Carbohydrate Metabolism II Flashcards
tricarboxylic acid cycle (TCA)
another name for the Citric Acid/Kreb’s Cycle
What is the main purpose of the citric acid cycle?
to oxidize acetyl-CoA to CO2 and generate high-energy electron carriers (NADH and FADH2) and GTP
What organic sources produce acetyl-CoA?
carbohydrates, fatty acids, ketogenic amino acids, ketone bodies, and alcohol
What 5 enzymes make up the pyruvate dehydrogenase complex?
pyruvate dehydrogenase (PDH)
dihydrolipoyl transacetylase (DHLTA)
dihydrolipoyl dehydrogenase (DHLDH)
pyruvate dehydrogenase kinase (PDHK)
pyruvate dehydrogenase phosphatase (PDHP)
What are the functions (briefly) of the enzymes in the PDH complex?
Pyruvate dehydrogenase, dihydrolipoyl transacetylase, dihydrolipoyl dehydrogenase all work to convert pyruvate to acetyl-coA
Pyruvate dehydrogenase kinase and pyruvate dehydrogenase phosphatase work to regulate the actions of PDH.
What molecules inhibit the PDH complex?
Accumulations of acetyl-CoA and NADH (which may occur if the ETC is not working properly or if itself is inhibited)
pyruvate dehydrogenase complex
A group of three (plus 2 regulatory enzymes) enzymes that decarboxylates pyruvate, creating an acetyl group and carbon dioxide. The acetyl group is then attached to coenzyme A to produce acetyl-CoA, a substrate in the Krebs cycle. In the process, NAD+ is reduced to NADH. The pyruvate dehydrogenase complex is the second stage of cellular respiration. This is an irreversible reaction
Where does pyruvate oxidation occur?
mitochondrial matrix
Coenzyme A (CoA-SH)
A coenzyme that functions as a carrier of acyl groups in metabolic reactions; contains a thiol (-SH) group which forms high energy bonds, releasing a significant amount of energy. In the citric acid cycle, hydrolysis of this bond helps drive the cycle forward.
pyruvate dehydrogenase
- oxidizes pyruvate, creating CO2
- requires thiamine pyrophosphate (vitamin B1, TPP) and Mg2+
dihydrolipoyl transacetylase
- oxidizes remaining two-carbon molecule (bonded to TPP) using lipoic acid, and transfers the resulting acetyl group to CoA, forming acetyl CoA
dihydrolipoyl dehydrogenase
-uses FAD to reoxidize lipoic acid, forming FADH₂
-FADH₂ can later transfer electrons to NAD⁺, forming NADH that can feed into the electron transport chain
Briefly describe how fatty acid (beta) oxidation forms acetyl-CoA.
- In the cytoplasm, activation causes a thioester bond to form between carboxyl groups of fatty acids and CoA-SH.
- The activated /fatty acyl-CoA/ cannot cross the inner mitochondrial membrane so it requires transport via carnitine. It (the fatty acyl group) is transferred to carnitine via a transesterfication reaction.
- Carnitine crosses the inner mitochondrial membrane with fatty acyl in tow .
- Carnitine transfers the fatty acyl to mitochondrial CoA-SH using transesterfication (again).
- Once acyl-CoA is formed in the matrix, beta-oxidation can occur to remove 2-C fragments from the carboxyl end (to produce acetyl-CoA).
Briefly explain how amino acid catabolism can form acetyl-CoA.
Ketogenic amino acids can be used to form acetyl-CoA by losing their amino group via transamination and forming ketone bodies. The conversion to ketone bodies allows them to be converted into acetyl-CoA.
Note: Usually ketones are produced by acetyl-CoA when the PDH complex is inhibited, but this reverse reaction can occur as well (especially during periods of starvation)
ketogenic amino acids
amino acids that can be converted into acetyl CoA and ketone bodies
lysine, leucine, isoleucine, phenylalanine, theronine, trypophan, tryosine
Briefly explain how alcohol can be used to form acetyl-CoA.
Alcohol consumed in moderate amounts is converted to acetyl-CoA by alcohol dehydrogenase and acetaldehyde dehydrogenase. This produces a lot of NADH, which actually inhibits the Krebs cycle, so the acetyl-CoA is usually used to synthesize FAs instead.
What 2 enzymes are primarily responsible for alcohol metabolism?
alcohol dehydrogenase
acetaldehyde dehydrogenase
alcohol dehydrogenase
an enzyme active in the stomach and the liver that converts ethanol (alcoholic beverages) to acetaldehyde
acetaldehyde dehydrogenase
an enzyme in the liver that converts acetaldehyde to acetic acid in alcohol metabolism
Write the overall reaction of the PDH complex.
Pyruvate + CoA-SH + NAD+ —–> acetyl-CoA + CO2 + NADH + H+
F0 subunit of ATP synthase
transmembrane subunit of ATP synthase which functions as an ion channel for protons to travel along their gradient back into the mitochondrial matrix. (From high concentration in the inner membrane space through the inner membrane back into the low concentrated mitochondrial matrix)
chemiosmotic coupling
Mechanism that uses the energy stored in a transmembrane proton gradient to drive an energy-requiring process, such as the synthesis of ATP through the ETC or the transport of a molecule across a membrane. (accepted mechanism for describing oxidative phosphorylation)
F1 subunit of ATP synthase
The multiprotein subunit of ATP synthase that has the ATP-synthesizing catalytic sites. It interacts with the F0 subunit of ATP synthase, coupling proton movement to ATP synthesis (phosphorylates ADP to ATP utilizing the energy released from the electrochemical gradient).
oxidative phosphorylation
The production of ATP using energy derived from the redox reactions of an electron transport chain; the third major stage of cellular respiration.
conformational coupling
A less-accepted mechanism of ATP synthase activity in which the proton gradient cause a conformational change in the ATPase (spins the F1 subunit like a turbine) that harnesses the gradient energy to form chemical bonds, and releases ATP from ATP synthase.
uncouplers
compounds that prevent ATP synthesis without effecting the ETC and thus decreasing the efficiency of the ETC/oxidative phosphorylation pathway
ADP builds up
ATP synthesis decreases
body sense lack of energy production - increases O2 production and NADH oxidation
energy from the electrons is released as heat (ex. fever that rises from toxic levels of salicylates such as aspirin)
What molecules are the key regulators of oxidative phosphorylation? How do they accomplish this?
O2 (oxygen) and ADP (think about oxidative=Oxygen, phophorylation=ADP to ATP)
O2: The ETC uses oxygen to accept the electrons and hydrogen from the electron carriers (NADH, FADH2) to form water. Thus, if it is limited, the rate of oxidative phosphorylation decreases and the concentrations of NADH and FADH2 increase, inhibiting the Krebs cycle (since it requires NAD+ and FAD+ to function).
ADP: If oxygen levels are normal, the rate of oxidative phosphorylation is dependent on ADP availability. If the cell is energetically satisfied (high ATP), than low amounts of ADP will be available and oxidative phosphorylation is not necessary. If high amounts of ADP are present, the opposite occurs. Then ADP will activate isocitrate dehydrogenase to increase the rate of the krebs cycle.
How is oxygen used in cellular respiration?
Oxygen is used to yield energy in the form of ATP and act as an acceptor for electrons and hydrogen, forming water. (It is the FINAL electron acceptor in cellular respiration/the ETC).
What is the difference between the ETC and oxidative phosphorylation? What links the two?
The ETC contains made intermembrane proteins within the inner mitochondrial membrane that undergo a series of redox reactions to transfer electrons to oxygen. The transfer of protons results in a proton-motive force (chemiosmotic gradient) between the matrix and inner membrane space.
Oxidative phosphorylation uses this proton gradient to generate ATP using only ATP synthase (not several proteins) to do so.
The standard free energy change of NADH reducing oxygen directly is significantly greater than any individual step along the electron transport chain. If this is the case, why does transferring electrons along the ETC generate more ATP than direct reduction of oxygen by NADH?
While NADH reduction of oxygen (directly) produces a great amount of energy, is releases it into the environment in a way that is insufficient for electron transport. In essence, by splitting the electron transfer into several different proteins/protein complexes, enough energy is released to create a proton gradient that would not otherwise result from the direct reduction of oxygen. The stronger the proton gradient, the greater the production of ATP. So, even those NADH releases a large amount of energy when reducing oxygen, the proton gradient established would not be as great and not as many ATP would be able to be produced.
Where does the citric acid cycle occur?
mitochondrial matrix
The citric acid cycle does not require the use of oxygen for any of its reactions. Why then, is it considered an aerobic process?
Without the presence of oxygen, the FADH2 and NADH produced as intermediates in the cycle will accumulate, since they cannot distribute their protons and electrons to oxygen, the terminal electron carrier in the electron transport chain.
Mnemonic for substrates of the citric acid cycle
“Please Can I Keep Selling Seashells For Money, Officer?”
-Pyruvate
-Citrate
-IsoCitrate
-alpha-Ketoglutarate
-Succinyl-CoA
-Succinate
-Fumarate
-Malate
-Oxaloacetate
Describe step 1 of the Krebs cycle.
Acetyl-CoA + oxaloacetate—> citryl-CoA—>citrate + CoA-SH
Acetyl-CoA is coupled to the cycle’s previous end product, oxaloacetate to undergo a condensation reaction to form citryl-CoA, an intermediate. The hydrolysis of citryl-CoA yields citrate and CoA-SH via citrate synthase.
Describe step 2 of the Krebs cycle.
Citrate—>cis-aconitate—>isocitrate
Citrate (an achiral molecule) is isomerized to one of four possible isomers of isocitrate via the following: Citrate binds to aconitase and a condensation reaction occurs yielding cis-aconitate. Water is added back in to form an isocitrate.
The switching of a hydrogen and hydroxyl group is useful for the later oxidative decarboxylation that occurs.
6Cs–>6Cs
oxidative decarboxylation
oxidation reactions in which a carboxylate (COO-) group is removed, forming carbon dioxide and producing NADH as a side product.
Describe step 3 of the Krebs Cycle.
Isocitrate–(NAD+->NADH+H+)–> oxalosuccinate–(CO2)–>alpha-ketoglutarate
Isocitrate is oxidized to oxalosuccinate by isocitrate dehydrogenase, and then dehydrogenated to produce alpha-ketoglutarate and CO2. This is the rate limiting step of the Krebs Cycle!
This step is the first to produce NADH, CO2, and reduces the molecule by 1 C
6C—>5C
Describe step 4 of the Krebs cycle.
alpha-ketoglutarate–(CoA-SH, NAD+–>NADH)–> succinyl-CoA
Alpha-ketoglutarate and CoA are catalyzed together to form succinyl-CoA via the alpha-ketoglutarate dehydrogenase complex (similar to PDH complex and contains its cofactors of TPP, lipoic acid, and Mg2+). CO2 is lost for a second time and NADH is formed.
Second step to produce NADH, CO2, and lose a C
5C–>4C
Describe step 5 of the Krebs Cycle.
Succinyl-CoA <—–(GDP+Pi–>GTP, CoA-SH)—->Succinate
Succinyl-CoA’s thioester bond is hydrolyzed to yield succinate and CoA, and is catalyzed by succinyl-CoA synthetase, which requires energy in the form of GTP.
The phosphorylation of GDP (GDP->GTP) is driven by the release of energy by thioester hydrolysis. The enzyme nucleoside-diphosphate kinase catalyzes the transfer of P from GTP to ADP to produce ATP. ONLY step in the citric acid cycle to produce ATP directly!
Step requires energy, produces GTP directly, only step to produce ATP (indirectly) in cycle, is reversible
4C->4C
Describe step 6 of the Krebs Cycle.
Succinate–(FAD->FADH2)–>fumarate
Succinate is oxidized to yield fumarate via succinate dehydrogenase, a flavoprotein (covalently bond to FAD) that is an integral protein of the mitochondrial inner membrane (direct part of ETC). FAD is reduced as succinate is oxidized to produce FADH2. FAD is used because succinate is not “strong enough” to reduce NAD+.
Only rxn in the mitochondrial inner membrane, only to use FAD as a carrier
4C->4C
Describe step 7 of the Krebs Cycle.
Fumarate —> Malate
Fumarase catalyzes the hydrolysis of the alkene (double bond) is fumarate to form L-malate using water.
deltaG in image is ~ -3 kJ/mol
Describe step 8 of the Krebs cycle.
Malate–(NAD+->NADH)—>oxaloacetate
Malate dehydrogenase catalyzes the oxidation of malate to oxaloacetate, reducing NAD+ to NADH in the process. Oxaloacetate will associate with acetyl-CoA in another round of the citric acid cycle.
Final step, final NADH formation
Write the net reaction of the pyruvate dehydrogenase complex.
Pyruvate + CoA-SH + NAD+ —-> acetyl-CoA + NADH + CO2 + H+
