6. Carbohydrate 4 Flashcards
What happens to pyruvate before it enters the TCA cycle?
• it is transported from the cytoplasm to the mitochondrial matrix across the mitochondrial membrane
• the pyruvate combines with coenzyme A to form acetyl coenzyme A in the presence of pyruvate dehydrogenase
(CH3COCOOH + CoA + NAD+ —-> CH3CO~CoA + CO2 + NADH + H+)
What is the function of a coenzyme
Helps enzymes perform their function - coenzyme A carry two atoms in an activated form
How do we know a reaction is unidirectional?
When CO2 is released
Is the convention f pyruvate to acetyl CoA reversible is irreversible?
Irreversible so is a key regulator
Describe the structure of pyruvate dehydrogenase?
PDH is a large multi-enzyme complex (5 enzymes)
What does pyruvate dehydrogenase require?
The different enzyme activities require various cofactors (FAD,
thiamine pyrophosphate and lipoic acid). B-vitamins provide
these factors, so reaction is sensitive to Vitamin B1 deficiency.
What is pyruvate dehydrogenase activated by?
- pyruvate
- CoASH
- NAD
- ADP
- insulin
- dephosphorylation
What is pyruvate dehydrogenase inhibited by?
acetyl-CoA
NADH
ATP
citrate
phosphorylation
describe an overview of the Tricarboxylic Acid (TCA) cycle
• Mitochondrial • A single pathway – • Acetyl (CH3CO-) converted to 2CO2 • Oxidative (requires NAD+, FAD) • Some energy produced (as ATP/GTP) • (Also produces precursors for biosynthesis)
describe the steps in the Tricarboxylic Acid (TCA) cycle
- acetyl CoA(2C) combines with oxaloacetate(4C) to form citrate (6C).
- the citrate (6C) is converted to another 6C molecule.
- it is then converted to isocitrate(6C) which is converted to a 5C molecule by releasing CO2 and reducing NAD to NADH
- the 5C is converted to a 4C molecule by addition of CoA and release of CO2 and reduction of NAD to NADH
- the 4C is converted to another 4C molecule by converting GDP to GTP(substrate level phosphorylation) and release of CoA.
- the 4C is converted to another 4C by reduction of FAD to FADH2
- the 4C is converted to another 4C by addition of H2O
the 4C is converted to another 4C by reduction of NAD to NADH. this 4C then can combine with another acetyl CoA
what are the products of the TCA cycle for each glucose molecule?
6 NADH
2 FADH2
2 GTP
can the TCA cycle be regulated?
yes, there are two steps that are irreversible.
which 2 steps are regulated?
- the conversion of the 6C molecule to 5C molecule by release of CO2 and reduction of NAD to NADH
- the conversion of the 5C molecule to 4C molecule by release of CO2 and reduction of NAD to NADH
which enzyme is involved in the conversion of the 6C to 5C?
Isocitrate dehydrogenase
which enzyme is involved in the conversion of the 5C to 4C?
a-ketoglutarate dehydrogenase
what stimulates Isocitrate dehydrogenase
ADP
what inhibits Isocitrate dehydrogenase
NADH, ATP
what inhibits a-ketoglutarate dehydrogenase?
NADH, ATP, succinyl-CoA
what is the TCA cycle regulated by?
Regulated by energy
availability,
i.e. ATP/ADP ratio and
NADPH/NAD+ ratio
what is an extra advantage of the TCA cycle?
TCA supplies biosynthetic processes glucose amino acids fatty acids haem
where does the TCA cycle occur?
mitochondria
the TCA cycle is the central pathway in the catabolism of?
Central pathway in the catabolism of sugars, fatty acids, ketone bodies, amino acids, alcohol
what is the aim of the TCA cycle?
Strategy - to produce molecules that readily lose CO2
- Produces precursors for biosynthesis
which bonds are broken in the TCA cycle?
Breaks C-C bond in acetate (acetyl~CoA); carbons oxidised to CO2
is the TCA cycle oxidative or reductive?
Oxidative producing NADH and FADH2
is energy released in the TCA cycle?
Some energy as GTP ( ATP) produced directly
which molecule is necessary for the TCA cycle to occur?
Does not function in absence of O2 (c.f. electron transport chain)
what happens to the intermediates of the TCA cycle?
Intermediates act catalytically - no net synthesis or degradation of
Krebs cycle intermediates alone
summarise Catabolism of glucose so far
- 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 happens in stage 4 of catabolism?
Electron transport and ATP
synthesis
• NADH & FADH2
re-oxidised
what is required for stage 4 of catabolism?
O2 required (reduced to H2O)
is energy produced in stage 4?
Large amounts of energy
(ATP) produced
where does the electron transport and ATP synthesis occur?
inner membrane of mitochondria
what happens during the electron transport?
• NADH and FADH2 dissociate into electrons and protons and the electrons are transferred to carrier molecules
• Electrons are transferred through series of carrier molecules (mostly within proteins) to O2, with release of energy.
• ~30% of energy used to move H+ across membrane into the intermembrane space through proton translocating complex
(a lot of the energy released as heat.)
• H+ concentration gradient (membrane potential) across inner mitochondrial membrane = proton motive force (pmf)
• 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
what two gradients are created in the membrane byy the movemnt of H+?
concentration gradient and electrical gradient
what is the difference between oxidative phosphorylation with NADH and FADH2?
- Electrons in NADH have more energy than in FAD2H,
- so NADH uses 3 PTCs, FADH2 uses only 2.
- The greater the p.m.f. more ATP synthesized
- 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)
describe the regulation of oxidative phosphorylation when ATP levels are high?
• When [ATP] is high, i.e. [ADP] is low, no substrate for
ATP synthase (synthetase)
• inward flow of H+ stops
• Concentration of H+
in the intermitochondrial space
increases
• Prevents further H+ pumping - stops electron transport
what are inhibitors of the oxidative phosphorylation?
cyanide, carbon monoxide
describe inhibition of electron transport
e.g. cyanide, carbon monoxide
• they bind to the oxygen acceptor molecule(cytochrome C) at the end of the electron transport chain more strongly than oxygen
• Block flow of electrons - no electron transport
• therefore, no p.m.f. - no oxidative phosphorylation
• Lethal
describe how uncouplers affect oxidative phosphorylation
e.g. Dinitrophenol, dinitrocresol, fatty acids
• Increase permeability of membrane to H+
• H+ enters mitochondria without driving ATP synthetase
• dissipates p.m.f.
• no phosphorylation of ADP (no oxidative phosphorylation)
• no inhibition of electron transport (continues)
describe how diseases may affect oxidative phosphorylation
Ox/Phos diseases
• Genetic defects in proteins encoded by mtDNA (some subunits of the PTCs and ATP synthase) –> decrease in electron transport and ATP synthesis
What happens to the rest of the energy ?
Lost as HEAT
WHAT does the Efficiency of coupling of oxidative
phosphorylation depend on?
• Efficiency depends on tightness of coupling
• Can be varied in some tissues
• Brown adipose tissue - degree of coupling controlled by
fatty acids (uncouplers) - allows extra heat generation
How is there extra heat generation in brown adipose tissue?
Contains thermogenin (UCP1) - naturally-occurring uncoupling protein.
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.
what is the function of family of UCPs?
role in heat generation by uncoupling but may
have other functions.
in which individuals are brown adipose tissue found?
Newborn infants to maintain heat, particularly around
vital organs.
Hibernating animals - to generate heat to maintain body
temperature
compare and contrast oxidative phosphorylation and substrate level phosphorylation
oxidative phosphorylation:
•Requires membrane-associated complexes (inner mitochondrial membrane)
•Energy coupling occurs indirectly through 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 formation of high energy of hydrolysis bond (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
