Transport into Mitochondria and peroxisomes Flashcards
The problem
Most mitochondrial and all peroxisomal proteins encoded by nuclear genome but how do they get to where they’re needed
What do mitochondria do
Primary function - energy production, make large amounts of ATP
First step - glycolysis, breakdown of glucose to pyruvate occurs in cytoplasm
Pyruvate enters mitochondria - CO2 removed from pyruvate, rest of molecule enter TCA cycle
1’ function to produce h+ gradient across inner mito membrane -> drives ATPase
Plays key role in apoptosis
Mitochondria will not be able to function without protein
Mitochondria - structure
Has double membrane
Outer membrane encloses organelle
Inner membrane highly folded, leads to high SA so more space in intermembrane space to hold more enzymes for ATP production
Folds known as cristae
Inner matrix contains enzymes responsible for energy production
(Also look at diagram to help visualise)
Outer membrane
Perforated with large channels (porins)
Allow entry molecules <5000 kDa
Contains enzymes involved in mitochondrial lipid synthesis
Also variation in number mitochondria between cells, so overall energy production different
Intermembrane space
Contains enzymes that use ATP to phosphorylate other nucleotides
H+ pumped into this space, create protein gradient to drive Ox phos
Inner membrane
Folded into cristae - maximises SA
Contains REDOX performing proteins of electron transport chain
Proteins for ATP synthesis
Transport proteins to move molecules in and out of matrix
Matrix
Internal space contains enzymes of Krebs cycle
Contains:
- Mitochondrial DNA
- Ribosomes (29 and 40)
- tRNAs
- Enzymes (TCS, b-oxidation)
- Metabolites (e.g. TCA urea cycle, Ca2+, K+, Mg2+)
Mitochondrial genetics
Mitochondria contain own genetic material:
- Circular chromosomes (double stranded)
- Approx 15-17 kbps
- Encodes 37 genes
Mitochondrial DNA inherited from mother
Mitochondrial life cycle - fusion, fission and mitophagy
Mitochondria can’t be synthesised de novo
New mitochondria must arise from existing ones
At any point, mitochondria are in dynamic flux between fission and fusion
Will eventually get damage and debry in mitochondria, have method to all segregation damaged bits to one pole of mitochondria then undergoes fission so you get one clean part and one damaged part
Damaged part then targeted for death and under goes mitophagy and is killed by autophagosome
Clean fuses back with another mitochondria if want to
If mitochondria comes under stress can recover some damaged mitochondria and refuse with it to create more energy in desperate times
How do you get proteins into mitochondria
Proteins translocate as synthesised
In contrast, proteins fully synthesised then translocated into mitochondria, still use sequences
Uses translocation proteins embedded in outer and inner membrane:
- TOM: translocator of the outer membrane
- SAM: Sorting and assembly machinery
- TIM translocator of inner membrane
- OXA: cytochrome Oxidase activity
How do you get proteins into mitochondrial matirx
N-terminal signal sequence ~ recognised by TOM complex
Protein translocate through TOM and TIM23
Translocates through TIM 23 into matrix
Signal is cleaved off
(look at diagram)
Mitochondrial precursor proteins are imported as unfolded polypeptide chains
Proteins could fold before docking with TOM complex
To stop this:
- Bind interacting proteins to newly synthesised polypeptide chain e.g. chaperones
- i.e. proteins imported into mitochondria unfolded
Directional processes usually need energy
Chaperones need energy to dissociate from polypeptide chain
Signal sequence is +ve charged
Electrochemical H+ gradient driven by electron transport has two effects:
1. ATP production
2. Membrane potential drives +ve charged signal sequence through the IMM
Getting proteins into mitochondrial outer membrane
Major proteins in OMM called porins
Are beta-barrel proteins
Problem: TOM cannot insert proteins into bilayers
Solution: Enter in intermembrane space, kept unfolded by chaperones
Then dock with SAM complex, when then inserts and folds
(look at diagram)
Getting proteins into mitochondrial inner membrane and intermembrane space is varied
Most common route for inner membrane use TOM and TIM23, but also uses Stop-transfer sequence to stop protein being fed through to matrix
2nd route:
- Proteins completely enter matrix space
- Signal sequence cleavage unmasks 2nd signal that causes insertion into OXA complex
OXA is same complex that inserts mitochondrially translated proteins
If want soluble protein in intermembrane space just cleave after membrane insertion
Multipass IMM proteins: snake through TOM as loop
Allows chaperones to bind to stop folding and guide towards TIM22
