that high citrate levels mean that biosynthetic precursors are abundant, making further glucose metabolism unnecessary.
What is the function of F-2,6-BP? What forms it?
It is a potent activator of both PFK and PK (feedforward stimulation). It is formed via the phosphorylation of F-6-P by PFK 2.
How does mannose enter the glycolytic pathway?
It is converted to Mannose-6-P via hexokinase, then converted to F6P via phosphomannose isomerase.
What is the function of phosphoglucose isomerase?
It isomerizes Glucose-6-phosphate to Fructose-6-phosphate
What is the function of phosphoglycerate kinase?
It phosphorylates 1,3-BPG to 3-PG
What step of glycolysis contains substrate level phosphorylation?
Step 10: PEP to pyruvate
What step of glycolysis produces NADH? Note the enzyme that catalyzes this reaction.
Step 6: G3P to 1,3-BPG (G3P dehydrogenase).
Which steps of glycolysis pertain to the investment phase?
Steps 1-5
Why can glycolysis not be sustainably run at extremely high rates?
The NADH produced can not be utilized quickly enough in the mitochondria to regenerate sufficient amounts of NAD+; thus, the reducing power becomes a limiting factor.
What occurs during phase 4 of glycolysis?
3PG is rearranged and dehydrated into phosphoenol pyruvate (PEP). Pyruvate kinase then transfers the phosphate, producing pyruvate and ATP.
What occurs during a phosphoryl shift reaction?
A phosphate group is transferred from one oxygen atom to another within the same atom via a mutase.
What occurs during an isomerization reaction?
A reaction in which an isomerase catalyzes the conversion of a ketose to an aldose or vice versa.
What is the fate of pyruvate produced from glycolysis under aerobic and anaerobic conditions?
Aerobic: oxidized to produce additional ATP via the TCA cycle and the ETC. Anaerobic: fermented to produce lactate or ethanol/CO2 along with NADH
What is the high Km of glucokinase for G-6-P crucial?
It allows for the brain and muscles to be given priority on blood glucose when its supply is limted.
What two ways does glucagon decrease the rate of glycolysis?
It blocks the feedforward stimulation of glycolysis via down-regulation of F-2,6-BP. It does so by phosphorylation of a single serine residue in PFK2, forming FBPase2 (thus, inhibiting PFK2). It also triggers a cascade that results in PK phosphorylation into a less active form.
How does glyceraldehyde enter the glycolytic pathway?
It is converted to G3P via triose kinase (or converted to glycerol indirectly, then converted back to G3P through an alternative pathway).
How does sucrose enter the glycolytic pathway?
It is converted to fructose and glucose, which then enter the pathway separately.
How does lactose enter the glycolytic pathway?
It is converted to galactose and glucose, which enter the pathway separately.
How does glycerol enter the glycolytic pathway?
It is converted to glycerol-3-phosphate via glycerol kinase. It is then converted to DHAP via glycerol phosphate dehydrogenase, where it is then converted to G3P via triose phosphate isomerase.
What is the function of phosphofructokinase and how does this relate to the regulation of glycolysis?
It phosphorylates F6-P to F1,6-P. Given that this is a relatively irreversible reaction, this step commits the molecules to the rest of the pathway. Thus, the activity of this enzyme determines how much glycolysis will occur.
What is the function of pyruvate kinase and how does this relate to the regulation of glycolysis?
It transfers the phosphate on PEP, producing pyruvate and ATP. Given that this is a relatively irreversible reaction, this step commits the molecules furthermore. Thus, this enzyme is highly regulated so as to regulate the amount of glycolysis occurring.
What are the three forms of pyruvate kinase and where are they found in the body?
L -> Predominantly in the liver M -> Predominantly in the muscle/brain A -> Predominantly in the other tissues
What is the difference between how the L isoenzyme of pyruvate kinase is regulated in comparison to the M isoenzyme and the A isoenzyme?
M: not reversibly phosphorylated A: controlled by covalent modification
What are the three main ways in which NAD+ is regenerated?
Oxidative phosphorylation (ETC) Lactate production Alcoholic fermentation (microbes)
What are the two main enzymes that regulate the glycolytic pathway?
Phosphofructokinase Pyruvate kinase
What are the five main types of reactions that occur during glycolysis?
Phosphoryl Transfer Phosphoryl shijft Isomerization Dehydration Aldol cleavage
What steps of glycolysis produce ATP? Note the enzymes that catalyze these reactions.
Step 7: 1,3-BPG to 3-PG (PG kinase) Step 10: PEP to Pyruvate (Pyruvate Kinase)
Which steps of glycolysis pertain to the payoff phase?
Steps 6-10
By what mechanism is ATP generated in glycolysis?
Substrate level phosphorylation.
What function do glycolysis intermediates serve besides further glycolysis?
They serve as precursors (skeletons) in the synthesis of a variety of biochemical building blocks (Ie. amino acids, lipids, pyrimidines, etc.).
What is substrate level phosphorylation?
When ADP is phosphorylated via a phosphate-containing substrate/intermediate.
What is the function of G3P dehydrogenase?
It adds an inorganic phosphate to G3P, forming 1,3-BPG and NADH.
What occurs during a dehydration reaction?
A dehydratase catalyzes the removal of a water molecule from an alcohol, forming a double bond between two adjacent carbons.
How many ATP molecules are produced per molecule of glucose during glycolysis?
2
How many NADH molecules are produced per molecule of glucose during glycolysis?
2
How is the L isoenzyme of Pyruvate kinase regulated?
Allosteric inhibition via ATP Allosteric inhibition via Alanine (abundance of amino acids) F-1,6-BP activates it Glucagon phosphorylates it into a less active form
What causes the release of glucagon?
Decreased blood glucose levels; thus, it prevents the liver from consuming glucose so as to give precedence to the brain and muscle tissue.
Why is glucose converted to G-6-P in the liver even when G-6-P levels are high?
Despite hexokinase being inhibiting by the high G-6-P levels, glucokinase can still carry out this conversion.
Be able to draw the following molecules and their phosphorylated forms: F-1,6-BP, DHAP, Glyceraldehyde, Glycerate and pyruvate.
Do it!
What are three possible fates of pyruvates?
Ethanol formation (alcohol fermentation in yeast) Lactate formation under conditions of high ATP demand or limiting oxygen Oxidative decarboxylation into acetyl CoA (for entry into TCA cycle)
What stimulates Phospho Fructo Kinase in the liver and through what type of interaction?
F-2,6-BP allosterically regulates PFK-1.
What occurs during phase 2 of glycolysis?
FBP is split by aldolase into G3P and DHAP (equilibrated by triose phosphate isomerase).
What inhibits Hexokinase?
G-6-P PFK inhibition (given that this causes F-6-P levels to increase; thus, its equilibrium with G-6-P causes G-6-P levels to also increase, inhibiting hexokinase).
What occurs during phase 3 of glycolysis?
G3P dehydrogenase adds a second phosphate to G3P, producing 1,3-bisphosphoglycerate (1,3-BPG) reducing an NAD+ molecule to NADH in the process. Phosphoglycerate kinase then transfers the phosphate, producing ATP and 3-phosphoglycerate (3PG).
How does galactose enter the glycolytic pathway?
Galactose is converted to Gal-1-P via galactokinase. Gal-1-P then acquires a Uridyl group from UDP-Glucose, where it is then epimerized to glucose (while attached to UDP) via Gal-1-P uridylyl transferase.
What is the overall reaction for glycolysis?
Glucose + 2ATP + 2ADP + 2Pi + 2NAD+ --> 2 pyruvate + 4 ATP + 2NADH
What occurs during phase 1 of glycolysis?
Glucose is phosphorylated and isomerized to F6P, which is then phosphorylated into FBP.
What effect does high H+ concentrations have on PFK?
H+ inhibits PFK, thus preventing excessive lactate formation and drops in blood pH.
What are the three enzymes that control the three irreversible steps of glycolysis?
Hexokinase Phospho Fructo Kinase Pyruvate Kinase
Why does the conversion of fructose to F-6-P via hexokinase more frequent in adipose tissue than the liver?
Hexokinase has a 20x greater affinity for glucose than fructose. Thus, in tissue with high [glucose], this conversion is much less frequent. However, adipose tissue has a much lower [glucose], thus allowing for the conversion to occur.
What inhibits phosphofructokinase?
High levels of ATP Excess acidity ([H+]) High [citrate]
What inhibits pyruvate kinase?
High levels of ATP.
What are the two main ways that fructose can enter the glycolytic pathway?
In the liver, it is converted to Fructose-1-P via fructokinase and then cleaved into into DHAP and glyceraldehyde. Triose kinase then converts the glyceraldehyde into G3P. Alternatively, fructose is converted to F-6-P via hexokinase.
What causes galactose to be highly toxic to some individuals?
Individuals lacking the enzyme Gal-1-P uridylyl transferase are subject to galactosemia, in which galactose metabolism is blocked, resulting in side reactions that form toxic products such as galactitol. This is often fatal.
What is the function of phosphoglycerate mutase?
It rearranges the phosphate on 3-PG to form 2-PG.
What is the function of enolase?
It removes an H20 molecule from 2-PG, forming PEP.
What is the function of triose phosphate isomerase?
It reversibly isomerizes DHAP to G3P. Given that G3P is consumed by G3P dehydrogenase, the equilibrium is constantly shifted towards G3P production.
What is the function of aldolase?
It splits F-1,6-BP into DHAP and G3P.
What drives the addition of the second inorganic phosphate to G3P in phase three of glycolysis?
The oxidation of the aldehyde to a carboxylic acid.
What is the importance of glycolysis?
The pathways and their associated enzymes are extremely conserved among living organisms and are a central part of the energy production pathways. It also provide carbon skeletons for many other biosynthetic reactions in the cell.
What is glycolysis?
The series of 10 reactions involved in splitting a six carbon sugar (glucose) into two inter-convertible three carbon molecules, which are then oxidized to form pyruvate.
What occurs during an aldol cleavage reaction?
The splitting of a carbon-carbon bond, forming an aldehyde in one of the two products.
What would happen if G3P were oxidized without a coupled phosphorylation?
There would be no net production of ATP in glycolysis