transport into mitochondria and peroxisomes Flashcards
What do the mitochondria do
-Primary function – energy production. Mitochondria make large amounts of ATP
-First step – glycolysis the breakdown of glucose to pyruvate occurs in the cytoplasm
-Pyruvate enters the mitochondria – CO2 removed from pyruvate. The rest of the molecule enters the TCA cycle
-1’ function to produce H+ gradient across inner mito membrane -> drives ATPase
-Key role in APOPTOSIS
whats the structure if mitochondria
-double membrane
-Outer membrane encloses the organelle
-The inner membrane is highly folded (high SA). The folds are known as cristae
-The inner matrix contains the enzymes responsible for energy production
what is the outer membrane structure of mitochondria
-Perforated with large channels (Porins). These allow entry of molecules < 5000 kDa
-Contains enzymes involved in mitochondrial lipid synthesis
what is the inter membrane structure of mitochondria
-Contains enzymes that use ATP to phosphorylate other nucleotides.
-H+ is pumped into this space (to create the proton gradient to drive Ox phos).
what is the inner membrane structure of mitochondria
-Folded into christae – this maximizes surface area.
-Contains the REDOX performing proteins of the electron transport chain.
-Proteins for ATP synthesis
-Transport proteins to move molecules in and out of the matrix
what is the matrix structure of mitochondria
-Internal space containing enzymes of the Krebs cycle
-Contains
-Mitochondrial DNA (Encoded in the nuclear genome, and They synthesise 13 polypeptides associated with OXPHOS)
-Ribosomes
-tRNAs ( and rRNAs)
-Enzymes (TCA, b-oxidation)
-Metabolites (eg TCA urea cycle, Ca++, K+, Mg++)
whats mitochondrial genetics
-The mitochondria contain their own genetic material
-Circular chromosome (double stranded)
-apprx 15-17kbps
(bacterial plasmids are 744bp -2.58Mb, average = 80kb)
-Encodes 37 genes
-The mitochondrial DNA is inherited from the mother
what stages are in the mitochondrial life cycle
-fusion
-fission
-mitophagy
-At any point of time, mitochondria are in a dynamic flux between fission and fusion
How do you get proteins into mitochondria
-A reminder about getting proteins into the ER-proteins translocate as they are synthesised ie unfolded
-In contrast, the proteins are fully synthesised and then are translocated into the mitochondria. Still uses signal sequences
-Uses translocation proteins embedded in the outer and inner mito membrane
-TOM: Translocator of the Outer Membrane
-SAM: Sorting and Assembly Machinery
-TIM: Translocator of the Inner Membrane
-OXA: Cytochrome OXidase acitivity
How do you get proteins into mitochondrial matrix
-N-terminal signal sequence – recognised by the TOM complex
-The protein translocates through TOM and TIM23
-Translocates through TIM23 into matrix
-Signal is cleaved off
how do you stop proteins from docking before fully synthesising with the TOM complex
-You bind interacting proteins to the newly synthesized polypeptide chain eg chaperones
-ie proteins imported into mitochondria UNFOLDED
why do Directional processes usually need energy
-Chaperones need energy (ATP) to dissociate the chaperones from the polypeptide chain
-The signal sequence is +ve charged
what is the electrochemical gradient driven by in directional processes and what are the effects it has
-driven by electron transport
1)ATP production
2)Membrane potential drives the +ve charged signal sequence through the IMM
how do you get proteins into mitochondrial outer membrane?
-Major proteins in OMM are called porins
-They are beta-barrel proteins
-Problem: TOM cannot insert proteins into bilayers
-Solution: Enter in intermembrane space, kept unfolded by Chaperones
-SAM complex inserts and folds
why is Getting proteins into mitochondrial inner membrane and intermembrane space is varied
-Most common route: For the Inner Mitochondrial Membrane: Uses TOM and TIM23
-2nd route:
=Protein completely enters matrix space
=Signal sequence cleavage unmasks a 2nd signal that causes insertion into OXA complex
- OXA is the same complex that inserts MITOCHONDRIALLY translated proteins
-If you want a soluble protein in the Intermembrane space you just cleave after membrane insertion
-Multipass IMM proteinsSnake through TOM as a loop- This allows chaperones to bind to stop folding and guide to wards TIM22
what are peroxisomes
-single membrane
-Do not contain DNA or ribosomes
-Found in all eukaryotic cells and carry out oxidative reactions
-Thought to be the remnant of an organelle found in the ancestors of eukaryotic cells
what is the peroxisome function
-Contain a variety of oxidative enzymes – catalase and urate oxidase
-Function to remove hydrogen atoms from various organic compounds according to the reaction
-RH2 + O2 -> R + H2O2
-This peroxide is used by peroxidases in other reactions : H2O2 + R’H2 -> R’ + 2H2O
-In the liver, peroxisomes have a role in detoxification – 25% of alcohol is converted in this pathway.
-Another important role is in b-oxidation of fatty acids
-Most peroxisomal membrane proteins are made in the cytosoland then insert into the membrane of preexisting peroxisomes
-New peroxisomes arise from preexisting ones, byorganellegrowth and fission ie like mitochondria
How do proteins get into peroxisomes
-Relatively little is known
-Signal sequence (C -terminus) (but can be a different sequence at N)
-Peroxins (using ATP) form a translocator in the membrane
-Proteins do not have to unfold to translocate
-Pex5 recognises signal sequences and accompanies cargo into peroxisome, is ubiquitylated and then is cycled back to the cytosol
-Mutation in Pex5 causes Zellweger syndrome – severe brain, liver and kidney abnormalities
