Exam 3 Flashcards
What is the main product of glycolysis?
ATP, NADH equivalents, and carbon left as pyruvate
Glycolysis converts glucose into these products.
Why so many phosphorylated intermediates?
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All products/reactants after glucose are phosphorylated. That is because…:
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Glucose is transported into cell via facilitated passive diffusion by a carrier (GLUT). This carrier is reversible. Phosphorylation changes charge and structure of glucose. This inhibits its transport through GLUT and out of cell.
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Energy released in hydrolysis of phosphodiester bond of ATP is partially retained in the phosphodiester bond of a product/reactant. High-energy intermediates (BPG and PEP) can then transfer phosphate to ADP to make ATP.
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Binding energy of phosphate intermediates to enzymes is increased because of phosphate. This helps drive catalysis.
Glycolysis converts glucose into these products.
Catabolism of di- and polysaccharides
- catabolism (digestion) begins in the mouth where salivary α-amylase (digest α1→4) hydrolyzes the glycosidic linkages of starch (physiological pH ~6.8)
- salivary α-amylase inactive in low pH of stomach, but pancreatic α-amylase secreted into small intestine is active and continues the digestion to produce maltose and maltotriose (di- and trisaccharides of glucose); these are converted to D-glucose by maltase
- also remaining are the branched saccharides (α1→6) — dealt with separately
In aerobic conditions, pyruvate enters the citric acid cycle.
Catabolism of di- and polysaccharides
- monosaccharides pass through intestinal cells to the bloodstream, which transports them to the liver or other tissues
- Membrane-bound hydrolases in the intestinal brush border hydrolyze disaccharides:
Catabolism of glycogen
- Storage of glucose primarily in skeletal muscle and hepatocytes (liver)
- glycogen phosphorylase catalyzes attack by inorganic phosphate on the terminal glucosyl residue at the nonreducing end of glycogen (Ch. 15)
- glucose 1-phosphate is released
- G1P is converted to G6P by phosphoglucomutase, then fed into glycolysis(reaction 2)
Catabolism of galactose
- created by hydrolysis of lactose to glucose + galactose in intestine
- enters glycolysis after several reactions
- galactose phosphorylated on C-1
- galactose 1-phosphate takes the uridine diphosphate (UDP) sugar-nucleotide from UDP-glucose (a product of this reaction) to generate glucose 1-phosphate for glycolysis and UDP-galactose
- UDP-galactose is oxidized on C-4 to a ketone, then reduced stereospecifically at C-4 to its epimer UDP-glucose
- UDP-glucose is used in step 2
- UDP acts as a coenzyme carrier of hexoses (helps with catalysis)
Fates of pyruvate post-glycolysis
- In aerobicconditions, it is oxidized to acetate (acetyl-CoA) →citric acid cycle
- if anaerobic: must keep glycolysis running to harvest net 2 ATP/glucose
- NAD+ will quickly become limiting if NADH + H+ not being used in oxidative phosphorylation (after citric acid cycle)
- = fermentation must regenerate NAD+
Fermentation: Fate of pyruvate under anaerobic conditions
- Extensive exercise, submerged plant tissues, solid tumors, erythrocytes (red blood cells lack organelles) have low levels of O2. Without O2, NADH generated during glycolysis can not be oxidized to NAD+ by mitochondria via the electron transport chain to produce additional ATP
- Pyruvate becomes the oxidizing agent for NADH oxidation. Produces lactate = lactic acid. Under this condition, there is no net gain of NAD+ and no generation of ATP by electron transport chain
Microbial fermentation
- yeast and other microorganisms ferment glucose to ethanol and CO2, rather than lactate
- product of glycolysis, pyruvate, is decarboxylated and reduced
- released CO2 is responsible for the bubbles in beer, champagne and dough rising
Gluconeogenesis
- conversion of pyruvate and related three- or four-carbon compounds to glucose
- occurs when glucose levels are really low and there is not enough glycogen in muscle and liver to supply it (e.g. during fasting, vigorous exercise, long lectures/tests, …)
- steps are almost the reverse of glycolysis; must bypass reactions that are nearly irreversible in the cell (reactions 1, 3, 10)
- bypasses are also irreversible, thus glycolysis and gluconeogenesis are both nearly irreversible
- note that the differences in steps mean that the chemical balance of glycolysis is not simply the reverse of gluconeogenesis
- a fraction of enzymes conduct glycolysis while another fraction conducts gluconeogenesis
- takes place in the liver, renal cortex (portion of kidney), and epithelial cells that line the inside of small intestine
- glucose passes into blood and carried to needed tissues
- energetically expensive, but essential given that the brain uses 120 g of glucose a day
Why isn’t gluconeogenesis = reverse of glycolysis?
- glycolysis: red arrows, top to bottom
- gluconeogenesis: blue arrows, bottom to top
Why isn’t gluconeogenesis = reverse of glycolysis? Part 2
3 reactions (red) in glycolysis are especially irreversible at cellular conditions, and thus must be bypassed for gluconeogenesis
Gluconeogenesis bypass 1 (= glycolysis R10)
- first bypass is the synthesis of phosphoenolpyruvate from pyruvate
- pyruvate is converted to oxaloacetate in mitochondria
- oxaloacetate is converted to phosphoenolpyruvate in the cytosol
- pyruvate carboxylase is first regulatory enzyme in gluconeogenesis. It requires acetyl-CoA as a positive effector (produced by FA oxidation). Lots of acetyl-CoA means lots of energy present from FA oxidation, thus turn on gluconeogenesis.
- PEP carboxykinase uses GTP to phosphorylate and decarboxylate oxaloacetate, forming PEP
Gluconeogenesis bypass 1 (= glycolysis R10) Part 2
- two competing pathways initiate gluconeogenesis, differing in NADH generation strategies
- [lactate] determines pathway 1 or 2
- enzyme to make oxaloacetate is only in mitochondrion
- cytosolic [NADH] is low and needs to be replenished for later in gluconeogenesis, but mitochondrial [NADH] is high
- so, either:
1. cytosolic [NADH] replenished via malate shuttling
2. cytosolic [NADH] replenished via oxidation of lactate to pyruvate
Regardless of 1 vs. 2, PEP continues with gluconeogenesis
Gluconeogenesis bypass 2 (= glycolysis R3)
- second bypass is the dephosphorylation (hydrolysis) of fructose 1,6-bisphosphate to fructose 6-phosphate
- note the generation of Pi
- catalyzed by fructose 1,6-bisphosphatase (FBPase-1)
- FBPase-1 is regulated via phosphorylation by a kinase
Gluconeogenesis bypass 3 (= glycolysis R1)
- third bypass is conversion of glucose 6-phosphate to glucose
- as with bypass 2, this is a dephosphorylation step, this time catalyzed by glucose 6-phosphatase
- note the generation of Pi
- enzyme only present in hepatocytes, renal cells and epithelial cells of small intestine. Thus gluconeogenesis is only possible in these cells!
Amino acids as a source of pyruvate
- almost all amino acids can be converted to pyruvate through an intermediate of the citric acid cycle, allowing conversion of protein -> AA -> glucose
- oxaloacetate is an intermediate of the citric acid cycle, and can be fed into gluconeogenesis
- useable AAs are called glucogenic
- note that Leu and Lys are not glucogenic
Pentose phosphate pathway
- needed by rapidly dividing cells (bone marrow, skin, intestinal mucosa); need pentoses to make DNA, RNA, ATP, NADPH, FADH2, and coenzyme A
- needed by tissues exposed directly to oxygen (RBCs, lens, cornea) because they have lots of damaging free radicals. Pentose phosphate pathway creates reducing atmosphere (high ratio of NADPH to NADP+, and high ratio of reduced to oxidized glutathione) that minimizes oxidative damage
- NADPH also needed for biosynthesis
Pentose phosphate pathway Part 2
- oxidizes and decarboxylates glucose 6-P
- end products are ribose 5-P, CO2, and NADPH
- the net result is the production of:
- NADPH, a reductant
- ribose 5-P, a precursor for nucleotide biosynthesis
[NADPH] regulates G6-P fate
- glucose 6-P can enter glycolysis or the pentose phosphate pathway
- when NADPH is forming faster than it is being used for biosynthesis and glutathione reduction, NADPH concentration rises and it inhibits the first enzyme in the pentose phosphate pathway (glucose 6-P dehydrogenase; previous slide)
- thus, high [NADPH] shifts use of glucose 6-P away from the pentose phosphate pathway and more toward glycolysis = feedback inhibition
Fates of glucose
- the complete oxidation of glucose to carbon dioxide and water proceeds with a standard free-energy change of -2,840 kJ/mol
- glycolysis is first part of this, handling oxidation of glucose to pyruvate
Summary of gluconeogenesis
- for each molecule of glucose formed from pyruvate, 6 high energy phosphate groups are required, 4 from ATP and 2 from GTP
- 2 molecules of NADH for the reduction of 2 molecules of 1,3-bisphosphoglycerate
- note the summation (—>): gluconeogenesis ≠ glycolysis
- gluconeogenesis is expensive
- glycolysis and gluconeogenesis are reciprocally regulated in cells that can do both
Summary of glycolysis
- Glycolysis: glucose -> ATP + NADH equivalents and carbon left as pyruvate
- Most sugars enter glycolysis as glucose or fructose
- Pyruvate enters citric acid cycle to turn into CO2 (aerobic) or ferments to lactic acid (anaerobic); EtOH in microbes
- Gluconeogenesis allows the synthesis of glucose from pyruvate in a pathway using many, but not all, steps of glycolysis
Feeder pathways for glycolysis
- Many carbohydrates are catabolized through glycolysis
- Some are converted to D-glucoseor glycolytic intermediates
- the most significant are the
- monosaccharides fructose, mannose, and galactose
- disaccharides maltose, lactose, trehalose, and sucrose
They are funneled into glycolysis at different points, allowing the cell to harvest energy from a wide range of dietary sugars efficiently
In aerobic conditions, pyruvate enters the citric acid cycle.
What are the fates of pyruvate under aerobic conditions?
Oxidized to acetate (acetyl-CoA) for the citric acid cycle
In aerobic conditions, pyruvate enters the citric acid cycle.
What are the fates of pyruvate under anaerobic conditions?
Fermentation to lactate or ethanol
NAD+ must be regenerated during anaerobic fermentation.
What role does NAD+ play in glycolysis?
Acts as an electron carrier
NAD+ is essential for the continuation of glycolysis.
What is gluconeogenesis?
Conversion of pyruvate and related compounds to glucose
Occurs when glucose levels are low.
Why is gluconeogenesis not simply the reverse of glycolysis?
It bypasses three irreversible reactions of glycolysis
These reactions must be bypassed due to their irreversibility.
What initiates gluconeogenesis?
Pyruvate is converted to oxaloacetate
This conversion occurs in the mitochondria.
What is the first regulatory enzyme in gluconeogenesis?
Pyruvate carboxylase
It requires acetyl-CoA as a positive effector.
What is the role of phosphoenolpyruvate (PEP) in gluconeogenesis?
PEP is formed from oxaloacetate and continues the pathway
PEP is an important intermediate in gluconeogenesis.
What carbohydrates can be catabolized through glycolysis?
Monosaccharides (fructose, mannose, galactose) and disaccharides (maltose, lactose, trehalose, sucrose)
These sugars enter glycolysis at different points.
What initiates the catabolism of di- and polysaccharides?
Salivary α-amylase in the mouth
It hydrolyzes glycosidic linkages of starch.
What is the main enzyme that converts glycogen to glucose-1-phosphate?
Glycogen phosphorylase
It catalyzes the attack by inorganic phosphate on glycogen.
How is galactose converted to glucose-1-phosphate?
Galactose is phosphorylated and undergoes several reactions
UDP-galactose is oxidized to its epimer, UDP-glucose.
What is the outcome of microbial fermentation of glucose?
Production of ethanol and CO2
This differs from lactate production in humans.
What is the primary method of energy generation during extensive exercise when O2 levels are low?
Anaerobic glycolysis generating lactate
This allows glycolysis to continue despite low oxygen.
What does the term ‘feeder pathways’ refer to in glycolysis?
Pathways through which carbohydrates enter glycolysis
This includes various sugars being converted into intermediates.
What is the significance of phosphorylated intermediates in glycolysis?
They enhance binding energy to enzymes and drive catalysis
This helps in the efficient progression of glycolysis.
What are the primary tissues that store glucose as glycogen?
Skeletal muscle and liver (hepatocytes)
These tissues are crucial for glucose storage and release.
What is the role of NADPH in cellular processes?
Acts as a regulator and reducing agent
It is involved in various biosynthetic reactions.
Fill in the blank: Glycolysis converts glucose to _______ and pyruvate.
ATP and NADH equivalents
True or False: Glycolysis can run backwards to produce glucose.
False
While gluconeogenesis is essentially the reverse of glycolysis, specific steps are bypassed.
What is the role of oxaloacetate in gluconeogenesis?
Oxaloacetate is an intermediate in gluconeogenesis
Where is oxaloacetate found?
Only in mitochondrion
What is the state of cytosolic [NADH] during gluconeogenesis?
Cytosolic [NADH] is low and needs to be replenished
How can cytosolic [NADH] be replenished?
Via malate shuttling or oxidation of lactate to pyruvate
What is the first bypass in gluconeogenesis?
PEP continues with gluconeogenesis
What is the second bypass in gluconeogenesis?
Dephosphorylation of fructose 1,6-bisphosphate to fructose 6-phosphate
What enzyme catalyzes the second bypass in gluconeogenesis?
Fructose 1,6-bisphosphatase (FBPase-1)
What regulates FBPase-1?
Regulated via phosphorylation by a kinase
What is the third bypass in gluconeogenesis?
Conversion of glucose 6-phosphate to glucose
What enzyme catalyzes the third bypass in gluconeogenesis?
Glucose 6-phosphatase
Where is glucose 6-phosphatase present?
In hepatocytes, renal cells, and epithelial cells of small intestine
What is required for each molecule of glucose formed from pyruvate in gluconeogenesis?
6 high energy phosphate groups: 4 from ATP and 2 from GTP
How many molecules of NADH are needed for gluconeogenesis?
2 molecules of NADH
What is the difference between gluconeogenesis and glycolysis?
Gluconeogenesis is expensive and not equivalent to glycolysis
How are glycolysis and gluconeogenesis regulated?
Reciprocally regulated in cells that can do both
What can almost all amino acids be converted to?
Pyruvate through an intermediate of the citric acid cycle
What are glucogenic amino acids?
Amino acids that can be converted to glucose
Which amino acids are not glucogenic?
Leucine (Leu) and Lysine (Lys)
What does glycolysis produce?
ATP + NADH equivalents and carbon left as pyruvate
What happens to pyruvate under aerobic conditions?
Enters the citric acid cycle to turn into CO2
What does gluconeogenesis allow?
Synthesis of glucose from pyruvate
What is the pentose phosphate pathway needed for?
Rapidly dividing cells to make DNA, RNA, ATP, NADPH, FADH2, and coenzyme A
What does the pentose phosphate pathway create?
A reducing atmosphere that minimizes oxidative damage
What are the end products of the pentose phosphate pathway?
Ribose 5-P, CO2, and NADPH
What happens when NADPH is forming faster than it is being used?
NADPH concentration rises and inhibits glucose 6-P dehydrogenase
What regulates the fate of glucose 6-P?
[NADPH] regulates G6-P fate
What is the standard free-energy change for the complete oxidation of glucose?
-2,840 kJ/mol
What is the total number of proteins involved in metabolic regulation in eukaryotes?
30,000
What do enzymes responsible for metabolic pathways have in common?
They are under tight regulation by the cell via multiple mechanisms
What can E. coli get all of its carbon from?
Glucose
What is glycogen stored as in vertebrates?
Granules in hepatocytes
What provides a quick source of energy in muscles?
Glycogen
What takes longer to catabolize than carbohydrates?
Fats
What is homeostasis?
A property of a system that regulates its internal environment to maintain a stable and relatively constant condition
What do cells and organisms maintain to achieve homeostasis?
A dynamic steady state
What happens to the concentrations of intermediate molecules during metabolic processes?
They remain approximately constant
What may result from the failure of homeostatic mechanisms?
Human diseases
What conditions can result from an excess of particular metabolic products?
Hyperglycemia and diabetes
What processes hold cellular parameters constant over time?
Metabolic regulation
What does metabolic control change?
The output of a metabolic pathway over time
What is the biochemical standard free energy denoted as?
∆G’o
What is the equilibrium constant at equilibrium represented as?
K_eq’
What is the mass action ratio denoted as?
Q
What do small K_eq’ values favor?
Reactants
What do large K_eq’ values favor?
Products
What is the role of glucagon?
Signals low [glucose] in blood and tells the liver to release glucose
What is the role of insulin?
Signals high [glucose] in blood and tells the liver to absorb glucose for storage
What happens to glucagon levels when blood glucose is low?
It is released to signal the liver to stop glucose consumption
Which enzymes are bypassed in glycolysis and gluconeogenesis regulation?
- Hexokinase
- Phosphofructokinase-1 (PFK-1)
- Pyruvate kinase
What are isozymes?
Enzymes evolved from a common gene that may differ in their use of cofactors and kinetic parameters
How is hexokinase IV regulated?
Inhibited by a regulatory protein specific to hepatocytes
What activates hexokinase IV at high [glucose]?
Regulatory protein releases it to return to cytosol
What does PFK-1 catalyze?
The committed step to glycolysis
What activates PFK-1?
AMP
What indicates that cells are meeting energy needs via the citric acid cycle?
High [citrate]
What is F2,6BP’s role in glycolysis?
Increases PFK-1 affinity for substrate and reduces its affinity for inhibitors
Which enzyme is activated by F1,6BP?
Pyruvate kinase
What is the effect of glucagon on liver pyruvate kinase?
It phosphorylates and inactivates the liver L isozyme
What hormone is released into the blood at low glucose levels?
Glucagon
What does glucagon activate to regulate glucose levels?
cAMP-dependent protein kinase A (PKA)
What is the effect of PKA on the liver L isozyme?
It phosphorylates and inactivates it
How does glucagon affect glucose use in the liver?
Slows the use of glucose for fuel
What does pyruvate carboxylase favor?
Gluconeogenesis
What does pyruvate dehydrogenase complex inhibit?
Creation of more acetyl-CoA
What indicates that a cell’s energy needs are met?
Acetyl-CoA
What is the primary regulatory mechanism for glycolysis and gluconeogenesis?
Reciprocal regulation
What catalyzes glycogen breakdown?
Glycogen phosphorylase
Where does glycogen breakdown predominantly occur?
Liver
What enzyme removes terminal glucose from glycogen?
Glycogen phosphorylase
What is the role of the bifunctional debranching enzyme?
Resolve branch points in glycogen
What is produced by phosphoglucomutase?
Glucose-6-P
What happens to glucose-6-P in the muscle?
Enters glycolysis
What happens to glucose-6-P in the liver?
Converted to glucose and released into blood
What transporter moves glucose-6-P into the Endoplasmic Reticulum?
G6-P transporter (T1)
What enzyme dephosphorylates G6-P in the liver?
Glucose 6-phosphatase
How is glucose transported out of the cytoplasm?
Via GLUT2
What molecule initiates glycogen synthesis?
Glycogenin
What does glycogenin do during glycogen synthesis?
Transfers glucose from UDP-glucose to itself
What is the role of glycogen synthase?
Transfers glucose residues to glycogen
What enzyme forms α1-6 linkages during glycogen synthesis?
Amylo transglycolase
What initiates the regulation of glycogen catabolism in muscle?
Epinephrine
What initiates the regulation of glycogen catabolism in the liver?
Glucagon
What is the active form of glycogen phosphorylase?
Glycogen phosphorylase a
What form of glycogen phosphorylase is less active?
Glycogen phosphorylase b
What hormone promotes the conversion of glycogen phosphorylase a to b?
Insulin
What is the effect of high glucose levels on glycogen phosphorylase a?
Induces conformational change reducing activity
What is the default state of glycogen synthase?
Glycogen synthase a
How can glycogen synthase a be inactivated?
Phosphorylation
What enzyme inactivates glycogen synthase?
Glycogen synthase kinase 3 (GSK3)
What is the role of insulin in glycogen synthesis?
Inhibits GSK3
What does glucose-6-P do to glycogen synthase b?
Binds to an allosteric site, enhancing dephosphorylation
What is the role of protein phosphatase 1 (PP1) in glycogen metabolism?
Dephosphorylates enzymes to promote glycogen synthesis
What is the result of insulin binding to its receptor?
Activation of receptor-bound Tyr protein kinase
What does PDK-1 activate in the insulin signaling pathway?
Protein kinase PKB
What does PKB phosphorylate to regulate glycogen synthesis?
Glycogen synthase kinase 3 (GSK3)
What is the effect of PTP1B on glycogen synthesis?
Inhibits the process
What happens to GSK3 when it is OFF?
It is unable to inactivate glycogen synthesis.
GSK3 is a kinase that typically phosphorylates and inactivates glycogen synthase.
What is the role of PP1 in glycogen synthesis?
PP1 can dephosphorylate glycogen synthase b to form glycogen synthase a.
Glycogen synthase a is the active form that can synthesize glycogen.
What inhibits glycogen synthase a?
It is inhibited by PTP1B.
PTP1B is a protein tyrosine phosphatase that negatively regulates insulin signaling.
What is the role of GM in insulin signaling?
GM serves to anchor proteins to glycogen particles and is phosphorylated by insulin to activate it.
This phosphorylation recruits the PP1-complex to promote glycogen synthesis.
How does insulin affect glycogen phosphorylase?
Insulin stimulates PP1 to inactivate glycogen phosphorylase.
This prevents glycogen breakdown.
What is the effect of epinephrine on glycogen metabolism?
Epinephrine stimulates PKA, which phosphorylates GM to disassemble the PP1-complex.
This results in the opposite effect of insulin, discouraging glycogen synthesis.
What happens in the liver after a meal?
Insulin is released, GSK3 is inactivated, and PP1 is activated, promoting glycogen synthesis and glycolysis.
This leads to the activation of glycogen synthase and inactivation of glycogen phosphorylase.
What role does hexokinase IV play in glucose metabolism?
Hexokinase IV is released from a regulatory protein in the nucleus, promoting glycolysis.
This occurs when there is extra glucose available.
What occurs in the liver when fasting?
Glucagon is released, activating PKA, which promotes glycogen breakdown and disfavors glycolysis.
This results in the activation of glycogen phosphorylase and inactivation of glycogen synthase.
What is the effect of glucagon on pyruvate kinase L?
Glucagon inactivates pyruvate kinase L from glycolysis.
This is part of the process that favors gluconeogenesis over glycolysis.
Fill in the blank: F2,6BP activates _______ in glycolysis.
PFK-1.
F2,6BP also inhibits FBPase-1 in gluconeogenesis.
True or False: Insulin promotes glycogen synthesis and glycolysis.
True.
Insulin signaling activates pathways that favor energy storage and usage.
What is the overall effect of glucagon on carbohydrate metabolism?
Promotes glycogen breakdown and disfavors glycolysis.
This helps maintain blood glucose levels during fasting.
