Topic 16: Glycolysis and Gluconeogenesis Flashcards
What are the 3 major fates of glucose
It may be stored as glycogen
It may be oxidized to a 3-carbon compound called pyruvate
It may be oxidized to pentose
Describe the 2 phases of glycolysis
The first 5 steps are the trapping and preparatory phase, where a glucose molecule is trapped in the cell and destabilized to allow fragmentation of the final molecules. The first phase is also known as the investment phase because ATP is required to destabilize glucose
Stage 1 begins with the conversion of glucose into fructose 1,6-biphosphate, which consists of three steps: a phosphorylation, an isomerization, and a second phosphorylation reaction
The the strategy of these initial steps in glycolysis is to trap the glucose in the cell and destabilize glucose to form a compound that can be readily cleaved into phosphorylated three-carbon units. Stage 1 is completed with the cleavage of fructose 1,6-biphosphate into phosphorylated three-carbon units. The resulting carbon units, glyceraldehyde-3-phosphate and dihydroxyacetonephosphate, are readily interconvertible
The preparatory phase requires investment of 2 molecules of ATP, raising the free energy of the intermediates. No oxidations have taken place, so no energy has been extracted from the original glucose molecule
In stage 2, ATP is harvested when the three-carbon fragments are oxidized to pyruvate. The final 5 steps in glucose are the pay-off phase. Each molecule of glyceraldehyde-3-phosphate is oxidized to pyruvate. These reactions yield 4 molecules of ATP and 2 molecules of NADH
The net ATP yield including steps 1-10 is two molecules of ATP per glucose molecule and two molecules of NADH
What is the first step of glycolysis
Glucose enters cells through specific transport proteins and has one principal fate in the cell: it is phosphorylated to form glucose-6-phosphate, G6P.
This step is notable for two reasons: (1) glucose 6-phosphate cannot pass through the membrane to the extracellular side, because it is not a substrate for the glucose transporters, and (2) the addition of the phosphoryl group facilitates the metabolism of glucose to phosphorylated three-carbon compounds with high phosphoryl-transfer potential.
The transfer of the phosphoryl group from ATP to the hydroxyl group on carbon 6 of glucose is catalyzed by hexokinase. Phosphoryl transfer is a fundamental reaction in biochemistry. Kinases are enzymes are enzymes that catalyze the transfer of a phosphoryl group from ATP to an acceptor.
What comes after G6P formation
The next step in glycolysis is the isomerization of glucose 6-phosphate to fructose 6-phosphate. Recall that the open-chain form of glucose has an aldehyde group at carbon 1, whereas the open-chain form of fructose has a ketogroup at carbon 2. Thus, isomerization of G6P to fructose 6-phosphate is conversion of an aldose to a ketose. This reaction is catalyzd by phosphoglucose isomerase
This isomerization is crucial because only three-carbon molecules are metabolized in the later stages of glycolysis. Glucose 6-phosphate is not readily cleaved into two three-carbon fragments, while fructose 6-phosphate is.
What is the next step after Fructose 6-phosphate formation
A second phosphorylation follows the isomerization step, trapping the sugar as the fructose isomer. Fructose 6-phosphate is phosphorylated by ATP to fructose 1,6-biphosphate. This is an irreversible reaction under cellular conditions, and is catalyzed by phosphofructokinase (PFK).
What is the last step of stage 1
Stage 1 is completed and stage two begins with 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.
What is the next step of stage 2
The initial reaction in this is the conversion of glyceraldehyde 3-phosphate to 1,3- bisphosphoglycerate (1,3 -BPG), a redox reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase.
Dehydrogenases are enzymes that catalyze redox reactions, often transferring a hydride ion from a donor molecule to NAD+ or transferring a hydride ion from NADH to an acceptor molecule
1,3-BPG is an acyl phosphate, which is a mixed anhydride of phosphoric acid and a carboxylic acid. Such compounds have a high phosphoryl-transfer potential, and the next step sees this, a transfer of its phosphoryl group to ADP to form ATP
What occurs to 1,3-bisphosphateglycerate
1,3-bisphosphoglycerate is an energy rich molecule with a greater phosphoryl-transfer potential than that of ATP. Thus, 1,3-BPG can be used to power the synthesis of ATP from ADP and orthophosphate. Phosphoglycerate kinase catalyzes the transfer of the phosphoryl group from the acyl phosphate of 1,3-bisphosphoglycerate to ADP. ATP and 3-phosphoglycerate are the products
Describe the last step
The first reaction is a rearrangement, where 3-phosphoglycerate is converted into 2-phosphoglycerate by phosphoglycerate mutase, which shifts the position of the phosphoryl group. In general, a mutase is an enzyme that catalyzes the intramolecular shift of a chemical group such as a phosphoryl group.
In the next reaction, 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).
Why does phosphoenolpyruvate have such a high phosphoryl-transfer potential? The phosphoryl group traps the molecule in its unstable enol form. When the phosphoryl group has been donated to ATP, the enol is able to undergo a conversion into the more stable ketone, namely, pyruvate!!
Hence, pyruvate is formed, and ATP is generated concomitantly. The irreversible transfer of a phosphoryl group from phosphoenolpyruvate to ADP is catalyzed by pyruvate kinase.
What are the three ways NAD+ is regenerated?
Alcohol and lactate fermentation, mitochondrial oxidation of pyruvate
Describe the process of alcohol fermentation
The first step is the decarboxylation of pyruvate, catalyzed by pyruvate decarboxylase, along with coenzyme thiamine pyrophosphate. The second step is the reduction of acetaldehyde to ethanol by NADH, catalyzed by alcohol dehydrogenase. Acetaldehyde is the organic compound that accepts the electrons in this fermentation. This reaction regenerates NAD
NAD+ and NADH do not appear in this equation even though they are crucial for the process. NADH generated by the oxidation of glyceraldehyde 3-phosphate is consumed in the reduction of acetaldehyde to ethanol. Thus, there is no net oxidation–reduction in the conversion of glucose into ethanol
Describe lactic acid fermentation?
Lactate is formed from pyruvate in a variety of microorganisms and in muscle cells in a process called lactic acid fermentation. Pyruvate is reduced NADH to form lactate in a reaction catalyzed by lactate dehydrogenase.
As in alcoholic fermentation, there is no net oxidation– reduction. The NADH formed in the oxidation of glyceraldehyde 3-phosphate is consumed in the reduction of pyruvate. The regeneration of NAD+ in the reduction of pyruvate to lactate or ethanol sustains the continued process of glycolysis under anaerobic conditions
Describe the regulation action of phosphofructokinase
Phosphofructokinase is the most important control site in the mammalian glycolytic pathway.
High levels of ATP allosterically inhibit the enzyme (a 340 kDa tetramer). ATP binds to a specific regulatory site distinct from the catalytic site. The binding of APT lowers the enzyme’s affinity for fructose-6-phosphate
AMP reverses the inhibitory action of ATP, and 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 PFK activity by augmenting the inhibitory effect of ATP
The pH might fall when fast-twitch muscle is functioning anaerobically, producing excessive quantities of lactic acid. The inhibition of glycolysis, and therefore of lactic acid fermentation, protects the muscle from damage that would result from the accumulation of too much acid.
One might wonder, why does ADP but not AMP stimulate the activity of phosphofructokinase? When ATP is being utilized rapidly, the enzyme adenylate kinase can form ATP from ADP by the following reaction: ADP + ADP ⇌ ATP + AMP. Thus, some ATP is salvaged from ADP, and AMP becomes the signal for the low-energy state
Describe the regulation action of hexokinase
Hexokinase is the enzyme catalyzing the first step of glycolysis. It is inhibited by its product, glucose-6-phosphate.
High concentrations of glucose 6-phosphate signal that the cell no longer requires glucose for energy, so no more glucose needs to be broken down. The glucose will then be left in the blood
A rise in glucose 6-phosphate concentration is a means by which phosphofructokinase communicates with hexokinase. When phosphofructokinase is inactive, the concentration of fructose 6-phosphate rises. In turn, the level of glucose 6-phosphate rises because it is in equilibrium with fructose 6-phosphate . Hence, the inhibition of phosphofructokinase leads to the inhibition of hexokinase
Why is phosphofructokinase the main regulator of glycolysis?
The reason becomes evident upon noting that glucose 6-phosphate is not solely a glycolytic intermediate. In the muscle for example, glucose 6-phosphate can also be converted to glycogen
The first irreversible reaction unique to the glycolytic pathway, the committed step, is the phosphorylation of fructose 6-phosphate to fructose 1,6- bisphosphate. Thus phosphofructokinase as the primary control site in glycolysis is highly appropriate. In general, the enzyme catalyzing the committed step in a metabolic sequence is the most important control element in the pathway because it regulates flux down the pathway.
