Glycolysis and gluconeogenesis
The major outcome of glycolysis...
-glucose is broken up to produce 2 molecules of pyruvate -NAD+ is converted to NADH -ADP + iP --> ATP (highly energetic) is the direct energy6 payoff of glycolysis
Which of the following glycolytic reactions is part of a substrate-level phosphorylation?
1,3-bisphosphoglycerte + ADP --> 3-phosphoglycerate + ATP In substrate-level phosphorylation, a phosphate group is moved onto a biochemical substrate and then transferred to ADP to form ATP. The high energy bond between the substrate and the phosphate group provides the energy needed to create ATP. The reaction depicted in choice C is the first step in the only substrate-level phosphorylation reaction of glycolysis.
Net equation for glycolysis
Glucose + 2 NAD+ + 2ADP + 2iP --> 2 Pyruvate + 2NADH + 2ATP + 2H2O
Glycolysis is globally regulated by the cellular energy pool. More specifically, it is upregulated by:
a low ratio of ATP to ADP A low ratio of ATP to ADP suggests that the cell does not have enough ATP. Since glycolysis is upregulated when the cell lacks energy, this situation would promote glycolysis.
what is pyruvate?
a three-carbon alpha-keto acid that participates in multiple pathways in the body
Glucokinase and hexokinase are isozymes that are expressed in different tissues. Hexokinase is inhibited by glucose 6-phosphate, whereas glucokinase is not. Which of the following choices best accounts for this regulatory difference?
cells that express glucokinase play a global role in glucose metabolism. Liver and kidney cells are responsible for regulating blood glucose levels. They are the primary cell types that express glucokinase. Because glucokinase is not inhibited by high concentrations of G6P, it keeps converting glucose to G6P even when there is a ton of G6P around. This allows these cells to respond to the concentration of glucose in their environment rather than focusing only on their own metabolic needs.
Which of the following reactions initiates the energy payoff phase of glycolysis?
glyceradlehyde 3-phosphate + NAD+ --> 1,3-bisphosphoglycerate + NADH The conversion of G3P to 1,3-bisphosphoglycerate is the first step of the energy payoff phase. Although this reaction does not directly produce ATP, it is the first step in a substrate-level phosphorylation event that generates ATP. The payoff step is shown in choice D.
Which of the following statements about glycolysis is NOT correct?
glycolysis is the primary source of energy for aerobic cells Although glycolysis does provide a small amount of energy for aerobic cells, it is not the primary source of energy. Aerobic cells get most of their energy from aerobic metabolism, which consists of the citric acid cycle and the electron transport chain.
Which of the following choices presents a way in which lactic acid fermentation differs from ethanol fermentation?
lactic acid fermentation consists of a single reaction
All of the following are gluconeogenic amino acids EXCEPT: tyrosine, lysine, serine, and glutamate
lysine
Which of the following scenarios would favor glucose catabolism through glycolysis? Select all that apply.
release of insulin Insulin promotes cellular absorption of glucose from the bloodstream. Glucose inside cells can either store it as glycogen or burn it for energy through glycolysis. Insulin promotes both of these pathways.
True or false: Fructose 1,6-bisphosphatase is a heavily regulated gluconeogenic enzyme.
true: Fructose 1,6-bisphosphatase catalyzes the conversion of fructose 1,6-bisphosphate to fructose 6-phosphate. It is inhibited by AMP and the regulatory compound fructose 2,6-bisphosphate.
What is the net ATP yield from the complete oxidation of a single molecule of glucose through glycolysis?
2: Although four molecules of ATP total are produced during one round of glycolysis, two ATP are invested in the earlier reactions. Therefore, the cell gains a net 2 ATP from the breakdown of 1 molecule of glucose in glycolysis.
Which of the following molecules are products of glycolysis? Select all that apply.
ATP and pyruvate
Which of the following events occur during glycolysis? Select all that apply.
ATP is generated and two molecules of pyruvate are produced 4 molecules of ATP are produced as one molecule of glucose is broken down in glycolysis. However, only two molecules of ATP are actually gained, since the cell burns two ATP during the energy investment phase. Two molecules of pyruvate are the end products of glycolysis, on which the pyruvate dehydrogenase complex may act to produce two molecules of acetyl-CoA and CO2.
Which of the following molecules does NOT inhibit pyruvate kinase?
Citrate High concentrations of citrate inhibit other enzymes in glycolysis, but not pyruvate kinase specifically.
All of the following enzymes function in the energy payoff phase of glycolysis. Place these enzymes in chronological order (from first to last): G3P dehydrogenase, pyruvate kinase, enolase, phosphoglycerate kinase, and phosphoglycerate mutase.
G3P dehydrogenase, followed by phosphoglycerate kinase, followed by phosphoglycerate mutase, followed by enolase, and finally pyruvate kinase. The energy payoff phase begins with the conversion of G3P into 1,3-bisphosphoglycerate, catalyzed by G3P dehydrogenase. Next, phosphoglycerate kinase converts 1,3-bisphophoglycerate into 3-phosphoglycerate by transferring a phosphate group to generate a molecule of ATP. After that, phosphoglycerate mutase converts 3-phosphoglycerate into 2-phosphoglycerate, which is quickly turned into phosphoenolpyruvate by the enzyme enolase. Lastly, pyruvate kinase converts phosphoenolpyruvate into pyruvate, generating one more ATP.
Which of the following steps of glycolysis is the first regulated step?
Glucose + ATP --> glucose 6-phosphate + ADP The very first step of glycolysis is the first regulated step. Recall that this reaction prevents glucose from leaving the cell, although it is not yet committed to glycolysis. High concentrations of glucose 6-phosphate (G6P) inhibit hexokinase, which is the enzyme that catalyzes this reaction. This prevents overproduction of G6P.
All of the following enzymes function in the energy investment phase of glycolysis. Match each enzyme to its product(s). Note: Enzymes may have more than one product.
Hexokinase is the first enzyme that acts in glycolysis. It adds a phosphate group onto glucose to form glucose 6-phosphate, which traps glucose inside the cell. Phosphofructokinase-1 converts fructose 6-phosphate into fructose 1,6-phosphate. This is an extremely important step because it is the point of no return in glycolysis. Once the cell produces fructose 1,6-phosphate, it has committed to running glycolysis. Next, the enzyme aldolase catalyzes another key reaction: the cleavage of fructose 1,6-phosphate. It generates two different three carbon molecules: glyceraldehyde 3-phosphate and dihydroxyacetone phosphate.
Each of the enzymes below is involved in either lactic acid or ethanol fermentation. Match each enzyme to the pathway in which it functions.
Lactic acid fermentation consists of a single reaction: the conversion of pyruvate to lactic acid by the enzyme lactate dehydrogenase. In the process, NADH is oxidized to NAD+. Ethanol fermentation is a bit more complicated. First, pyruvate decarboxylase converts pyruvate into acetaldehyde, releasing a molecule of carbon dioxide. Next, alcohol dehydrogenase converts acetaldehyde into ethanol, oxidizing NADH to NAD+.
Which of the following compounds are products of lactic acid fermentation? Select all that apply.
NAD+ and lactic acid NAD+ is the most important product of lactic acid fermentation. Cells use fermentation to regenerate NAD+ from the NADH that was created in glycolysis. Pyruvate is converted into lactic acid through lactic acid fermentation. Lactic acid is more of a side product, since generating NAD+ is the real goal, but it can be metabolized through a variety of other metabolic pathways.
True or false: Ethanol fermentation is only performed by prokaryotes.
false: Although ethanol fermentation is much more common among prokaryotes than eukaryotes, certain eukaryotes can perform these reactions. The most notable example is yeast, which are single-celled eukaryotes that produce ethanol through ethanol fermentation. They are used to generate fermented food products, like alcoholic beverages.
True or false: The purpose of fermentation is to replenish the supply of NADH for glycolysis.
false: Fermentation replenishes the supply of NAD+, not NADH. Glycolysis gradually converts all of the cell's NAD+ into NADH, which needs to be converted back into NAD+ for glycolysis to continue. Fermentation provides one way to do this.
True or false: Gluconeogenesis begins in the cytoplasm and reaches completion in the mitochondria.
false: The first reaction of gluconeogenesis occurs in the mitochondria. All of the other reactions take place in the cytoplasm.
Which of the following molecules upregulates phosphofructokinase-1 (PFK-1)?
fructose 2,6-bisphosphate Fructose 2,6-bisphosphate is a key regulatory molecule that activates PFK-1.
Glucokinase is an isozyme of hexokinase. Which of the following choices identify ways in which glucokinase differs from hexokinase? Select all that apply.
glucokinase has an especially low affinity for glucose and glucokinase is not inhibited by its byproduct Glucokinase has a lower affinity for glucose than hexokinase does, which makes it less likely to catalyze the first reaction of glycolysis. As a result, glucokinase tends to function best at extremely high concentrations of glucose. Unlike hexokinase, glucokinase is not inhibited by glucose 6-phosphate. Glucokinase remains functional at extremely high glucose concentrations. This allows cells that express glucokinase (liver, pancreas) to focus on lowering blood glucose concentrations rather than on meeting their own energetic demands.
Which of the following choices present(s) facts about gluconeogenesis that are NOT true of glycolysis? Select all that apply.
gluconeogenesis occurs in the cytosol and mitochondria and gluconeogenesis is an anabolic pathway
Negative feedback occurs when a product of a pathway feeds back to inhibit an enzyme in the pathway that produced it. Which of the following glycolysis enzymes are regulated by negative feedback? Select all that apply.
hexokinase and pyruvate kinase
Which of the following conditions would correlate with inhibition of glycolysis in muscle cells? Select all that apply.
high levels of NADH and a high concentration of G6P
Which of the following choices best explains why fermentation is used by aerobic cells?
it produces NAD+ in the absence of oxygen Aerobic cells normally use the citric acid cycle and electron transport chain to regenerate NAD+, which is necessary for glycolysis to function. In the absence of oxygen, these aerobic pathways cannot be used. Lactic acid fermentation is a secondary way to regenerate NAD+ so that cells can continue producing small amounts of energy from glycolysis when oxygen is not available.
Which of the following types of enzyme-catalyzed reactions occur during ethanol fermentation? Select all that apply.
oxidation-reduction and decarboxylation Oxidation-reduction (redox) reactions involve electron transfer between two species. This description fits the second reaction of ethanol fermentation, in which alcohol dehydrogenase converts acetaldehyde into ethanol. Acetaldehyde gains electrons and gets reduced, while NADH loses electrons and becomes oxidized. The first reaction of ethanol fermentation is a decarboxylation reaction. The enzyme pyruvate decarboxylase cleaves a carbon from pyruvate, releasing it as a molecule of CO2.
Which of the following enzymes catalyzes the committed step of glycolysis?
phosphofructokinase-1
Which of the following are components of the bifunctional enzyme that is responsible for the metabolism of fructose 2,6-bisphosphate? Select all that apply.
phosphofructokinase-2 and fructose 2,6-bisphosphatase Phosphofructokinase-2 is one of the two enzymatic components of this bifunctional enzyme. PFK-2 generates fructose 2,6-bisphosphate to activate glycolysis. Fructose 2,6-bisphosphatase breaks down fructose 2,6-bisphosphate to inhibit glycolysis.
Which of the following statements are true of the bifunctional enzyme complex that regulates fructose 2,6-bisphosphate? Select all that apply.
phosphofructokinase-2 creates frucgtose 2,6 bisphosphate and fructose 2,6-bisphosphatase breaks down fructose 2,6-bisphosphatase PFK-2 phosphorylates fructose 6-phosphate to generate fructose 2,6-bisphosphate, which is an important regulatory compound in glycolysis. This statement is true. Fructose 2,6-biphosphatase removes a phosphate from fructose 2,6-bisphosphate, yielding fructose 6-phosphate.
Insulin and glucagon are key hormones that regulate glucose metabolism on a global level. Insulin:
promotes the synthesis of fructose 2,6-bisphosphate, which inhibits gluconeogenesis The regulatory compound fructose 2,6-bisphosphate inhibits fructose 1,6-bisphosphatase, which is a gluconeogenic enzyme. Recall that insulin promotes satiety, glucose storage, and glucose breakdown in response to high blood glucose levels. Therefore, insulin promotes the synthesis of fructose 2,6-bisphosphate, which inhibits fructose 1,6-bisphosphatase, putting the brakes on gluconeogenesis.
Each of the following enzymes catalyzes a reaction in gluconeogenesis designed to bypass an irreversible reaction of glycolysis. Place each enzyme in chronological order, from first to last: fructose 1,6-bisphosphatase, phosphoenolpyruvate carboxykinase, pyruvate carboxylase, and glucose 6-phosphatase.
pyruvate carboxylase --> phosphoenolpyruvate carboxykinase --> fructose 1,6-bisphosphatase --> glucose 6-phosphatase The first two reactions of gluconeogenesis are designed to bypass the last (irreversible) reaction of glycolysis. The first reaction occurs in a mitochondrion, where the enzyme pyruvate carboxylase converts pyruvate into oxaloacetate. Oxaloacetate enters the cytosol via the malate-aspartate shuttle, where it is converted into PEP by PEP carboxykinase. The next workaround reaction is catalyzed by fructose 1,6-bisphosphatase, which converts fructose 1,6-bisphosphate into fructose 6-phosphate. Lastly, glucose 6-phosphatase removes a phosphate group from glucose 6-phosphate, generating a molecule of glucose.
The "committed step" is a regulatory mechanism employed in many metabolic pathways. One feature of a reaction that occurs at a committed step is that it is irreversible. In the committed step the cell commits to producing products from that pathway. Which of the following reactions is considered the commitment step of glycolysis?
the conversion of fructose 6-phosphate to fructose 1,6-bisphosphate The production of fructose 1,6-bisphosphate is the committed step of glycolysis. This reaction is catalyzied by phosphofructokinase. Once the cell produces fructose 1,6-bisphosphate, it must complete glycolysis because this compound cannot be used for anything else. Consequently, this is the most heavily regulated and rate-limiting step of glycolysis.
Which of the following glycolytic reactions require energy in the form of ATP? Select all that apply.
the conversion of glucose into glucose 6-phosphate and the conversion of fructose 6-phosphate into fructose 1,6- bisphosphate Hexokinase catalyzes the conversion of glucose to glucose 6-phosphate, transferring a phosphate from ATP. This reaction requires 1 molecule of ATP. Phosphofructokinase-1 converts fructose 6-phosphate into fructose 1,6-bisphosphate, transferring a phosphate group from ATP. This is another energy investment step and is heavily regulated in cells.
True or false: A high concentration of citrate inhibits glycolysis via negative feedback.
true: Citrate is a key intermediate in the citric acid cycle, which occurs after glycolysis in aerobic respiration. High levels of citrate indicate that the cell has plenty of raw materials to produce energy through this pathway, so it does not need to continue breaking down glucose through glycolysis.
True or false: Glycolysis is an anaerobic pathway.
true: Glycolysis does not require oxygen, unlike the citric acid cycle and electron transport chain within mitochondria. As a result, it can be used by aerobic cells when insufficient oxygen is present. Glycolysis is also the main route of energy production for anaerobic cells like bacteria.
