Chapter 9 and 10: Carbohydrate Metabolism Flashcards
(119 cards)
1
Q
GLUT 2
A
Low affinity transporter in hepatocytes and pancreatic cells; after a meal, blood traveling through the hepatic portal vein from the intestine is rich in glucose; GLUT 2 captures the excess glucose primarily for storage
2
Q
What happens when the glucose concentration drops below the Km for GLUT 2?
A
Much of the remainder leaves the liver and enters the peripheral circulation
3
Q
What is the Km for GLUT 2?
A
Quite high (~15 mM)
4
Q
What does the liver do?
A
The liver will pick up excess glucose and store it only after a meal, when blood glucose levels are high; in the β-islet cells of the pancreas, GLUT 2, along with the glycolytic enzyme glucokinase, serves as the glucose sensor for insulin release
5
Q
GLUT 4
A
Adipose tissue and muscle; responds to the glucose concentration in the peripheral blood
6
Q
What does insulin do?
A
The rate of glucose transport is increased in GLUT 4; insulin stimulates the movement of additional GLUT 4 transporters to the membrane by a mechanism involving exocytosis
7
Q
What is the Km of GLUT 4?
A
5 mM (normal glucose concentration in the blood is 5.6 mM or between 4-6 mM); the transporters become saturated when blood glucose levels are just a bit higher than normal
8
Q
What does muscle store excess glucose as? Adipose tissue?
A
Glycogen; dihydroxyacetone phosphate (DHAP) —> glycerol phosphate to store incoming fatty acids as triacylglycerols
9
Q
Glycolysis
A
Cytoplasmic pathway that converts glucose into two pyruvates, releasing a modest amount of energy captured in two SLP and one oxidation reaction; if the cell has mitochondria and oxygen, the energy-carriers produced in glycolysis (NADH) can feed into the aerobic respiration pathway to generate energy for the cell; also provides intermediates for other pathways
10
Q
What are GLUT transporters specific for?
A
Glucose (not phosphorylated glucose AKA glucose-6-phosphate); this traps the glucose inside the cell once it has entered via facilitated diffusion or active transport
11
Q
Hexokinase
A
Widely distributed in tissues and is inhibited by its product glucose-6-phosphate
12
Q
Glucokinase
A
Found only in liver cells and pancreatic β-islet cells; in the liver, glucokinase is induced by insulin
13
Q
Where is hexokinase found?
A
Present in most tissues
14
Q
Where is glucokinase found?
A
Present in hepatocytes and pancreatic β-islet cells (along with GLUT 2, acts as the glucose sensor)
15
Q
What is the Km of hexokinase? Glucokinase?
A
Low Km (reaches maximum velocity at a low [glucose]); high Km (acts on glucose proportionally to its concentration)
16
Q
What is hexokinase inhibited by?
A
Glucose-6-phosphate
17
Q
What is glucokinase induced by?
A
Insulin in hepatocytes
18
Q
Phosphofructokinase 1 (PFK-1)
A
PFK-1 is the rate-limiting enzyme and main control point in glycolysis; phosphorylates fructose 6-phosphate to form fructose 1,2-bisphosphate using ATP
19
Q
What inhibits PFK-1?
A
ATP and citrate; activated by AMP (when the cell has high ATP)
20
Q
Phosphofructokinase 2
A
PFK-2 converts a tiny amount of fructose 6-phosphate to fructose 2,6-bisphosphate (F2,6-BP); F2,6-BP activates PFK-1; insulin stimulates PFK-1 and glucagon inhibits PFK-1 via an indirect mechanism involving PFK-2 and fructose 2,6-bisphosphate
21
Q
What does activation of PFK-2 do?
A
PFK-2 is found mostly in the liver; by activating PFK-1, it allows these cells to override the inhibition caused by ATP so that glycolysis can continue, even when the cell is energetically satisfied; the metabolites of glycolysis can thus be fed into the production of glycogen, fatty acids, and other storage molecules rather than just being burned to produce ATP
22
Q
Glyceraldehyde-3-phosphate dehydrogenase
A
Catalyzes an oxidation and addition of inorganic phosphate (Pi) to its substrate, glyceraldehyde 3-phosphate; results in the production of a high-energy intermediate 1,3-bisphosphoglycerate and the reduction of NAD+ to NADH
23
Q
3-Phosphoglycerate Kinase
A
Transfers the high-energy phosphate from 1,3-bisphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate; substrate-level phosphorylation
24
Q
Pyruvate kinase
A
The last enzyme in aerobic glycolysis, it catalyzes a substrate-level phosphorylation of ADP using the high-energy phosphoenolpyruvate (PEP); activated by fructose 1,6-bisphosphate from the PFK-1 reaction; referred to as feed-forward activation, meaning that the product of an earlier reaction of glycolysis stimulates or prepares a later reaction in glycolysis
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What is the key fermentation enzyme in mammalian cells?
In the absence of oxygen, fermentation will occur; lactate dehydrogenase which oxidizes NADH to NAD+, replenishing the oxidized coenzyme for glyceraldehyde-3-phosphate dehydrogenase
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What is fermentation in yeast cells?
Conversion of pyruvate to ethanol and CO2; result is the same as mammalian cells: replenishing NAD+
27
What are the important intermediates of glycolysis?
```
Dihydroxyacetone phosphate (DHAP) - used in hepatic and adipose tissue for triacylglycerol synthesis; formed from fructose 1,6-bisphosphate; can be isomerized to glycerol 3-phosphate, which can then be converted to glycerol
1,3-bisphosphoglycerate (1,3-BPG) and phosphoenolpyruvate (PEP) are high-energy intermediates used to generate ATP by SLP (the only ATP gained in anaerobic respiration)
```
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Irreversible enzymes
Keeps the pathway moving in one direction; glucokinase/hexokinase; PFK-1; pyruvate kinase
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What enzymes do RBCs have in particular?
Bisphophoglycerate mutase - produces 2,3-BPG from 1,3-BPG in glycolysis; (mutases are enzymes that move a functional group from one place in a molecule to another); 2,3-BPG binds allosterically to the β-chains of HbA and decreases its affinity for oxygen
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Irreversible steps of glycolysis
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How Glycolysis Pushes Forward the Process: Kinases
Hexokinase
Glucokinase
PFK-1
Pyruvate kinase
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What are the important enzymes in galactose metabolism?
Galactokinase - phosphorylates galactose to trap it in the cell
Galactose-1-phosphate uridyltransferase - converts galactose 1-phosphate to glucose 1-phosphate
Epimerase - enzyme that catalyzes the conversion of one sugar epimer to another
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Fructokinase
Phosphorylates fructose to trap it in the cell
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Aldolase B
Cleaves fructose 1-phosphate into glyceraldehyde and DHAP
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Uses of acetyl-CoA
Entry into the citric acid cycle if ATP is needed or for fatty acid synthesis if sufficient ATP is present
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Pyruvate dehydrogenase complex (PDH)
Irreversible; activated by insulin in the liver; complex of enzymes carrying out multiple reactions in succession
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What are the three possible fates of pyruvate?
Conversion to acetyl-CoA by PDH, conversion to lactate by lactate dehydrogenase, or conversion to oxaloacetate by pyruvate carboxylase
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What cofactors and coenzymes are required by PDH?
Thiamine pyrophosphate, lipoic acid, CoA, FAD, and NAD+
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What is PDH inhibited by?
Acetyl-CoA; build up of acetyl-CoA shifts pyruvate conversion into oxaloacetate
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Glycogen
Branched polymer of glucose, represents a storage form of glucose; stored in the cytoplasm as granules - central protein cores with polyglucose chains radiating outward to form a sphere; glycogen granules composed entirely of linear chains have the highest density of glucose near the core; if the chains are branched, the glucose density if highest at the periphery of the granule, allowing more rapid release of glucose on demand
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Purpose of glycogen in the liver? Muscle?
Source of glucose that is mobilized between meals to prevent low blood sugar; stored as an energy reserve for muscle contraction
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Glycogenesis
Synthesis of glycogen granules; begins with a core protein called glycogenin; glucose addition to a granule begins with glucose 6-phosphate which is converted to glucose 1-phosphate which is activated by coupling to a molecule of uridine diphosphate (UDP) which permits integration into the glycogen chain by glycogen synthase; activation occurs when glucose 1-phosphate interacts with uridine triphosphate (UTP), forming UDP-glucose and pyrophosphate
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Glycogen synthase
Rate-limiting enzyme of glycogen synthesis and forms the α-1,4 glycosidic bond found in the linear glucose chains of the granule; stimulated by glucose 6-phosphate and insulin; inhibited by epinephrine and glucagon through a protein kinase cascade that phosphorylates and inactivates the enzyme
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Branching enzyme (glycosyl α-1,4:α-1,6 transferase)
Hydrolyzes one of the α-1,4 bonds to release a block of oligoglucose (a few glucose molecules bound together in a chain) which is then moved and added in a slightly different location; forms an α-1,6 bond to create a branch
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Glycogen phosphorylase
Breaks α-1,4 glycosidic bonds, releasing glucose 1-phosphate from the periphery of the granule; cannot break α-1,6 bonds and therefore stops when it nears the outermost branch points; activated by glucagon in the liver and AMP and epinephrine in skeletal muscle; inhibited by ATP
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Debranching enzyme (glucosyl α-1,4:α-1,4 transferase and α-1,6 glucosidases)
Breaks an α-1,4 bond adjacent to the branch point and moves the small oligoglucose chain that is released to the exposed end of the other chain; forms a new α-1,4 bond; hydrolyzes the α-1,6 bond, releasing a single free glucose (only free glucose released in glycogenolysis)
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Isoforms
Slightly different versions of the same protein
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Gluconeogenesis
Performed by the liver and kidneys (but mainly the liver); pathways are promoted by glucagon and epinephrine which act to raise blood sugar levels and are inhibited by insulin which acts to lower blood sugar levels; after 24 hours of starvation, it becomes the sole source of glucose
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What are important substrates of gluconeogenesis?
Glycerol 3-phosphate (from stored fats, or triacylglycerols, in adipose tissue)
Lactate (from anaerobic glycolysis)
Glucogenic amino acids (from muscle proteins)
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Glucogenic amino acids
Include all except lysine and leucine; can be converted into intermediates that feed into gluconeogenesis
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Ketogenic amino acids
Can be converted into ketone bodies which can be used as alternative fuel, particularly during periods of prolonged starvation
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Propionyl-CoA
Glucogenic; formed when fatty acids with an odd number of carbon atoms is metabolized
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Alanine aminotransferase
Converts alanine into pyruvate
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Glycerol-3-phosphate dehydrogenase
Converts glycerol 3-phosphate into dihydroxyacetone phosphate (DHAP)
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Pyruvate carboxylase
Mitochondrial enzyme that is activated by acetyl-CoA (from β-oxidation); the product oxaloacetate (OAA) is a citric acid cycle intermediate and cannot leave the mitochondrion; OAA is reduced to malate which leaves through malate-aspartate shuttle and is oxidized to OAA in the cytoplasm
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What inhibits pyruvate dehydrogenase?
Acetyl-CoA
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What is the source of acetyl-CoA?
Burning of fatty acids; used to produce glucose in the liver during gluconeogenesis; stop the forward flow of the CAC; produce massive amounts of OAA that can eventually lead to glucose production for the rest of the body
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Phosphoenolpyruvate carboxylase
PEPCK in the cytoplasm; induced by glucagon and cortisol; converts OAA to PEP in a reaction that requires GTP; PEP continues in the pathway to F-1,6-BP; the combination of pyruvate carboxylase and phosphoenolpyruvate carboxylase circumvent the action of pyruvate kinase by converting pyruvate back into PEP
58
Fructose-1,6-bisphosphatase
Cytoplasmic enzyme; key control point of gluconeogenesis and represents the rate-limiting step of the process; reverses the action of PFK-1, the rate-limiting step of glycolysis by hydrolyzing phosphate from F-1,6-BP to produce F-6-P; activated by ATP and inhibited by AMP and F-2,6-BP
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Glucose-6-phosphatase
Found only in the lumen of the ER in liver cells; glucose-6-phosphate is transported into the ER and free glucose is transported back into the cytoplasm where it can diffuse out of the cell via GLUT transporters; absent in muscle cells —> muscles don’t produce blood glucose; glucose-6-phosphatase is used to circumvent glucokinase and hexokinase
60
What is glucose produced by the liver used for?
Hepatic gluconeogenesis does not represent an energy source for the liver; requires the expenditure of ATP that is provided by β-oxidation of fatty acids; hepatic gluconeogenesis is always dependent on β-oxidation of fatty acids in the liver
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Pentose phophate pathway (PPP)
AKA the hexose monophosphate shunt; occurs in the cytoplasm of ALL cells; two major functions: production of NADPH and serving as a source of ribose 5-phosphate for nucleotide synthesis
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Glucose-6-phosphate dehydrogenase
Induced by insulin (abundance of sugar entering the cell will be shunted into both fuel utilization pathways - glycolysis and aerobic respiration and fuel storage pathways - fatty acid synthesis, glycogenesis and the PPP); inhibited by NADPH and activated by NADP+
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Second part of the PPP
Starts with ribulose-5-phosphate; a series of reversible reactions that produce an equilibrated pool of sugars for biosynthesis, including ribose 5-phosphate for nucleotide synthesis; F-6-P and G3P are products that can enter glycolysis; pentoses can also be made from glycolytic intermediates without going through the G6PD reactions via interconversions by transaldolase and transketolase
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Functions of NADPH
Electron donor in a number of biochemical reactions; potent reducing agent
Biosynthesis, mainly of fatty acids and cholesterol; assisting in cellular bleach production in certain WBCs, thereby contributing to bactericial activity; maintenance of a supply of reduced glutathione to protect against ROS (especially against free radical oxidative damage caused by peroxides)
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What do free radicals do?
Attack lipids (including phospholipids in the cell membrane); makes the membrane weak, causing cell lysis (especially in RBCs)
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Glutathione
Reducing agent that can help reverse radical formation before damage is done to the cell
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Citric acid cycle
Krebs cycle/TCA cycle (tricarboxylic acid cycle); occurs in the mitochondria; the main function of the CAC is to oxidize acetyl-CoA to CO2 and H2O; this cycle produces the high-energy electron-carrying molecules NADH and FADH2
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Acetyl-CoA
Can be obtained from the metabolism of carbohydrates, fatty acids, and amino acids
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PDH complex enzymes
PDH, dihydrolipoyl transacetylase, dihydrolipolyl dehydrogenase, pyruvate dehydrogenase kinase, and pyruvate dehydrogenase phosphatase; the reaction is exergonic (-33.4 kJ/mol); inhibited by NADH and acetyl-CoA
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Coenzyme A (CoA)
Thiol, containing an -SH group; when acetyl-CoA forms, it does so via covalent attachment of the acetyl group to the -SH group, resulting in the formation of a thioester, which contains sulfur instead of the typical oxygen ester -OR
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Pyruvate dehydrogenase (PDH)
Pyruvate is oxidized, yielding CO2, while the remaining 2C molecule binds covalently to thiamine pyrophosphate (TPP or vitamin B1); TPP is a coenzyme held by noncovalent interactions to PDH; Mg2+ is also required
72
Dihydrolipoyl transacetylase
2C molecule bonded to TPP is oxidized and transferred to lipoic acid, a coenzyme that is covalently bonded to the enzyme; lipoic acid’s disulfide group acts as an OA, creating the acetyl group which is bonded to lipoic acid via thioester linkage; dihydrolipoyl transacetylase catalyzes the CoA-SH interaction with the newly formed thioesterlink, causing transfer of an acetyl group to form acetyl-CoA
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Dihydrolipoyl dehydrogenase
Flavin adenine dinucleotide (FAD) is used as a coenzyme in order to reoxidize lipoic acid, allowing lipoic acid to facilitate acetyl-CoA formation in future reactions; as lipoic acid is reoxidized, FAD is reduced to FADH2 which is reoxidized to FAD, while NAD+ is reduced to NADH
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Fatty acid oxidation (β-oxidation)
Occurs in the intermembrane space; activation causes a thioester bond to form between the carboxyl groups of fatty acids and CoA; activated fatty acyl-CoA is transported to the intermembrane space of the mitochondrion; the fatty acyl group is tranferred to carnitine via a transesterification reaction; carnitine is a molecule that can cross the inner membrane with a fatty acyl group in tow; it transfers the fatty acyl group to a mitochondrial CoA-SH via another transesterification reaction; β-oxidation involving the removal of 2C fragments from the carboxyl end can then occur
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Amino acid catabolism
Amino acids must lose their amino group via transamination; their carbon skeletons can then form ketone bodies which can be converted to acetyl-CoA
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Ketones
Reverse of ketone synthesis from acetyl-CoA (when PDH is inhibited)
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Alcohol
When alcohol is consumed in moderate amounts, alcohol dehydrogenase and acetylaldehyde dehydrogenase convert it to acetyl-CoA; however, NADH buildups which inhibits the Krebs cycle —> acetyl-CoA formed through this process is used primarily to synthesize fatty acids
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CAC Step 1
Citrate Formation; acetyl-CoA and oxaloacetate undergo a condensation reaction to form citryl-CoA, an intermediate; hydrolysis of citryl-CoA yields citrate and CoA-SH; catalyzed by citrate synthase - enzymes that form new covalent bonds without requiring significant energy input; second step energetically favours the formation of citrate
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CAC Step 2
Citrate isomerized to isocitrate; achiral citrate is isomerized to one of four possible isomers of isocitrate; citrate binds at 3 points to aconitase; water is lost from citrate, yielding cis-aconitate; water is added back to form isocitrate
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Aconitase
Metalloprotein that requires Fe2+
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CAC Step 3
α-ketoglutarate and CO2 formation
isocitrate is first oxidized to oxalosuccinate by isocitrate dehydrogenase; oxalosuccinate is decarboxylated to produce α-ketoglutarate and CO2
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Isocitrate dehydrogenase
Rate-limiting enzyme of the CAC
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CAC Step 4
Succinyl-CoA and CO2 formation; reactions carried out by α-ketoglutarate dehydrogenase complex; α-ketoglutarate, succinyl-CoA, and CoA come together and produce a molecule of CO2
Involves TPP, lipoic acid, Mg2+ with reduction of NAD+ to NADH; similar to PDH complex
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Dehydrogenase
Subtype of oxidoreductases that transfer a H- ion to an electron acceptor
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CAC Step 5
Succinate formation; hydrolysis of the thioester bond on succinyl-CoA yields succinate and CoA-SH and is coupled to the phosphorylation of GDP to GTP; reaction is catalyzed by succinyl-CoA synthetase - creates a new covalent with energy input; nucleoside diphosphate kinase catalyzes the phosphate transfer from GTP to ADP
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CAC Step 6
Fumarate formation; doesn’t occur in the mitochondrial matrix but on the inner membrane; catalyzed by succinate dehydrogenase; succinate undergoes oxidation to yield fumarate
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Succinate dehydrogenase
Flavoprotein because it is covalently bonded to FAD; integral protein on the inner membrane; as succinate is oxidized to fumarate, FAD is reduced to FADH2
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CAC Step 7
Malate formation; enzyme fumarase catalyzes the hydrolysis of the alkene bond in fumarate, giving rise to malate (only L-malate forms)
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CAC Step 8
Oxaloacetate formed anew; the enzyme malate dehydrogenase catalyzes the oxidation of malate to oxaloacetate; a third and final molecule of NAD+ is reduced to NADH
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Pyruvate Dehydrogenase Complex reaction
Pyruvate + CoA-SH + NAD+ —> acetyl-CoA + NADH + CO2 + H+
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Citric Acid Cycle reaction
Acetyl-CoA + 3NAD+ FAD + GDP + Pi + 2H2O —> 2CO2 + 3NADH + 3H+ + FADH2 + GTP + CoA-SH
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ATP Production
30-32 ATP
4NADH —> 10 ATP
1 FADH2 —> 1.5 ATP
1 GTP —> 1 ATP
Total: 12.5 ATP per pyruvate or 25 ATP per glucose + (2 ATP and 2 NADH = 5 ATP from glycolysis)
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PDH complex regulation
Pyruvate dehydrogenase kinase - phosphorylates PDH and inhibits acetyl-CoA production (high ATP)
Pyruvate dehydrogenase phosphatase - dephosphorylates PDH and reactivates acetyl-CoA production (high ADP)
Acetyl-CoA - negative feedback effects on its own production
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Which enzymes of the CAC are control points?
Citrate synthase, isocitrate dehydrogenase (RLE), α-ketoglutarate dehydrogenase complex
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Citrate synthase
ATP, NADH, citrate, and succinyl-CoA are allosteric inhibitors
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Isocitrate dehydrogenase
ATP and NADH inhibit; ADP and NAD+ allosterically activate and enhance its affinity for substrates
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α-ketoglutarate dehydrogenase complex
Succinyl-CoA and NADH function as inhibitors of this enzyme complex
ATP is inhibitory and slows the rate of the cycle when the cell has high levels of ATP
Stimulated by ADP and Ca2+
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Is the formation of ATP endergonic or exergonic? Electron transport?
Endergonic; exergonic; therefore, coupling these reactions can allow for the energy from one reaction to fuel the other
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Complex I
NADH-CoQ oxidoreductase
The transfer of electrons from NADH to coenzyme Q (CoQ) is catalyzed by this first complex; has over 20 subunits including an iron-sulfur cluster and a flavoprotein that oxidizes NADH; the flavoprotein has a coenzyme called flavin mononucleotide (FMN) covalently bound to it
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What happens at complex I
NADH transfers e- to FMN, forming FMNH2
FMNH2 transfers e- to iron-sulfur cluster —> iron-sulfur cluster reduced
Reduced iron-sulfur cluster transfers e- to CoQ (ubiquinone) to form CoQH2
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What are the reactions at complex I?
NADH + H+ + FMN —> NAD+ + FMNH2
FMNH2 + 2 Fe-S oxidized —> FMN + 2 Fe-S reduced + 2H+
2 Fe-S reduced + CoQ —> 2 Fe-S oxidized + CoQH2
(NOTE 4 H+ are passed into the intermembrane space)
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Complex II
Succinate-CoQ oxidoreductase
| Receives e- from succinate; succinate dehydrogenase is part of complex II; no proton pumping occurs here
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What are the reactions at Complex II?
Succinate + FAD —> fumarate + FADH2
FADH2 + Fe-S oxidized —> FAD + Fe-S reduced
Fe-S reduced + CoQ —> Fe-S oxidized + CoQH2
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Complex III
CoQH2-cytochrome c oxidoreductase AKA cytochrome reductase
| Facilitates the transfer of electrons from CoQ to cytochrome c in a few steps
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What are cytochromes
Proteins with heme groups in which iron is reduced to Fe2+ and oxidized to Fe3+
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What are the reactions at complex III?
CoQH2 + 2 cytochrome c [with Fe3+] —> CoQ + 2 cytochrome c [with Fe2+] + 2H+
Only one electron is transferred per reaction but CoQ has 2 e- to give —> 2 cytochrome c required
107
Q cycle
2 e- are shuttled from a molecule of CoQH2 (ubiquinol) near the intermembrane space to a molecule of ubiquinone (CoQ) near the mitochondrial matrix; another 2 e- are attached to heme moieties, reducing two molecules to cytochrome c; a carrier containing iron and sulfur assists this process; 4 protons are displaced to the intermembrane space, increasing the gradient of the PMF
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Complex IV
Cytochrome c oxidase; facilitates the cumulating step of the ETC: transfer of electrons from cytochrome c to oxygen; this complex includes subunits of cytochrome a, cytochrome a3, and Cu2+ ions; together cytochrome a and a3 make up cytochrome oxidase; cytochrome oxidase gets oxidized as oxygen becomes reduced and forms water; 2 protons are pumped across the membrane
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Overall reaction at complex IV
2 cytochrome c [with Fe2+] + 2H+ + 1/2O2 —> 2 cytochrome c [with Fe3+] + H2O
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Proton-motive force
As [H+] increases in the intermembrane space:
- pH decreases in the intermembrane space
- voltage difference between the intermembrane space and the matrix
Forms a electrochemical gradient: a gradient that has both chemical and electrostatic properties
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ATP synthase
Harness energy from the electrochemical gradient to form ATP from ADP and an inorganic phosphate
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NADH shuttles
NADH formed in the cytosol through glycolysis cannot directly cross into the mitochondrial matrix; requires a shuttle mechanism - transfers high energy electrons of NADH to a carrier that can cross the inner mitochondrial membrane
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Glycerol-3-phosphate shuttle
The cytosol contains one isoform of glycerol-3-phosphate degydrogenase which oxidizes cytosolic NADH to NAD+ while forming glycerol-3-phosphate from DHAP; on the outer face of the mitochondrial membrane, there exists another isoform of glycerol-3-phosphate dehydrogenase that is FAD-dependent; mitochondrial FAD is the OA and ends up being reduced to FADH2; once reduced, FADH2 proceeds to transfer its e- to the ETC via complex II —> results in 1.5 ATP per NADH
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Malate-aspartate shuttle
Cytosolic oxaloacetate is reduced to malate via malate dehydrogenase with the oxidation of cytosolic NADH to NAD+; once malate crosses into the matrix, mitochondrial malate dehydrogenase reverses the reaction to form mitochondrial NADH which passes through complex I; recycling the malate requires oxidation to oxaloacetate which can be transaminated to restart the cycle via aspartate transaminase
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Chemiosmotic coupling
Allows the chemical energy of the gradient to be harnessed as a means of phosphorylating ADP, thus forming ATP; describes a direct relationship between the proton gradient and ATP synthesis
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F1 portion
Utilizes energy released from the electrochemical gradient to phosphorylate ADP to ATP
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F0 portion
Functions as an ion channel, so protons travel through F0 along their gradient back to the matrix
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Conformational coupling
Relationship between the proton gradient and ATP synthesis is indirect; ATP is released from the synthase as a result of conformational change caused by the gradient
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Respiratory control
The accumulation of NADH inhibits the CAC; O2 and ADP are the key regulators of oxidative phosphorylation
