Lecture 19-20/Textbook Ch. 16: Glycolysis Flashcards
Glycolysis is the
sequence of reactions that converts one molecule of glucose into two molecules of pyruvate while generating ATP
Glycolysis serves two major functions in the cell (2):
- this set of reactions generates ATP
- the molecules formed during glycolysis are used as precursors for amino acid and fatty acid synthesis providing building blocks for biosynthesis
The two pathways are reciprocally regulated so that glycolysis and gluconeogenesis do not
- take place in the same cell at the same time to a significant extent, thereby preventing the waste in energy that would result if glucose were being broken down at the same time as it is being synthesized.
During a sprint, when the ATP needs outpace oxygen delivery, as would be the case for Bolt, glucose is metabolized to —–. When oxygen delivery is adequate, glucose is metabolized more efficiently to ——.
- lactate
- carbon dioxide and water
Why is glucose such a prominent fuel, rather than some other monosaccharide (2):
- glucose is the most stable hexose because the hydroxyl groups and the hydroxymethyl group are all in the equatorial position, minimizing steric clashes
- glucose has a low tendency, relative to other monosaccharides, to nonenzymatically glycosylate proteins. In their open-chain forms, monosaccharides contain carbonyl groups that can covalently modify the amino groups of proteins. Glucose has a strong tendency to exist in the ring formation and, consequently, relatively little tendency to modify proteins.
In eukaryotic cells, glycolysis takes place in the
—–. Glucose is converted into ——.
- cytoplasm
- two molecules of pyruvate with the concomitant generation of two molecules of ATP
Glycolysis can be thought of as comprising two stages: Stage 1 (3)
ATP+Begings with+ completed with+ATP
- Stage 1 is the trapping and preparation phase.
- No ATP is generated in this stage.
- Stage 1 begins with the conversion of glucose into fructose 1,6-bisphosphate, which consists of three steps: a phosphorylation, an isomerization, and a second phosphorylation reaction.
- Stage 1 is completed with the cleavage of the fructose 1,6- bisphosphate into two phosphorylated three-carbon fragments (Glyceraldehyde 3-Phosphate)
- ATP is consumed to donate phosphoryl groups
Glycolysis can be thought of as comprising two stages: Stage 2 (2)
- ATP is harvested when the three-carbon fragments (Glyceraldehyde 3-Phosphate) are oxidized to pyruvate
- ATP is produced via substrate-level phosphorylation
Glycoylsis chemical equation
Glucose enters cells through specific transport proteins and has one principal fate inside the cell:
it is phosphorylated by ATP to form glucose 6-phosphate.
Why is Glucose phosphorlyated upon entry to the cell (2)?
(1) glucose 6-phosphate cannot pass through the membrane to the extracellular side, because it is not a substrate for the glucose transporters
(2) the addition of the phosphoryl group facilitates the metabolism of glucose to phosphorylated three-carbon compounds with high phosphoryl-transfer potential.
hexokinase (3)
What it does+requires+ when glucose binds
- The transfer of the phosphoryl group from ATP to the hydroxyl group on carbon 6 of glucose is catalyzed by hexokinase.
- requires Mg2+ (or another divalent metal ion such as Mn2+) for activity. The divalent metal ion forms a complex with ATP.
- Hexokinase consists of two lobes, which move toward each other when glucose is bound. The cleft between the lobes closes, and the bound glucose becomes surrounded by protein, except for the carbon atom that will accept the phosphoryl group from ATP. The closing of the cleft in hexokinase is a striking example of the role of induced fit in enzyme action
Kinases
Kinases are enzymes that catalyze the transfer of
a phosphoryl group from ATP to an acceptor.
The isomerization of glucose 6-phosphate to fructose 6-phosphate (3)
conversion+ catalyzed by+ why?
- a conversion of an aldose into a ketose.
- The reaction is catalyzed by phosphoglucose isomerase
- Glucose 6-phosphate is not readily cleaved into two threecarbon fragments, while fructose 6-phosphate is (G6P is not readily cleaved into two three-carbon fragments because it is a stable molecule. Its structure makes it less reactive and less prone to spontaneous cleavage into smaller molecules compared to other phosphorylated sugars.)
A second phosphorylation reaction follows the isomerization step (3):
changed into+ATP+enzyme+nature+enzyme
- Fructose 6-phosphate is phosphorylated by ATP to fructose 1,6-bisphosphate
- ATP donates phosphoryl group
- This reaction, which is irreversible under cellular conditions, is catalyzed by phosphofructokinase (PFK), an allosteric enzyme that is the key regulatory enzyme for glycolysis
aldolase
The cleavage of fructose 1,6-bisphosphate into two triose phosphates, glyceraldehyde 3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP). The products of the remaining steps in glycolysis consist of three-carbon units rather than six-carbon units. This reaction, which is readily reversible, is catalyzed by aldolase.
Triose phosphate isomerase
Enzyme that catalyzes the interconversion of dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3 phosphate
What happens to gylceraldehyde 3-phosphate in stage 2 of glycolysis? (3)
+ general equation of what happens+phosphoryl-transfer potential
- The initial reaction in this sequence is the conversion of glyceraldehyde 3-phosphate into 1,3-bisphosphoglycerate (1,3-BPG), an oxidation–reduction reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase.
- 1,3-bisphosphoglycerate has a high phosphoryl-transfer potential, one of its phosphoryl groups is transferred to ADP in the next step in glycolysis.
Dehydrogenases
enzymes that catalyze oxidation–reduction reactions, often transferring a hydride ion (H-) from a donor molecule to NAD+ or transferring a hydride ion from NADH to an acceptor molecule.
The reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase can be viewed as the sum of two processes:
- The oxidation of the aldehyde (in this case, glyceraldehyde 3-phosphate) to a carboxylic acid by NAD+
- The joining of the carboxylic acid (3- phosphoglycerate) and orthophosphate to form the acylphosphate product, 1,3-bisphosphoglycerate
The reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase to form 1,3-bisphosphoglycerate from glyceraldehyde 3-phosphate explain the energetics (4):
2 rxn favorability+sucession+must be+active site
- The first reaction is thermodynamically quite favorable, whereas the second reaction is quite unfavorable.
- If these two reactions simply took place in succession, the second reaction would not take place at a biologically significant rate, because of its very large activation energy
- These two processes must be coupled so that the favorable aldehyde oxidation can be used to drive the formation of the acyl phosphate.
- Gly-3-P Hydrogenase has active-site cysteine which forms high-energy thioester intermediate that makes step 2 favourable
1,3-Bisphosphoglycerate is an energy-rich molecule with a —– phosphoryl-transfer potential than that of ATP
greater
Phosphoglycerate kinase
products?
Catalyzes the transfer of the phosphoryl group from the acyl phosphate of 1,3-bisphosphoglycerate to ADP. ATP and 3-phosphoglycerate are the products.
Additional ATP Is Generated with the Formation of Pyruvate, explain the steps (3):
- The first reaction is a rearrangement. 3 Phosphoglycerate is converted into 2-phosphoglycerate by phosphoglycerate mutase, which shifts the position of the phosphoryl group (C-3 to C-2).
- The dehydration of 2-phosphoglycerate catalyzed by enolase introduces a double bond, creating an enol phosphate, an unstable class of molecule in relation to an alcohol such as 2-phosphoglycerate. Enolase catalyzes the formation of the enol phosphate phosphoenolpyruvate (PEP). This dehydration markedly elevates the transfer potential of the phosphoryl group.
- The irreversible transfer of a phosphoryl group from phosphoenolpyruvate to ADP is catalyzed by pyruvate kinase.
Diverse fates of pyruvate:
- Ethanol and lactate can be formed by reactions that include NADH. (Ethanol/lactic acid fermentation)
- Alternatively, a two-carbon unit from pyruvate can be coupled to coenzyme A. (Tricarboxylic Acid cycle)
Ethanol Fermentation
enzyme+requires…
- Decarboxylation of pyruvate. This reaction is catalyzed by pyruvate decarboxylase, which requires the coenzyme thiamine pyrophosphate.
- The second step is the reduction of acetaldehyde to ethanol by NADH, in a reaction catalyzed by alcohol dehydrogenase. Acetaldehyde is thus the organic compound that accepts the electrons in this fermentation. This reaction regenerates NAD+
Lactic Acid Fermentation
Pyruvate accepts the electrons from NADH to form lactate in a reaction catalyzed by lactate dehydrogenase.
If fermentation was disrupted….
- glycolysis will be halted
Regulation of Glycolysis occurs at (3)
steps+regulation by+ occurs
- the 3 irreversible steps: Hexokinase, Phosphofructokinase, pyruvate kinase
- Regulation by allosteric effectors or covalent modification
- Occurs differently in different tissues
Feedforward stimulation +ex
- A metabolic produced earlier in the pathway activates an enzyme that catalyze a reaction further down the pathway
- ex: the activation of pyruvate kinase (PK) by fructose-1,6-biphosphate (FBP) in glycolysis
Phosphofructokinase regulation in skeletal muscle by ATP and AMP for Glycolysis (5):
High levels of ATP+ATP binds/what happens when it binds+AMP+ATP?AMP ratio+decrease in PH
- High levels of ATP allosterically inhibit the enzyme
- ATP binds to a specific regulatory site that is distinct from the catalytic site. The binding of ATP lowers the enzyme’s affinity for fructose 6-phosphate.
- AMP reverses the inhibitory action of ATP. AMP competes with ATP for the binding site but, when bound, does not inhibit the enzyme.
- Consequently, the activity of the enzyme increases when the ATP/AMP ratio is lowered.
- A decrease in pH also inhibits phosphofructokinase activity by augmenting the inhibitory effect of ATP
Why does AMP but not ADP stimulate the activity of phosphofructokinase?
- When ATP is being utilized rapidly, the enzyme adenylate kinase can form ATP from ADP. Thus, some ATP is salvaged from ADP, and AMP becomes the signal for the low-energy state
Liver phosphofructokinase regulation (4)
ATP+PH+Citrate+F-26BP
- can be regulated by ATP as in muscle, but such regulation is not as important since the liver does not experience the sudden ATP needs that a contracting muscle does
- low pH is not a metabolic signal for the liver enzyme, because lactate is not normally produced in the liver
- phosphofructokinase is inhibited by citrate, an
early intermediate in the citric acid cycle. A high level of citrate in the cytoplasm means that biosynthetic precursors are abundant, so there is no need to degrade additional glucose for this purpose - the concentration of fructose 6- phosphate rises when blood-glucose concentration is high because of the action of hexokinase and phosphoglucose isomerase, and the abundance of fructose 6-phosphate accelerates the synthesis of F-2,6-BP. An abundance of fructose 6 phosphate leads to a higher concentration of F-2,6-BP. Fructose 2,6-bisphosphate stimulates glycolysis by increasing phosphofructokinase’s affinity for fructose 6-phosphate and diminishing the inhibitory effect of ATP.
Liver Pyruvate kinase regulation (3)
inhibited by + activated by + blood glucose is low
- inhibited by alanine and ATP
- activated by fructose 1,6-biphosphate
- When the blood glucose concentration is low, the glucagon-triggered cyclic AMP cascade leads to the reversible phosphorylation of pyruvate kinase with ATP, which diminishes its activity. This prevents the liver from consuming glucose when it is more urgently needed by brain and muscle.
