Carbohydrates III Flashcards
What is the entry stage point into stage 3
Acetyl CoA rather than pyruvate is the entry point into stage 3 (tricarboxylic acid (TCA) cycle)
What is the enzyme that regulates entry into the TCA cycle
Pyruvate dehydrogenase
is a key site of regulation of entry into the TCA cycle.
What does CoA stand for
Coenzyme A
What type of reaction are co-enzymes involved in
Catalytic Reaction
CoA (CoASH) what does the SH stand for
It stands for the Thioesters (with acyl groups) THE SULPHYDRYL GROUP
What groups are contained within an Actetly CoA
Contains a functional sulfhydryl group that reacts with carbonyl groups and forms acyl thioesters – such as acetyl CoA
What happens when you don’t have enough vitamins
Complex structures derived from vitamins cant be formed
Where is Pyruvate dehydrogenase (PDH) found
It is found in the Mitochondrial matrix
where does the pyruvate have to travel for the TCA cycle to occur
pyruvate transported from the cytoplasm across the mitochondrial membrane
What is PDH made from
PDH is a large multi-enzyme complex (5 enzymes)
What is the benefit of a large multi-enzyme complex
intermediates move from enzyme to enzyme without ever having to be released thus more effecient
Why do we need Vitamin B1
The different enzyme activities require various coenzymes (FAD, thiamine pyrophosphate and lipoic acid). B-vitamins provide these factors, so reaction is sensitive to Vitamin B1 deficiency.
Is the reaction CH3COCOOH + CoA + NAD+ –> CH3CO~CoA + CO2 + NADH + H+ a regulatory step and why ?
YES, Reaction is irreversible, so is a key regulatory step.
Pyruvate dehydrogenase (PDH) regulation?
What is it activated by -4
What is it Inhibited by - 4
activated by:
Pyruvate CoA
NAD+
ADP
Insulin
-This is called dephosphorylation
inhibited by:
acetyl-CoA
NADH
ATP
citrate
- This is called phosphorylation
What enzyme is used in phosphorylation
Kinase Enzyme
What enzyme is used in dephosphorylation
phosphatase Enzyme
What is another name for TCA cycle
KREBS CYCLE/ CITRIC ACID CYCLE
6 KEY points of info about Stage 3
(The Tricarboxylic Acid (TCA) cycle (Kreb’s cycle))
- Mitochondrial
- A single pathway –
- Acetyl (CH3CO-) converted to 2CO2
- Oxidative (requires NAD+, FAD)
- Some energy produced GTP
- (Also produces precursors for biosynthesis)
How many electrons are there within the chemical bonds of the Acetyl group
8
Where does the TCA cycle transfer the elections
NAD+ and FAD
why are there so many steps in the TCA cycle (2)
To conserve the energy present in the chemical bonds and use it efficiently for generating ATP.
Produces many intermediates which will be used in biosynthetic reactions.
What does glucose turn into - 2 steps
Glucose —-> 2 x acetyl groups into TCA —->
6 NADH 2 FADH2 2 GTP
What is GTP
Guanosine triphosphate - the energy currency that comes out of the TCA cycle
What regulates the TCA cycle
Regulated by energy availability,
i.e.
ATP/ADP ratio
NADH/NAD+ ratio
What is Isocitrate dehydrogenase?
isocitrate dehydrogenase (IDH) is an enzyme that is best known for its role in the Krebs cycle, catalyzing the oxidative decarboxylation of isocitrate
What Upregulates and Downregulated Isocitrate dehydrogenase
Upregulated by a high level of ADP
Downregulated by High levels of NADH
What does the TCA supply to the biosynthetic process - (4)
- Fatty Acids
- Amino Acids
- Heam
- Glucose
Why does the TCA cycle not work without O2
Tightly coupled to electron transport chain – so does not function in absence of O2
(TCA cycle will come to a halt due to a lack of NAD+ and FAD.)
Summary – Catabolism of glucose TCA cycle -4
- Broken all C - C bonds
- Oxidised all the C-atoms to CO2
- Broken all the C - H bonds
- Transferred all the H atoms (H+ + e-) to
NAD+ → NADH+ H+
FAD → FADH2
What is the ∆Go for oxidation of glucose to CO2 and H2O
- 2870 kJ.mole-1
Where is the energy accounted for, or oxidation of glucose to CO2 and H2O
- ATP from Glycolysis (4 ATP) 2 ATP net (= - 62 kJ.mole-1) TCAcycle2GTP = 2ATP (=-62kJ.mole-1)
Total from substrate level phosphorylation = - 124 kJ.mole-1 - Still need to account for - 2746 kJ.mole-1
- Chemical bond energy of the e- in NADH and FAD2H
- ‘High energy’ electrons in NADH & FADH2 transferred to O2 with the release of large amounts of energy.
This energy is used to drive ATP synthesis
Key Points of stage 4 - Catabolism (4)
-Mitochondrial
-Electron transport and ATP synthesis
-NADH & FADH2 re-oxidised O2 utilised (reduced to H2O)
-Large amounts of energy (ATP) produced
What is the use of reducing power in ATP synthesis?
Two processes
1. Electrons on NADH and FADH2 are transferred through a series of carrier molecules to oxygen (ELECTRON TRANSPORT)
Releases energy in steps
2. Free energy released is used to drive ATP synthesis (OXIDATIVE PHOSPHORYLATION)
How does Mitochondrial electron transport work
(simplified)
- Electrons are transferred through series of carrier molecules to O2, with release of energy.
- [H+] gradient (an electrochemical gradient) is generated across the inner mitochondrial membrane = proton motive force (pmf)
- ~30% of energy used to move H+ across inner membrane (a lot of the energy is released as heat.)
What happens in the Proton translocating complexes
Energy from the electrons is used to translocate protons from the matrix into the intermembrane space.
This creates a Conc gradient of protons and generates an electrical gradient as you have a lot of positively charged molecules building up on one side of the membrane.
What is the F1F0-ATPase or ATP synthase (synthetase)
It is present in the inner membrane of eukaryotic mitochondria and acts as the powerhouse of the cell by synthesizing ATP
How does Proton translocating ATPase work
- Return of protons (H+) is favoured energetically by the electrochemical potential (electrical and chemical gradient)
- Protons (H+) can only return across membrane via the ATP synthase and this drives ATP synthesis
Quick summary of Oxidative phosphorylation:
- Occur in the mitochondria
- Electrons are transferred from NADH and FAD2H to molecular oxygen
- Energy released is used to generate a proton gradient (proton motive force (pmf))
- Energy from the dissipation of the proton motive force is coupled to the synthesis of ATP from ADP
When does NADH enter the Oxidative Phosphorylation pathway
At Complex 1 (Proton translocating complex 1)
When does FADH2 enter the Oxidative Phosphorylation pathway
Complex 2
What is the consequence of FADH2 entering later at complex 2 compared to NADH entering at Complex 1
Less proton transport via electrons of FADH2
You get less ATP synthesised
What is the reason for NADH uses 3 PTCs and FADH2 using only 2?
- Electrons in NADH have more energy than in FADH2,
- so NADH uses 3 PTCs, and FADH2 uses only 2.
How much ATP is generated from
NADH
FADH2
- Oxidation of 2 moles of NADH synthesis of 5 moles of ATP (P/O = 2.5)
- Oxidation of 2 moles of FADH2 synthesis of 3 moles of ATP (P/O = 1.5)
what happens when there is a greater p.m.f. (proton motive force (pmf))
more ATP synthesized
How is the ATP: ADP ratio regulating Oxidative Phophorylation
- When [ATP] is high, i.e. [ADP] is low, then there is no substrate for ATP synthase (synthetase)
- So inward flow of H+ stops
- Concentration of H+ in the intermitochondrial space
increases - Prevents further H+ translocation - stops electron
transport
What happens to oxidative phosphorylation when ADP is low
- Normally oxidative phosphorylation and electron transport are tightly coupled
- Both regulated by mitochondrial [ATP]
- High ATP = Low ADP
- When [ADP] is low, = no substrate for ATP synthase (synthetase)
- So inward flow of H+ stops
- Concentration of H+ in the intermitochondrial space increases – stops H+ pumping and electron transport
How does Inhibition of oxidative phosphorylation occur (electrons)
give examples
Inhibitors block electron transport,
-e.g. cyanide (CN-) prevents acceptance of electrons by O2
-e.g Carbon monoxide prevents protons from biding to the proton translocating complex
What do Uncouplers do in Oxidative Phosphorylation
- Uncouplers increase the permeability of the mitochondrial inner membrane to protons
- dissipate the proton gradient, thereby reducing the proton motive force
- No drive for ATP synthesis
What do OXPHOS diseases do
Genetic defects in proteins encoded by mtDNA (some subunits of the PTCs and ATP synthase)
Reduction in electron transport and ATP synthesis
What is the efficiency of coupling of oxidative phosphorylation
∆Go for NADH/O = - 220 kJ.mole-1
2.5 ATP = + 78 kJ.mole-1
(142 kJ.mole-1 lost)
∆Go for FADH2/O = - 152 kJ.mole-1
1.5 ATP = + 47 kJ.mole-1
(105 kJ.mole-1 lost)
What happens to the rest of the energy in oxidative phosphorylation?
- What happens to the rest of the energy?
- Lost as HEAT
- Efficiency depends on the tightness of the coupling
- Brown adipose tissue - degree of coupling controlled by fatty acids (uncouplers) - allows extra heat generation
Where is brown tissue found
1.Newborn infants
to maintain heat, particularly around vital organs
2.Hibernating animals
to generate heat to maintain body temperature
What is thermogenin (UCP1)
naturally-occurring uncoupling protein.
What happens when it is cold
In response to cold, noradrenaline (norepinephrine) activates :
1. Lipase which releases fatty acids from Triacylglycerol
2. Fatty acid oxidation NADH/FADH2 electron transport
3. Fatty acids activate UCP1
4. UCP1 transports H+ back into mitochondria
So, Electron Transport uncoupled from ATP Synthesis. Energy of
p.m.f. is then released as extra heat.
Difference between
OXIDATIVE PHOSPHORYLATION and SUBSTRATE LEVEL PHOSPHORYLATION
OXIDATIVE PHOSPHORYLATION
-Requires membrane-associated complexes (inner mitochondrial membrane)
-Energy coupling occurs indirectly through the generation & subsequent utilisation of a proton gradient (pmf)
-Cannot occur in the absence of O2
-Major process for ATP synthesis in cells requiring large amounts of energy
SUBSTRATE LEVEL PHOSPHORYLATION
-Requires soluble enzymes (cytoplasmic & mitochondrial matrix)
-Energy coupling occurs directly through the formation of high-energy of hydrolysis bonds (phosphoryl-group transfer)
-Can occur to a limited extent in the absence of O2
-Minor process for ATP synthesis in cells requiring large amounts of energy
What is the net yield of ATP from glucose?
Glycolysis - 7
PDH - 5
TCA - 20
TOTAL = 32 ATP
