Mitochondria and Peroxisomes Flashcards
Importance of mitochondrial function
Mitochondrial functions are critical for cellular and organismal life. They have been central in driving the evolution of complex eukaryotic organisms.
Mitochondrial function has been implicated as a contributing factor to ageing and the development of ageing-related diseases such as cancer, cardiovascular diseases and dementia.
Why are mitochondria medically important?
Investigating mitochondrial disorders - a large group of inherited monogenic diseases affecting mitochondrial function.
The side effects of many commonly used antibiotics happen because they can inhibit mitochondrial function.
Importance of peroxisomes
Peroxisomes are far simpler organelles when compared to mitochondria.
But they have important roles in cellular metabolism.
Why are peroxisomes medically important?
Investigating peroxisomal disorders - a group of inherited diseases caused by mutations in peroxisomal proteins
What is the endosymbiotic theory related to the origin of mitochondria?
Mitochondria are present in all eukaryotic organisms
Approximately 2 billion years ago, the earth was colonised by bacteria while there was no complex life.- no eukaryotes, no multicellular life, and thus no animals or plants.
The endosymbiotic theory proposes that all the mitochondria that have existed can be traced back to one single prokaryotic cell
The prokaryote was engulfed by a primitive form of eukaryotic cell - the two cells formed a symbiotic relationship
The prokaryote is divided inside its host, producing daughter cells and increasing their number so that when the host cell divides, these daughter cells are also passed on
The prokaryote supposedly provided a functional advantage to the host cell, which then gave rise to all eukaryotic organisms that have lived
What is the importance of mitochondria and their evolutionary ancestor?
It enabled the evolution of complex life
Which eukaryotic cells do not contain mitochondria?
Some eukaryotes have lost mitochondria in evolution but retain a similar organelle
Mature red blood cells destroy their mitochondria
What is the structure of mitochondria?
Double membrane - inner and outer membranes
Intermembrane space - between the inner and outer membranes
Mitochondrial matrix - located within the inner membrane (this is where a large number of metabolic reactions occur)
Small circular genome - mitochondria have their own DNA
Where are the large protein complexes responsible for oxidative phosphorylation located?
Embedded in the inner mitochondrial membrane
What are the clues that mitochondria originate from a prokaryotic ancestor?
Presence of two membranes and a small circular genome (mitochondria have their own DNA)
What is the organisation of mitochondria highly dependent on?
Cell type
Examples of when mitochondria form networks
In skin fibroblast cells, mitochondria are interconnected and form networks with one another
This network is highly dynamic, whereby the mitochondria are constantly moving in the cell
Individual mitochondria can separate from the network, divide and fuse with other mitochondria (by fission and fusion)
Mitochondria are also highly abundant in cardiac cells (cardiomyocytes) and reside in distinct zones - distinct sub-cellular populations may perform zonal-specific functions necessary for cardiomyocyte function
How are mitochondria transported?
Mitochondria are transported on cytoskeletal microtubules
Mitochondria bind to dynein and kinesin proteins by adaptor proteins (milton and miro)
Milton and miro are located on the surface of mitochondria
Why is the transport of mitochondria important in neurons?
Mitochondria are channeled along axons and delivered to synapses where they are required for neuronal signalling and function
How is the prokaryotic origin of mitochondria reflected in their behaviour?
Like bacteria, mitochondria can divide and fuse with one another (by fission and fusion)
The process of increasing the number of mitochondria involves growing the mass of existing mitochondria, which are then able to undergo fission
Although it is known that the process of fusion is important for mitochondria, the physiological role is less clear
Why do macromolecules such as DNA and proteins become damaged in the mitochondria over time?
Due to oxidative insults
What process is involved in removing damaged mitochondria from the cell?
These damaged mitochondria are removed from the cell in the process of mitophagy (a form of autophagy)
What are the functions of mitochondria?
- To produce ATP in oxidative phosphorylation
- Anabolic synthesis of nucleotides which are required for DNA replication and growth
- Calcium homeostasis which is critical for muscle contraction
- Production of amino acids such as glutamate, which have a function as neurotransmitters
- Important in apoptosis - programmed cell death
- Involved in immune responses - there are protein receptors that detect invading viral RNA molecules and these protein receptors are located on the outer membrane of mitochondria (this activates an innate immune response)
Mitochondrial function has emerged as an important target for cancer therapy
What is the role of mitochondria in cellular energy metabolism?
Mitochondria play an important role in cellular energy metabolism as it is the site where most cellular ATP is generated
Nutrients in food are oxidised to produce acetyl coenzyme A, which is processed in the citric acid cycle (TCA/ Krebs cycle)
NADH and FADH are produced which serve as the electron donors in the electron transport chain - these electron donors are oxidised by the ETC and result in production of ATP
Citric acid cycle is the final common pathway for oxidation of fuel molecules, what are these fuel molecules?
Carbohydrates, fatty acids, and amino acids
Most fuel molecules enter the citric acid cycle as acetyl coenzyme A, where is this derived from?
Acetyl coenzyme A is derived from the metabolism of carbohydrates in the cytoplasm and from fatty acids metabolised by beta-oxidation in the mitochondria
The acetyl coenzyme A is completely oxidised to the carbon dioxide in the citric acid cycle
What are the two important functions of the citric acid cycle?
- To produce reduced electron donors NADH and FADH2 - these are used to produce ATP in oxidative phosphorylation (catabolic state - where energy is released during the breakdown of molecules)
- To provide biosynthetic precursors for the biosynthesis of fatty acids and amino acids (anabolic state - where energy is consumed in the formation of larger molecules)
Where does the citric acid cycle occur?
Citric acid cycle takes place in the mitochondrial matrix
What are and how many intermediates are there in the citric acid cycle?
9 intermediates, with the first one being citrate (made by combining acetyl coenzyme A and oxaloacetate)
Explain the stages in the citric acid cycle
- Oxaloacetate (4C) combines with acetyl coenzyme A (2C) to form citrate (6C)
- This reaction is catalysed by citrate synthase
- Then isomerisation of citrate into isocitrate
- This is a 2 step reaction catalysed by the enzyme aconitase
- The 2 step reaction generates the intermediate, cis-aconitase
- Isocitrate is oxidised into alpha-ketoglutarate (5C) by the enzyme isocitrate dehydrogenase
- In this reaction, 1 molecule of NADH and 1 molecule of CO2 are produced
- Alpha-ketoglutarate is converted into Succinyl-CoA (4C) by the enzyme alpha-ketoglutarate dehydrogenase
- In this reaction, 1 molecule of NADH and 1 molecule of CO2 are produced
- Succinyl-CoA is converted into succinate by the enzyme Succinyl CoA synthetase
- In this reaction, 1 molecule of GTP is produced
- Succinate is then converted into fumarate by the enzyme succinate dehydrogenase
- In this reaction, 1 molecule of FADH2 is produced
- Fumarate is converted into malate by the enzyme fumarase
- Malate is converted into oxaloacetate by the enzyme malate dehydrogenase
- This reaction produces 1 molecule of NADH
What is special about succinate dehydrogenase as an enzyme compared to the other citric acid enzymes?
Succinate dehydrogenase is also involved in oxidative phosphorylation; this enzyme is found in the inner membrane of mitochondria
All the other citric acid enzymes are soluble enzymes inside the mitochondrial matrix
What are the products generated for 1 molecule of acetyl coenzyme A metabolised in the citric acid cycle?
3 x NADH
1 x FADH2
2 x CO2
1 x GTP
The NADH and FADH2 are then used in the electron transport chain to produce ATP
What are the 2 key steps of oxidative phosphorylation?
- Generation of a proton motive force due to building up of protons in the intermembrane space (Electron transport chain/ Respiratory chain) - this force consists of a transmembrane potential and pH gradient
- Chemiosmosis (ATP synthase) - the flow of protons from inter-membrane space into mitochondrial matrix via ATP synthase, generating ATP
Which protein complexes are involved in oxidative phosphorylation?
5 protein complexes embedded in the inner mitochondrial membrane:
- NADH dehydrogenase
- Succinate dehydrogenase
- Cytochrome BC1 complex
- Cytochrome C oxidase
- ATP synthase
Which protein complexes are involved in the transfer of electrons and generation of the proton motive force in oxidative phosphorylation?
Complexes 1 to 4 i.e. NADH dehydrogenase, succinate dehydrogenase, cytochrome BC1, cytochrome c oxidase
These 4 complexes are part of the electron transport chain, part of respiratory chain complex
What are the 2 small electron carriers in oxidative phosphorylation?
Ubiquinone and cytochrome C
Why is ATP synthase not in the respiratory chain complex?
ATP synthase is not involved in electron transfer
Role of NADH in oxidative phosphorylation
Electrons from NADH enter complex 1
In this step, NADH is converted into NADP+ and donates electrons to complex 1
As complex 1 becomes charged with electrons, protons are transported by the complex from the matrix to the intermembrane space
This generates a transmembrane potential - intermembrane space is more positively charged than the matrix
Role of FADH2 in oxidative phosphorylation
Electrons from FADH2 enter the ETC at complex 2
In this reaction, FADH2 is converted to FAD
Complex 2 does not pump protons and thus does not contribute to the proton motive force
Electrons from complex 1 or 2 are transported to the electron carrier, ubiquinone, then onto complex 3
Electrons from complex 3 transported to complex 4 by electron carrier cytochrome C
Both complex 3 and 4 pump protons across the membrane and contribute towards proton motive force
Where does the final step of the ETC occur?
At complex 4, when the electron is donated to the terminal electron acceptor, oxygen
Oxygen is thus reduced to water
The high membrane potential resulting from build up of positively charged protons in the intermembrane space
These protons flow back into the matrix via complex 5, ATP synthase, which harnesses the energy from flow of protons to generate ATP from phosphorylation of ADP with Pi
What is the sum of ATP generated from each molecule of NADH and FADH2?
1 x NADH = 2.5 x ATP
1 x FADH2 = 1.5 x ATP
What is the structure of peroxisomes?
Single membrane bilayer
Unlike mitochondria, they do not contain their own DNA
What are the 2 main functions of peroxisomes?
- Some fatty acids are broken down in the peroxisomes in the process of beta-oxidation
These long-chain fatty acids are taken up and metabolised in peroxisomes
- Detoxification of the harmful molecule, hydrogen peroxide
Hydrogen peroxide is a reactive oxygen species that can react with and damage macromolecules such as DNA and proteins
This form of oxidative damage has caused cellular dysfunction and contributed to the A G process
Where does beta-oxidation take place?
Mitochondria
Mitochondria are unable to break down very long-chain fatty acids
What are the other functions of peroxisomes?
- Metabolism of bile acids which are important in the digestion and synthesis of cholesterol
What is the name of the enzyme that detoxifies hydrogen peroxide?
Catalase
Catalase is very abundant in peroxisomes but not elsewhere in the cell
What are 2 of the proteins involved in peroxisomal fission?
Peroxisomes and mitochondria can divide by fission; this process requires a set of proteins that constitute the fission machinery
Examples of proteins involved are DNM1L and FIS1
What are the shared features of mitochondria and peroxisomes?
- Both are important in cellular metabolism, in particular, the metabolism of fatty acids
- Both are able to divide and the protein machinery required is located on the outside of both organelles
- Antiviral signalling - mitochondrial antiviral signalling complex (MAVs) is found on the outer membrane of mitochondria, but also on the membrane of peroxisomes
What is mitochondrial DNA and how do mitochondrial diseases arise?
Some mitochondrial diseases are caused by mutations in DNA that is present in mitochondria
Mitochondrial DNA - a small circular DNA molecule, that encodes several subunits of oxidative phosphorylation complexes
