Module 8 - Citric Acid Cycle + Acetyl-CoA Flashcards
What is aerobic respiration and what process does it involve
Most important process to produce energy for a cell
Involves: glycolysis, pyruvate oxidation, the citric acid cycle and electron transport
○Sugars, fats and protein enter these pathways and can break down to produce ATP energy for an cell
What is the citric acid cycle and where does it happen,
- aka the tricarboxylic acid cycle
- takes place in the matrix or fluid of the mitochondria where mitochondrial DNA is found and where fatty acid breakdown takes place
- a series of eight rxns break down the two-carbon acetylCoA to the one-carbon molecule of CO2, while at the same generating three molecules of NADH, one molecule of FADH2 and GTP
How many chemical reactions take place in citric acid cycle, what are the products and reactants
● Involves eight chemical reactions that use acetyl coA and oxaloacetate to produce carbon dioxide, NADH, GTP and FADH2
Give a quick overview of Citric acid cycle
Step 1: 2C molecule and 4C molecule combine to form 6C molecule which undergoes biochemical changes an at end original 4C molecule is produced
- Each time C is lost CO2 is released
- 2 CO2 are formed during conversion of 6C to 4C
What is the 2C molecule and how is it formed. Where is it formed? What are the products?
● The 2-carbon molecules come from pyruvate (product of glycolysis in the cell, transported into the mitochondrial matrix where it is oxidized to acetyl CoA by pyruvate dehydrogenase → produces one NADH and one CO2 for each of the two pyruvate molecules made from glucose.)
What does the citic acid cycle start w/
●Acetyl CoA is the starting point of the cycle, the acetyl group from acetyl CoA is transferred to oxaloacetate from citrate.
How many reactions until succinate is formed and what does it produce by then
●4 different rxns lead to the formation of succinate
○Produce two co2, two NADHS and one ATP
What is Succinate recycles back into and how many rxns does that take and what does it produce during that time
● Succinate is then recycled back to oxaloacetate through three more reactions = produce FADH2 and one more NADH = important for fully functioning cells
What are the two stages by which pyruvate is oxidized
○1st the conversion of the three-carbon pyruvate to a two-carbon molecule of acetylCoA,
○2nd the oxidation of acetylCoA to carbon dioxide through the citric acid cycle
How does pryruvate become acetylCoA
● The oxidation of pyruvate to acetylCoA is catalyzed by the enzyme, pyruvate dehydrogenase (PDH) which is located in the mitochondrion
What happens to NADH and FADH2. What does this do to the energy
● All reducing equivalents NADH and FADH2 will go on to the final stage of metabolism which is the electron transport chain which reduces the potential energy
What is the strucutre of the mitochondria
- has double membrane, outer mitochondrial membrane surrounds the entire structure while the inner mitochondrial membrane is invaginated to increase the surface area of the membrane
- invaginations are called cristae and the inner compartment is called matrix
- Space bwn the inner and outer membrane is called intermembrane space
- outer mitochondrial membrane is highly permeable with large channels that span the membrane are called porins which allow compounds less than approx 5000 daltons in size to pass
- the inner mitochondrial membrane is relatively impermeable, allowing only small uncharged compounds like CO2 and water to cross the membrane
How do larger or charged molecules cross the inner membrane
○Larger or charged molecules like pyruvate, protons or ATP will cross with transport proteins
Describe the inner membrane compared to the outer membrane of the mitochondria
- inner contains a much higher concentration of proteins as it contains these specific transport proteins as well as the protein complex involved in the electron transport chain,
○the relative permeability of the inner mitochondrial membrane allows a proton gradient to be established for the electron transport chain
Where are enzymes for citric acid cycle located
Inner membrane, Matrix
What does the Matrix of the mitochondria contan
-PDH pyruvate dehydrogenase complex Citric acid cycle enzymes - Fatty B oxidation enzymes -AA oxidation enzymes -Many other enzymes -ATP, ADP, PI, Mg, Ca, K
What is Coenzyme A.
What partof it is reactive
- a coenzyme that participates in the pyruvate dehydrogenase rxn, it is a derivative of pantothenic acid, which is a B vitamin
- Free thiol group is the reactive part of coenzyme A which will form an energy-rich thioester bond with the two-carbon acetyl group that will be derived from pyruvate
- In reduced form it is also called CoASH
What is Coenzyme A
- a coenzyme that participates in the pyruvate dehydrogenase rxn, it is a derivative of pantothenic acid, which is a vitamin B
- Free thiol group is the reactive part of coenzyme A which will form an energy-rich thioester bond with the two-carbon acetyl group that will be derived from pyruvate
- In reduced form it is also called CoASH
- functions as a carrier that serves to activate the two-carbon acetyl group through the formation of that high-energy thioester bond
What is Pyruvate Dehydragenase,
What does it do
- Stage 1
- Catalzyes the irreversible oxidative decarboxylation of pyruvate to acetyl-CoA
- Oxidative: because electrons are reducing NAD+ to NADH
- decarboxylation reaction because a one-carbon carbon dioxide molecule is removed from the three carbon pyruvate.
What is the leaving group in pyruvate that makes it a 2C acetyl group
carboxyl group of pyruvate is good ‘leaving group’ which leaves in the form of carbon dioxide
What does CoASH attach itself to and what is its fxn
● Free thiol group from CoASH is forming that energy-rich thioester bond (in red) with two-carbon acetyl group derived from pyruvate
- effectively activating the 2C acetyl group
What is the structure of Pyruvate Dehydrogenase and what else does it require
- multi-enzyme complex Consists of 3 Core enzymes and 2 Regulatory enzymes
- three core enzyme subunits, named E1, E2 and E3, which participated in the oxidative decarboxylation of pyruvate
- Two regulatory enzymes: PDH kinase (inactives PDH by phosphorylation) and PDH phosphase (activates PDH by dephosphorylation)
- ALSO REQUIRES: 5 Co-factors
Whats so important about the E2 enzymes-
Contains Lipoic acid which is an important coenzyme that is covalently attached to E2 enzyme that serves as a ‘swinging arm’ for the acetyl group as it goes from one enzymatic rxn to the next in the overall rxn of the pyruvate dehydrogenase complex.
What are the names of the 3 core enzymes
E1: Pyruvate dehydrongenase
E2: Dihydrolipoamide transacetlase
E3: Duhydrolipoamide dehydrogenase
What are the 5 Coenzymes required for PDH and what are their functions
○NAD+ electron carrier
○FAD+ electron carrier
○Thiamine pyrophosphate (TPP) coenzyme of pyruvate decarboxylase has two thiol groups that can undergo reversible oxidation to a disulfide bond
○Lipoic acid: serves as electron (H) carrier and acyl carrier
○CoA serves as a carrier of the activated acyl group
What are the advantages of Multienzymes complex
- Increased rate or efficency of rxn due to minimized diffusion distance for intermediates between enzymes
- Minimized side reactions between successive enzymes in the pathway (In PDH this is lipoic acid)
- Rxns catalyzed by multienzyme complexes can be coordinately regulated
What does PDH kinase and PDH phophatase do
Kinase phosphorylated - bring PDH to its inactive PDH-b form
Kinase phosphatase - bring PDh to its active PDH-a form
PDH kinase is allosterically regulated by:
Acetyl-CoA, ATP, and NADH which activated PDH kinase and promotes phosphorylation
-●Elevated level of acetylCoA and NADH signal to PDH that they have accumulated in the call and therefore, there is no need to further breakdown pyruvate until they can be utilized further downstream.
●Elevated levels of ATP signal that the energy status of the cell is high and that no further pyruvate breakdown is necessary.
- Negactive allosteric regulator
PDH phosphatase is allosterically regulated by:
Insulin and Ca2+, activates PDH phosphatase to promote dephosphorylation
- insulin is released following the consumption of a meal, signals that glucose is abundant and can be broken down for energy and any excess glucose can be converted to acetylCoA which is the precursor for fat synthesis.
○Through this mechanism, excess glucose will be converted to fat for longer-term storage
●Ca2+ is released during exercise to signal muscle contraction
●By activating PDH phosphatase, it will promote activation of PDH to its active form to increase flux of pyruvate into the citric acid cycle for the continued generation of ATP to support the exercising muscle.
●Balance of positive and negative factors will determine whether PDH exists in an active a-form or less active b-form
What does elevated levels of pyruvate, ADP and NAH+ do
inhibit the kinase and therefore promote the active form of PDH and PDHa.
What does competitive inhibition look like in regards to PDH
- regulated by feedback inhibition when concentrations of acetyl-CoA and NADH are elevated.
●These serve to feedback and inhibit PDH when they accumulate in the cell.
What is the net reaction of the citric acid cycle and the equation
3 NADH, 1 FADH2, 1 GTP, 2 CO2m and free coenzyme A
2NAH+ + FAD + GDP +Pi + acetyl CoA ~ 3NADH + FADH2 + GTP + CoA + CO2
How many enzymes are used in the citric acid cycle
● Are 8 eight enzymes in the citric acid cycle and all of these are compartmentalized in the mitochondria.
