Bioenergetics Flashcards
Under aerobic conditions, where is 95% of cellular ATP produced?
In the mitochondria
Describe the outer membrane of the mitochondria?
- Smooth and freely permeable to molecules
- No ionic or electrical gradients
Describe the inner membrane of the mitochondria?
- Folded into christae
- Permeable to a small number of molecules only via specific transporters
- A good electrical insulator, capable of maintaining large ionic gradients
- Contains more protein than lipid –> respiratory enzymes, transporter proteins
What are the 2 spaces in mitochondria?
- Matrix space (in the middle)
2. Intermembrane space (between inner and outer membranes)
What does the matrix space contain?
- Wide variety of enzymes
- High conc of substrates, cofactors and ions
- Mitochondrial DNA, RNA and ribosomes (ribosomes resemble prokaryotes)
What does the intermembrane space?
- Has metabolite and ion conc similar to cytosol
- Contains cytochrome C
After glycolysis, where is pyruvate transported?
Across the inner mitochondrial membrane from cytosol to mitochondrial matrix
What reaction links glycolysis to the Krebs cycle (link reaction)?
Pyruvate dehydrogenase (PDH) catalyses conversion of pyruvate to acetyl CoA
What can a disruption of PDH function be caused by?
Lack of thiamine
What can a disruption of PDH function lead to?
Beriberi –> neurological and CVS symptoms
Where can a lack of thiamine be seen?
Alcohol addicts as alcohol affects absorption of thiamine –> can cause Wernicke-Korsakoff syndrome
Can acetyl CoA be converted back to glucose?
No
Conversion of pyruvate to acetyl CoA commits the C atoms of glucose to energy production or lipid synthesis
When is PDH inhibited?
When energy levels are high (as glucose not needed for ATP at this moment)
What is the 1st stage in the Krebs/citric acid cycle?
Synthesis of 6-C compound (citrate) which then loses 2 C as CO2 to become 4-C compound (succinyl CoA)
What is the 2nd stage in the Krebs/citric acid cycle?
Oxidation of 4-C compound to regenerate oxaloacetate and initiate another round of the cycle
What are the main outputs of the Krebs cycle?
Reduced coenzymes NADH and FADH2 –> electron carriers
CO2 –> waste product
1 GTP –> 1 ATP
What is the Krebs cycle inhibited by?
ATP and NADH
What is the Krebs cycle activated by?
ADP and NAD+
What are the key enzymes involved in the Krebs cycle?
- Citrate synthase
- Isocitrate dehydrogenase
- α-ketoglutarate dehydrogenase
What is effect of type 1 diabetes on glycolysis and gluconeogenesis?
Glycolysis is inhibited so pyruvate levels are low
Gluconeogenesis is not inhibited so oxaloacetate and malate is being removed to form glucose
In the absence of insulin (in type 1 diabetes) what does the body switch over to using?
Fatty acids from adipose tissue -> oxidised to acetyl CoA
What is effect of type 1 diabetes on acetyl CoA levels?
Acetyl CoA levels are high but limited oxaloacetate to react with it (can’t enter Krebs)
So ketones are synthesised instead
By which process is ATP synthesised?
Oxidative phosphorylation
What is oxidative phosphorylation made up of?
Where do these processes take place?
Two tightly coupled processes:
- Electron transport (oxidation)
- ATP synthesis (phosphorylation)
In and across the inner mitochondrial membrane
Describe the electron transport (oxidation) stage of oxidative phosphorylation?
The energy (reduction potential) of the electrons in NADH/FADH2 is used to create a proton gradient across the inner mitochondrial membrane – OXIDATION
Describe the ATP synthesis (phosphorylation) stage of oxidative phosphorylation?
The energy from the proton gradient is used to phosphorylate ADP to synthesise ATP – PHOSPHORYLATION
What are the components of the electron transport chain?
Complex I, II, III, IV
Ubiquinone
Cytochrome c
As electrons are transferred from one electron carrier to the next, what happens when they reach Complex IV?
They are donated to oxygen, reducing it to water (this removes them from the chain)
Cyanide blocks this step (inhibits Complex IV) and cells die from lack of oxygen
As electrons are transferred between complexes, what happens?
Each reduction step (transfer of electrons) releases energy
What is the energy released from electron used for?
To pump protons across the inner mitochondrial membrane (from mitochondrial matrix to intermembrane space)
For each pair of electrons from NADH, how many electrons are translocated?
What does this create?
10
A large proton gradient across the well insulated inner membrane
What is job of ATP synthase?
Converts the energy stored in a proton gradient into chemical energy stored in the bond energy of ATP
What is ADP needed for?
ADP must be transported into mitochondria to supply the ATPase
What allows the ADP in and ATP out?
Antiporter in the inner mitochondrial membrane
What is required for movement of ADP?
Phosphate
How many protons are required to cross the inner membrane for the synthesis of one ATP?
4
Can NADH cross the inner membrane?
No –> NADH produced in the cytosol cannot be directly reoxidised by electron transport
How is NADH transported across the inner membrane?
NADH oxidised in the cytosol to NAD+
Crosses membrane
NAD+ reduced in the membrane to NADH
Electron transport and ATP synthesis are tightly coupled.
If electron transport chain isn’t working, what happens?
Lack of reduced substrates or more commonly lack of oxygen - there is no proton gradient to drive ATPase enzyme
If ATP synthesis is blocked, what happens? Why does the electron transport stop?
By lack of substrate (ADP) or inhibition of ATPase enzyme
Proton gradient builds up to the level where the complexes have insufficient energy to pump more protons across the membrane
Energy cannot be released from the electron carriers, so they can’t accept any more electrons – electron transport stops
What is function of uncouplers?
Some compounds can uncouple oxidation from phosphorylation
Overall effect is to dissipate the proton gradient – electron transport can continue without ATP synthesis
What are uncouplers?
Weak acids which are soluble in the membrane
What happens once uncouplers penetrate the inner mitochondrial membrane?
They diffuse freely
At the inter-membrane space, what do uncouplers associate with?
Protons –> driven by the relatively high H+ conc
At the matrix surface, what do uncouplers do?
Release protons –> driven by relatively low H+ conc
When is uncoupling used?
Uncoupling is used to generate heat in newborns –
non-shivering thermogenesis
When electron transport is uncoupled from ATP synthesis energy is released as HEAT
Describe the difference in adipose tissue in babies
Babies possess brown adipose tissue – has more mitochondria/ different appearance to white adipose tissue
Mitochondria in brown adipose tissue contain thermogenin (uncoupling protein-1)
When core body temperature drops, what is released? What is effect on thermogenin?
When core body temperature drops, sympathetic nervous system release of noradrenalin leads to increased concentrations of free fatty acids in the cytosol, which activate thermogenin
What does brown adipose tissue decrease with?
Age and obesity
What is beige adipose tissue?
Beige (‘brown in white’) adipose tissue – can switch between brown and white forms
What can activating beige/brown adipose tissue be used for?
Activating beige/brown adipose tissue may be a valuable therapeutic target to promote triglyceride clearance & weight loss
What was Dinitrophenol (DNP) used as?
Slimming pill but dangerous side effects (death included)
Where are reduced coenzymes produced?
In the mitochondria by the Krebs cycle
What is function of electron transport chain?
The electron transport chain oxidises the cofactors, and transfers the electrons down a series of electron carriers before donating them to oxygen, reducing it to water
What is the energy derived from electrons used to create?
The energy derived from the electrons is used to create a proton gradient across the inner mitochondrial membrane
What does the proton gradient provide?
This proton gradient provides the energy to drive the synthesis of ATP from ADP & Pi by the enzyme F0F1-ATPase
