Mod 8 Flashcards
The mitochondria
Specializes in energy production in terms of ATP generation
has a double membrane
Outer mem surrounds entire structure
- Highly permeable with porin channels
- Lets things smaller than 5000 daltons big to pass
intermembrane space
Inner mem has cristae to increase surface area
- Relatively impermeable, allow only small uncharged compounds like CO2 and water to pass
mitochondrial matrix
What doe the permeability difference of the two mito matrices allow for
Difference in permeabilities allow proton gradient to be established for the ETC
Where are most CA cycle enzymes located
in the mito matrix
Pyruvate oxidation - what is it
- in mitochondria
- conversion of the three-carbon pyruvate to a two-carbon molecule of acetylCoA
- catalyzed by enzyme pyruvate dehydrogenase (PDH)
Two stages of pyruvate oxidation
- Decarboxylation of pyruvate to form acetyl-CoA
- Oxidation of the acetyl group of acetyl-CoA to form carbon dioxide
Pyruvate dehydrogenase reaction
step 1 of pyruvate oxidation
Catalyzed by pyruvate dehydrogenase
Causes irreversible oxidative decarboxylation of pyruvate to acetyl-CoA
Pyruvate dehydrogenase net reaction
pyruvate + CoA-SH + NAD+
–> (using PDH)
NADH + CO2 + acetyl-CoA
pyruvate loses CO2 and instead binds with S-CoA
Oxidative processes
draw e- away (LEO)
reduce things
ex. NAD+ to NADH
CoASH enzyme
In reduced form, coenzyme A is also referred to as CoASH
Derivative of pantothenic acid which is a B vitamin
Reactive part is the free thiol group
Forms an energy-rich thioester bond with the two-carbon acetyl group that will be derived from pyruvate
Pyruvate Dehydrogenase structure
Multi-enzyme complex
Made up of three core enzyme subunits = E1, E2, E3
and two regulatory enzymes = PDH kinase and PDH phosphatase
Lipoic acid coenzyme is covalently attached to the E2 enzyme and acts as a ‘swinging arm’ for the acetyl group as it goes from one enzymatic complex to the next
PDH kinase and PDH phosphatase
When phos by PDH kinase = inactivated
When dephos by PDH phosphatase = activated
(like glycogen synthase)
PDH 5 co-factors required
- NAD+
- TPP
- Lipoic acid
- CoA
- FAD
3 advantages of multienzyme complexes (such as pyruvate dehydrogenase)
- increased rate/efficiency (due to reduction in diffusion distance for intermediates)
- complex channels intermediates between successive enzymes in a pathway, minimizing side reactions
(like the swinging arm) - Reactions catalyzed by multienzyme complexes can be coordinately regulated
PDH kinase allosteric regulation
done by effectors
activate PDH kinase
AcetylCoA, ATP, and NADH
= inactivate PDH
deactivate it
Pyruvate and ADP
= activate PDH
PDH phosphatase allosteric regulation
done by effectors
Insulin and Ca2+
Activate PDH phosphatase
= activate PDH
Competitive inhibition of PDH
AcetylCoA and NADH are competitive inhibitors when in high concentration
What form of PDH is active
a-form
b-form phos = inactive
CA cycle (gen)
2-carbon acetylCoA is broken down into two molecules of CO2
CA cycle net reaction
3 NAD+, FAD, GDP, Pi, acetylCoA
–>
3 NADHs, 1 FADH2, 1 GTP, 2 CO2s, 1 free CoA (no acetyl)
How many enzymes in CA cycle and where are they located
Eight enzymes in the cycle, compartmentalized within the mitochondria
All soluble within the matrix EXCEPT succinate dehydrogenase which is a membrane protein in the inner mitochondrial membrane
Why is the CA cycle aka the tricarboxylic acid cycle aka the krebs cycle
named after scientist krebs
product of first reaction is citrate
citrate is a tricarboxylic acid
cycle begins and ends with oxaloacetate
NADH/FADH2 etc in the CA cycle
Drawing electrons away from the AcetylCoA and passing them to reducing equivalents NAD+ and FAD
ETC regenerates NADH and FADH2
What happens to the CA cycle CO2
CO2 waste product will eventually leave cell → bloodstream → exhaled
Three things that regulate key CA cycle enzymes
SCA
- Availability of substrates
- Need 1:1 ratio of oxaloacetate and acetylCoA for reaction 1
- Adequate free NAD+ - Competitive inhibition by products
Ex. accumulation of NADH inhibits key enzymes like dehydrogenases that generate these reducing equivalents - Allosteric regulation
- Positive or negative
CA cycle allosteric regulation
done by three things
Allosteric effectors (ex. Ca2+)
- positive regulation
- bc signals muscle contraction = fuel will be needed
ATP
- negative regulation
- signals high energy = doesn’t need to oxidize any more acetylCoA
ADP
- positive regulation
- Signals low energy so more ATP needed = increased CA cycle flux
3 Common themes in CA cycle metabolic regulation
ERF
- Energy status of the cell (ADP/ATP)
- Redox state of the cell (NADH/NAD+)
- Feedback inhibition by products
Amphibolic
both catabolic and anabolic in nature
In what way is the CA cycle amphibolic
both catabolic and anabolic in nature
Catabolic bc they break down and oxidize acetylCoA that originated from glucose or fat to generate ATP and reducing equivalents
Anabolic citric acid cycle intermediates are also the starting materials for biosynthetic pathways
Examples of where CA intermediates go
citrate for the production of fatty acids and steroids
succinylCoA for the synthesis of heme and chlorophyll
alpha-ketoglutarate and oxaloacetate for aa synthesis and purines (ak) and pyrimidines (ox)
Anaplerosis
the process of replenishing citric acid cycle intermediates (siphoned off) so the cycle can begin again
Synthesis of oxaloacetate from pyruvate
quantitatively the most important anaplerotic reaction
pyruvate + CO2 + ATP + H2O
–>
oxaloacetate + ADP + Pi
Regulation of pyruvate dehydrogenase and pyruvate carboxylase
to ensure a 1:1 ratio of oxaloacetate:acetylCoA need to coordinate pd and pc
If acetylCoA > oxaloacetate, citrate synthase will not function maximally due to substrate limitation
= Excess acetylCoA inhibits PDH, diverting pyruvate to the pyruvate carboxylase reaction
= forms oxaloacetate
Fat breakdown
When low glucose levels (at rest or between meals)…
- Glucagon released stimulates lipase to breakdown fats to provide energy
- Process of beta oxidation
- Lipase enzyme hydrolyzes the ester bonds and therefore releases the fatty acids from the glycerol
= fatty acids + glycerol
Fatty acid journey after separation from triacylglycerol
- Leave the adipose tissue and enter the bloodstream carried by albumin
- Taken up by the liver and muscle for energy but NOT by the brain
- Activated
- Then broken down into acetyl-CoA via beta oxidation
Glycerol journey after separation from triacylglycerol
- Leaves the adipose tissue to be taken up by the liver
Liver = site of gluconeogenesis
Glycerol = substrate for gluconeogenesis - enters the gluconeogenic pathway at the midpoint
- Converted to glucose via gluconeogenesis and released back into the bloodstream to be taken up by tissues for energy
Fatty acid activation
in cytosol
Activate by attaching a long chain hydrocarbon’s acyl group to CoA (_S-CoA)
Catalyzed by enzyme acylCoA synthetase
uses ATP and forms AMP + PPi
Beta oxidation of fatty acids
in the matrix
Carbons are removed from fatty acid (acyl group) two at a time adjacent to the beta carbon
Cleaving the C-C bond = generates an acetylCoA molecule
1 round of B-oxy is 4 reactions
- One rxn generates a FADH2
- One generates NADH
Oxidative process so generates reducing agents
so like 16 carbon chain
–> 8 acetylCoA, 7 FADH2 and NADH
Beta oxidation net reaction
fatty acid, FAD, NAD+, CoASH
–>
many rounds
acetyl-CoA, FADH2, NADH
Fate of beta oxidation products
AcetylCoA either enters CA cycle or (in liver) converted to ketone bodies = energy production
FADH2/NADH will donate their e- to the ETC to generate ATP through oxidative phosphorylation
Ketogenesis
Glucose will be low bc the liver glycogen stores have been depleted
Brain can’t use fatty acids for energy, so converts them to ketone bodies in the liver
Ketone bodies
- once carb stores in liver are used up, liver will convert fats to ketone bodies via an acetylCoA intermediate
Ketone bodies enter bloodstream cross the blood-brain barrier, are then taken up by the brain and broken back down into acetylCoA which enters the CA cycle and produces energy
What ketone bodies can our body produce (3)
Acetone
Acetoacetate
Beta-hydroxybutyrate
Fat storage
as+in
- Stored as triacylglycerols aka triglycerides
- Made of three fatty acids esterified with glycerol
- Made of 2+ different types of fatty acids
- Stored in adipocytes = specialized cells
Is fat storage just reverse beta oxidation
basically
but doesn’t share any of the beta oxidation enzymes
2 carbons originating from acetylCoA are added (one at a time) to an elongating fatty acyl chain
When does fat storage occur
- Fat comes from excess carb intake
High glucose will be first used to replenish glycogen stores
Then the rest will be broken down through glycolysis and pyruvate dehydrogenase to generate acetylCoA
- When acetylCoA not needed to generate energy, it will do fat synthesis instead of the CA cycle
Are anabolic processes ox or red
specifically fatty acid synthesis
reductive
We need to donate e- to the newly synthesized fatty acids
E- come from reducing equivalents (NADP+ is the reducing equivalent of the reduced form NADPH) - so from NADP+
Where does fat synthesis occur
cytosol
Tricarboxylate transport system must move acetylCoA out of the mito matrix into the cytosol via a citrate intermediate
What does fatty acid synthesis use
+ example
enzyme fatty acid synthase
NADPH
1 ATP / acetylCoA round
ex. Takes 8 acetylCoA molecules and condenses them bit by bit into a 16-carbon fatty acid called palmitate
Enzymes called elongases and desaturases
make longer chain fatty acids and create double bonds in unsaturated fatty acids
Triacylglycerol synthesis
Once the fatty acids are synthesized, they are esterified with glycerol to form triacylglycerols or are used to make membrane lipids
Fatty acid synthesis net reaction with palmitate
8 acetylCoA, 14 NADPH, 7 ATP
–>
palmitate, 8 CoA, 14 NADP+, 7 ADP, 7 Pi
Fatty acid metabolism → high glucose
Ex. after a carb-rich meal
Insulin (hormone) is released
Activates fat synthesis in the liver and inhibits fat breakdown in adipose tissue
Fatty acid metabolism → low glucose
Ex. when fasting or between meals
Glucagon released ⇒ glucagon is the counter regulatory hormone to insulin
Glucagon stimulates fat breakdown by stimulating lipase in adipose tissue
Fats then broken down to acetylCoA and used for energy
AcetylCoA can be diverted to ketone body synthesis during starvation/extended fasting
AcetylCoA –> cholesterol
all cholesterol carbons are derived from acetyl-CoA
uses HMG-CoA reductase
Those with high LDL cholesterol
Enzyme HMG-CoA reductase is the target of statin drugs
help lower cholesterol
Amino acids –> pyruvate or acetylCoA
The breakdown of amino acids derived from protein can lead to the production of pyruvate or acetylCoA as well as to other intermediates in glycolysis or the citric acid cycle
So body breaks down protein for energy
