Mitochondria/peroxisomes Flashcards
General functions of mitochondria (6)
- aerobic ATP production
- lipid/phospholipid metabolism
- steroid hormone synthesis
- Ca2+ homeostasis
- Heat production
- Apoptosis initiation
2 theories of mitochondrial biogenesis (and which one is more supported)
- ENDOSYMBIOSIS:
- descendants of ancient prokaryotes
-engulfed by phagocytosis to become part of bacterium - AUTOGENOUS:
-event in eukaryotes which caused separation of genomic parts and encircling them in a plasma membrane
!! Endosymbiotic is supported because: mitochondria have their own genome, can synthesise some of their own substances and replicate similarly to bacteria
General structure of mitochondria
3 REGIONS:
1. outer membrane
2. intermebrane space
3. matrix
2 important characteristics of mitochondria
- Shape can change via fusion/fission
- Organelle can move throughout cytoplasm bcos of attachment to microtubules
how can mitochondria be separated and visuallised?
SEPARATED: centifugation, diff speeds have an output of different regions
VISUALISED: either fluorescent microscopy (allows visualisation of the motility as well as the structure) or TEM
Structure of outer membrane
-enzymes used for lipid synthesis
-VDAC channels for ATP to exit mitochondria and into cell cytoplasm
-PORINS: Transmembrane proteins which allow it to be mostly permeable
-membrane receptors for proteins and peptides that translocate into intermembrane space
Structure of intermembrane space
-enzymes that use ATP generated in inner membrane
-cytochrome C for apoptosis
!! size is variable and depends on activity levels
Structure of inner membrane
2 SUBDIVISIONS:
1. Inner boundary
-contains double phospholipid CARDIOLIPIN (with 4 fatty acid tails) that makes membrane impermeable to ions
-!!! no cholesterol
- Cristae
-contains elementary particles: transmembrane proteins needed for respiratory chain/ ATP synthesis
-dehydrogenases for oxidative phosphorylation
2 possible organisations of cristae
- LAMELLAR: perpendicular to longitudinal axis
!! most common - TUBULAR: shorter and tubular invaginations into inner membrane
!! present in steroid synthesising cells
structure of matrix
-contains circular mtDNA
-contains mRNA, tRNA, ribosomes
-contains Ca2+ and Mg2+ ions stored in granules
-contains enzymes needed for Krebs and Ox, phosphorylation
Cell types that mitochondria are in higher concentration in (than average) [6]
- cardiomyocytes
- skeletal muscle fibers
- liver cells
- sperm cells
- neurons
- kidney tubular cells
Mitochondria organisation in cardiomyocytes
-mitochondria positioned inbetween actin and myosin filaments –> along myofibrils
3 different types:
1. subsarcolemmal mitochondria (SSM):located just under sarcolemma
- interfibrillar mitochondria (IFM): major population, between the myofibrils
- perinuclear mitochondria (PNM): located in region adjacent to nucleus
Mitochondria in sperm cells
-present within the ‘collar’
-provides energy needed for motility
Mitochondria in neurones and function
-biogenesis occurs in the soma, and mitochondria are then moved (via cytoskeletal attachment) to the synapse
FUNCTION:
-supply ATP for neurotransmitters
-act as buffering for local Ca2+ concentration
RELATIONSHIP BETWEEN MITOCHONDRIA AND ACTIVITY OF SYNAPSE: higher activity means mitochondria with higher volume, higher cristae density and lamelarity, higher cytochrome C expression
Role of mitochondria in Ca2+ homeostasis and its purpose
Ca2+ homeostasis regulates:
-cardiac energy metabolism
-apoptosis
-oxygen free radical production
-autophagy
-oxidative phosphorylation and redox balance
!! regulated by mitochondria via influx/efflux mechanisms
Pathologies associated with mitochondrial abnormalities
- Morphological abnormalities:
-childhood myopathy: larger mitochondria with paracrystalline structures of regular cristae (dense packing of inner membrane proteins) - Mutational defects (present a slight amount in everyone but become a disease when they cross the threshold of expression):
-autism
-muscular dystrophy
-parkinsons/alzheimers
Orthodox vs condensed mitochondral configuration
ORTHODOX:
-healthy cells
-matrix occupies the majority of volume
-cristae are prominent
-occurs when ADP is low
CONDENSED:
-unfolded cristae
-decreased volume
-increased intermembrane space
-facilitates secretion of cytochrome C
-occurs due to H+ accummulation at high ADP
Role of mitochondria in heat production
-occurs in brown adipose tissue (get their colour because of high abundance of mitochondria and capilaries)
-H+ leak in the brown fat cells occurs via uncoupling proteins which generates ATP
Importance of the fission:fusion ratio in mitochondria
determines overall morphology of mitochondrion
- FUSION>FISSION: produces interconnected mitochondria
!! occurs in actively metabolising cells for energy dissipation - FISSION>FUSION: produces mitochondrial fragments. Can also eliminate damanged mitochondria via mitophagy
!! occurs in quiescent cells
Proteins involved in the fusion and fission mechanisms
! proteins of the DYNAMIN family
- FUSION:
- Mfn 1/2 (mitofusin) protein mediates fusion of outer membrane
- Opa1 mediates fusion of inner membrane - FISSION:
-Drp1 squeezes mitochondria to help separation
Outline of mitophagy
MITOPHAGY: process by which damaged mitochondria are lysed
- damaged organelle
- phagophore
- autophagosome
- phagolysosome
- lysis
- recycling of fragments
structure of mtDNA
-double membrane
-circular (nucleoid)
-each nucleoid contains 4/5 copies of the mtDNA
-linked to cristae membrane
-maternal inheritance
Where do the proteins needed by the mitochondria come from?
- they synthesise it themselves
- imported from the cytosol (main method): either through TIM complex (on inner membrane) or TOM complex (on outer membrane)
!! signaling sequences on the proteins are picked up by complexes
4 ways that mitochondria can be exhnaged between cells
- tunelling nanotubes
- extracellular vesicles
- gap junction channels
- cell fusion
INTRINSIC (mitochondrial) apoptotic pathway cause and detailed mechanism
CAUSE: triggered by intracellular stress signals (usually derived from DNA damage). Relies on proteins of the Bcl-2 family (which can be either pro or anti apoptosis depending on the number of BH domains they contain)
PROCESS:
1. Peristimulus, the cytosolic antiapoptotic molecules (of Bcl2 family) are active: hence prevent cell death
- Arrival of apoptotic stimulus inactivates antiapoptotic molecules and hence pro-apoptotic functions are allowed to interact
- interactions induce formation of the MOMP (mitochondrial outer membrane permeabilization) pore)
- Pore allows exit of certain molecules from mitochondrial space into cytoplasm (eg. cytochrome C or p53)
- Cytochrome C binds to cytoplasmic APAF1 which is found in its inactive state
- Binding causes a conformational shape change which causes APAF1 to exhibit a CARD domain
- CARD domain exposure induces formation of an apoptosome (macromolecular structure)
- recruiting of caspase 9 (initiator which shows a CARD domain) by apoptosome
- activated caspase 9 activates executioner caspases triggering proteolytic cascade
- executioner caspases cleave and degrade substrates (eg. cytoskeletal elements) and activate endonucleases
- cytoskeleton breaks down, forming a cytoplasmic bud which forms the apoptotic body
- recognition of apoptotic body and internalization by professional phagocytes
Process by which mitochondria replicate/divide
-segmentation
-similar to bacteria (endosymbiotic theory proof)
-squeezing of the organelle from its center into 2
what is the morphology of mitochodria in each phase of the cell cycle
G1: various morphologies as they become replicated
G1-S transition : fusion and elongation (high metabolic demand for DNA replication)
G2/M phases: undergo fission to form individual organelles spatially distributed (for equal distribution of mitochondria to the daughter cell in mitosis)
2 organelles that dont possess mitochondria
- RBCs
- terminal stage keratinocytes
General functions of a peroxisome (5)
- lipid synthesis & storage
- production of bile acids
- remove free radicals and reactive oxygen species
- synthesis of plasmalogens (phospholypids where one fatty acid chain is attached to glycerol present in the brain and heart)
- role in immune response
structure of peroxisomes
-membrane bound organelle
-enclosed granular matrix
-40% enzyme content = catalase
-sometimes present: paracrystalline central structure made of urate oxidase
-variable shape (rod/circular)
-increased numbers in cells with higher metabolic activity
biogenesis of peroxisomes
!! either synthesised de novo or derived from pre-existing peroxisomes
- Classical model: growth and division of preexisting peroxisomes give rise to new ones
- Alternative model: membrane proteins are acquired from SER and matrix proteins are acquired from cytoplasm
-initiated by the budding off of pre-perixosomal vesicles from SER which undergo fusion
-some synthesis of proteins on free ribosomes of cytoplasm
-proteins needed for assembly contain PTS (peroxisomal targeting signal) – PTS1 & PTS2
-proteins inserted into membrane using chaperones
how can peroxisomes be visualised?
- fluorescent miroscopy via probes
- stains that target specific enzymes within the matrix (eg. DAB)
main differences between lysosomes and peroxisomes (5)
- microscopically: L is pale and P is dark
- L has lytic enzymes, P has oxidative enzymes
- L derived from Golgi/ER, P derived from ER and can self replicate
- L are larger than P
- L doesnt generate energy, P generates ATP
