Mitochondrial function + disfunction

What are some functions of mitochondria?

• Oxidises fats, proteins and carbohydrates produce ATP for cellular processes.

• involved in steroid hormone production and the metabolism of neurotransmitters.

• contributes to the synthesis of nucleotides needed to make DNA and RNA.

• takes up and releases Ca²⁺ to regulate intracellular calcium levels and signalling.

• provides energy and metabolic intermediates required for cell growth and division.

• regulates programmed cell death + apoptosis

what is the structure of mitochondria?

what are the 3 stages of Cellular respiration?

• Stage 1: Production of acetyl-CoA

• Stage 2: Oxidation of acetyl-CoA

• Stage 3: Electron transport and chemiosmosis

what is stage 1?

• Carbohydrates, fats and proteins are broken down into molecules that can be converted into acetyl-CoA.

• Acetyl-CoA enters the next stage of cellular respiration.

what is stage 2?

• Acetyl-CoA is oxidised in the citric acid cycle (TCA/Krebs cycle), producing CO₂, NADH and FADH₂.

• NADH and FADH₂ carry high-energy electrons to the next stage.

what is stage 3?

• Electrons pass along the electron transport chain in the inner mitochondrial membrane, releasing energy used to pump H⁺ ions across the membrane.

• H⁺ ions flow back through ATP synthase, producing ATP by oxidative phosphorylation.

• Oxygen acts as the final electron acceptor, forming water.

What happens in the electron transport chain?

what can NAD+ accept?

1 proton + 2 electrons

what are flavoproteins?

proteins that contain a vitamin B₂-derived cofactor called FAD or FMN, which helps them transfer electrons during metabolic reactions.

• can accept ½ electrons giving FMNH₂ or FADH₂

what are Iron–sulphur (Fe–S) clusters?

structures containing iron and sulphur atoms that transfer electrons between proteins during oxidation–reduction reactions.

what is their role in respiration?

found in several electron transport chain complexes, particularly Complex I, II and III, where they help pass electrons along the chain.

how many electrons do they accept?

accept or release 1 electron at a time.

what is Ubiquinone (Coenzyme Q)?

a mobile, lipid-soluble electron carrier located in the inner mitochondrial membrane that transfers electrons from Complex I and Complex II to Complex III.

• Only electron carrier not bound to a protein complex

what does it do?

• accepts electrons and H⁺ to become reduced ubiquinol (QH₂), carrying electrons through the respiratory chain.

what are Cytochromes?

proteins containing a haem group with an iron atom that transfers electrons during oxidation–reduction reactions.

• Haem prosthetic groups – oscillates between Fe³⁺ and Fe²⁺ after accepting an electron

• Cytochromes carry 1e^-

what is complex I in the ETC?

NADH dehydrogenase

what are the main functions of complex I?

• Oxidises NADH → NAD⁺, accepting electrons from NADH.

• Transfers 2 electrons to ubiquinone (Q), reducing it to ubiquinol (QH₂).

• Pumps 4 H⁺ from the matrix into the intermembrane space.

• Contributes to the proton gradient that drives ATP synthesis.

• Can produce reactive oxygen species (ROS) through electron leakage.

where is it located?

It is an integral membrane enzyme in the inner mitochondrial membrane.

what is the shape of it?

It is L-shaped and has 2 arms - matrix + membrane arm.

what does the matrix arm do?

contains FMN and Fe–S clusters involved in electron transfer

what does the membrane arm do?

couples electron transfer to proton pumping

what 3 other things does the structure have + what do they do?

• FMN-containing flavoprotein = accepts electrons from NADH.

• Iron–sulphur (Fe–S) clusters = transfer electrons through the complex, one electron at a time.

• Fe–S protein N2 is the terminal Fe–S electron acceptor before ubiquinone.

what is the process of electron transfer?

1. NADH is oxidised into NAD⁺ + 2e⁻

2. FMN accepts 2e⁻ and is reduced to FMNH₂.

3. Fe–S clusters transfer electrons one at a time towards N2.

4. N2 transfers electrons to ubiquinone (Q), which accepts 2e⁻ and 2H⁺ from the matrix to form ubiquinol (QH₂).

5. 4H⁺ are pumped from the matrix into the intermembrane space.

what are the 2 processes that complex I catalyses simultaneously?

• Electron transfer: Transfers 2e⁻ from NADH to ubiquinone (Q), reducing it to QH₂.

• Proton pumping: Pumps 4H⁺ from the matrix into the intermembrane space.

what is complex III?

Coenzyme Q – cytochrome c reductase

where is ti located?

in the intermembrane space.

explain the structure of complex III?

It is a dimer (structure made of two molecules joined together) of identical monomers.

Each monomer has 11 different subunits including:

• Cytochrome b with 2 haem groups

• Rieske Fe-S protein (Fe₂S₂)

• Cytochrome c₁ with its heme group

Which Complex III proteins project outwards + interact with cytochrome c?

Cytochrome c₁ and the Rieske iron–sulphur protein project towards the intermembrane space.

what are the 2 binding sites that complex III has?

• Has two distinct ubiquinol-binding sites:

  • Qₒ (Qᴾ) site: faces the intermembrane space (positive side).

  • Qᵢ (Qᴺ) site: faces the mitochondrial matrix (negative side).

what is the function of complex III?

catalyses two coupled processes:

1. Electron transfer: Oxidises ubiquinol (QH₂) and transfers electrons to cytochrome c.

2. Proton translocation: Contributes to moving protons into the intermembrane space, helping generate the proton gradient for ATP synthesis.

what is the Q cycle?

explains how electrons pass through Complex III.

what is the process of the Q cycle?

Q cycle in Complex III:

1. QH₂ binds to the Qₒ site and releases 2H⁺ into the intermembrane space.

2. Its 2 electrons follow different pathways:

   • Electron 1 → Rieske Fe-S protein → cytochrome c₁ → cytochrome c.

   • Electron 2 → cytochrome b → Qᵢ site, helping reduce Q.

3. The cycle repeats, transferring electrons to 2 cytochrome c molecules.

4. Net result: 4H⁺ are released into the intermembrane space per pair of electrons, helping create the proton gradient for ATP synthesis.

what happens if cytochrome c leaks out of the mitochondrial membrane?

it can trigger apoptosis.

what is complex IV?

cytochrome c oxidase

what does it do?

• Transfers electrons from cytochrome c to oxygen.

  • Oxygen: Acts as the final electron acceptor and combines with electrons and H⁺ to form water (H₂O).

• It also Pumps 4H⁺ from the matrix into the intermembrane space per 2 electrons.

  • Helps maintain the proton gradient needed for ATP synthesis.

what is superoxide?

O₂•⁻ is a reactive oxygen species formed when oxygen accepts just one electron.

how does complex IV reduce O₂ to H₂O without releasing superoxide?

• Two reduced cytochrome c molecules donate electrons to the CuA centre.

• Electrons pass through haem a to the haem a₃–CuB centre, where oxygen binds.

• Four electrons are delivered in total, allowing the O–O bond to break safely.

• 4H⁺ from the matrix combine with O₂ and 4e⁻ to form 2H₂O (chemical protons).

• Complex IV also pumps 4H⁺ into the intermembrane space per O₂ reduced, helping maintain the proton gradient for ATP synthesis.

what is CuA and haem a₃–CuB centre?

- CuA centre: A copper-containing centre in Complex IV that receives electrons from cytochrome c and passes them into the complex.

- Haem a₃–CuB centre: The catalytic centre containing haem a₃ (with iron, Fe) and copper (CuB). Oxygen binds here and is reduced to water.

CuA = receives electrons; haem a₃–CuB = reduces oxygen to water.

what is complex II?

Succinate dehydrogenase

what does it do?

It is an enzyme complex in the inner mitochondrial membrane that transfers electrons into the electron transport chain. - it does not pump protons.

what does the ETC do?

it generates a proton gradient to drive ATP synthase.

what powers the ETC?

REDOX reactions.

what is redox potential?

measure of how likely a molecule is to accept or donate electrons.

• Positive redox potential: More likely to accept electrons (be reduced).

• Negative redox potential: More likely to donate electrons (be oxidised).

For a spontaneous reaction, what are the values of ΔG0’ and ΔE0’?

ΔG0’ is always negative and ΔE0’ is always positive.

THE BIT YOU SKIPPED

COME BACK TO IT!!

what is Mitochondrial Dysfunction and 3 main causes?

when mitochondria do not work properly.

1. Impaired electron transport + ATP synthesis due to defects in ETC complexes or ATP synthase reduce ATP production.

   • Can result from a defect in a single enzyme.

2. Inadequate number of mitochondria due to Impaired mitochondrial biogenesis (making new mitochondria) or dynamics (fission and fusion).

• Fewer functional mitochondria means less energy production.

3. Accumulation of damaged mitochondria

• Impaired mitophagy means damaged mitochondria are not removed properly.

• Damaged mitochondria can produce more ROS and less ATP.

what is the most common single-enzyme mitochondrial disorder + what are they caused by?

Complex I disorders - caused by mutations.

what does this affect?

Impairs electron transport and ATP production.

what is the fatality rate for these dysfunctions?

• Around 50% fatal before age 2.

• Around 75% fatal before age 10.

what is Heteroplasmy?

the presence of more than one type of mitochondrial genome within a cell, including normal and mutant mtDNA.

what does a higher proportion of mutant mtDNA mean?

It can lead to more severe disease, depending on the tissue and mutation.

what is are Monogenic mitochondrial disorders?

diseases caused by a mutation in a single gene that affects mitochondrial function, particularly oxidative phosphorylation and ATP production.

where can the mutations occur in? (2 places)

1. Nuclear DNA - inherited from parents based on Mendelian inheritance patterns (like recessive and dominant)

2. Mitochondrial DNA: maternally inherited; symptoms can vary because of heteroplasmy

why is mitochondrial dysfunction so bad?

Mitochondria are present in almost all cells, so many organs can be affected.

• Neurons and muscles are commonly affected because they have high energy demands

what are some Treatment strategies for harmful mtDNA variants?

• Pre-implantation genetic testing: embryos created through IVF can be tested to help select embryos with a lower proportion of mutant mtDNA.

• Mitochondrial donation: uses a donor egg containing healthy mitochondria to reduce the risk of passing on harmful mtDNA variants.

what are 2 processes which occur to maintain mitochondrial health?

Fusion and fission.

what is mitochondrial Fusion + what does it allow?

It is when mitochondria join together.

• Mixes mitochondrial contents, including mtDNA and proteins.

• Allows healthy components to help compensate for damage, maintaining function.

• Can support ATP production.

what is mitochondrial Fission + what does it allow?

when one mitochondrion splits into two

• Helps increase mitochondrial number + distribute mitochondria during cell division.

• Separates damaged mitochondrial parts so they can be removed by mitophagy.

• Allows rapid adaptation to cellular needs.

how does Mitochondrial fusion take place?

• Mfn1 and Mfn2: fuse the outer mitochondrial membranes.

• OPA1: fuses the inner mitochondrial membranes.

• Mixes mitochondrial contents, helping compensate for damage.

how does Mitochondrial fission take place?

• Drp1 is activated by phosphorylation or sumoylation.

• Drp1 is recruited to mitochondria and binds receptors.

• Forms a cuff around the mitochondrion, constricting and severing both membranes.

• Produces two separate mitochondria.

what is sumoylation?

It is when a small protein called SUMO attaches to another protein, changing its activity, location or interactions.

what does OPA1 do?

maintains cristae structure and supports ETC function.

what can disrupting OPA1 lead to?

• OPA1 disruption can impair the ETC and release cytochrome c.

• Cytochrome c release activates apoptosis (programmed cell death).

what is mitophagy?

Selective degradation and recycling of damaged mitochondria.