HomeWork | Due: Friday, September 27, 2024 Flashcards
The structure and chemical composition of mitochondrion (including the features and marker enzyme of each part)
A mitochondrion is a double-membrane bound organelle having an outer membrane, an inner membrane folded into cristae, and a cell matrix, where the majority of the chemical reactions required for energy production occur. The membranes are primarily composed of phospholipids and embedded proteins.
- Outer membrane
A relatively permeable membrane containing porin (channel-forming protein) channels that allow small molecules (<5000 daltons) to pass through freely. The porins in the outer membrane are ideal for transport. - Intermembrane space
The narrow space between the outer and inner membranes containing several enzymes which use ATP to phosphorylate other nucleotides. - Inner membrane
Highly folded into cristae, creating a large surface area for the embedded enzymes involved in oxidative phosphorylation and ATP synthesis. There is a high concentration of specific proteins involved in the electron transport chain which is responsible for carrying out oxidation reactions. There is also synthase which makes ATP in the matrix, as well as, transport proteins which allow the passage of metabolites. - Matrix
The innermost compartment enclosed by the inner membrane, containing enzymes (Tricarboxylic acid, TCA) for the Kreb’s cycle, mitochondrial DNA (mtDNA) for mtDNA expression and ribosomes for protein synthesis.
The requirements for the targeting of polypeptides to the matrix or intermembrane space of mitochondrion
To target polypeptides to the mitochondrial matrix or intermembrane space, the key requirement is the presence of a specific amino acid sequence called a mitochondrial targeting signal (MTS) at the N-terminus (amine terminus) of the polypeptide, which acts as a signal to guide the protein to the mitochondria.
For the intermembrane space, additional sorting signals may be needed depending on the exact location within the space, often involving a bipartite signal sequence where the first part targets the matrix and the second directs it to the intermembrane space upon further processing.
Targeting signals are most commonly found at the N-terminus of the polypeptide chain and are typically composed of a short sequence of amino acids with a high proportion of positively charged residues and a hydrophobic region, forming an amphipathic alpha-helix. The function of targeting signals is to be recognized by receptors on the outer mitochondrial membrane, initiate the import process where the polypeptide is unfolded and translocated through the membrane complex.
Chaperone Proteins - Cytosolic chaperones help maintain the polypeptide in an unfolded state for efficient import
Translocation Complexes - The outer and inner mitochondrial membranes have specific protein complexes that facilitate the recognition and transport of the polypeptide
ATP Hydrolysis - energy from ATP is required for the translocation process across the mitochondrial membranes
- Targeting to matrix
1. A single N-terminal MTS is usually enough to target a protein to the matrix
2. Once inside the mitochondria, the MTS is cleaved off by a processing peptidase - Targeting to the Intermembrane space
1. Proteins required in the intermembrane space often have a longer MTS that contains additional information beyond the matrix targeting signal which allows further translocation across the inner membrane and release into the intermembrane space (bipartite signal sequence)
2. In some cases, internal signal sequences within the polypeptide chain can also direct targeting to the intermembrane space (internal sorting signals)
The structure and mechanism of ATP synthase/elementary particle
ATP synthase plays a critical role in generating the majority of cellular ATP and acts primarily as a rotary mechanism,as well as, for the coupling of proton flow and ATP synthesis.
It is a complex enzyme composed of two main parts: the membrane embedded “F0” component which acts as a proton channel, and the water soluble “F1” component that protrudes into the mitochondrial matrix (responsible for ATP synthesis as the catalytic subunit).
- Mechanism
1. A proton gradient is established across the mitochondrial membrane with more protons outside the membrane than inside
2. Protons flow through the F0 complex causing the “c” ring to rotate
3. The rotation of the central stalk involves the rotating “c” ring being connected to the central “gamma” subunit in F1, causing it to rotate
4. As the “gamma” subunit rotates, it induces conformational changes in the alpha and beta subunits, cycling through binding Adenosine Diphosphate (ADP) and Inorganic Phosphate (Pi) forming Adenosine Triphosphate (ATP) and releasing the ATP molecule.
Mitochondrial reproduction: human mtDNA and its features, semi-autonomous organelle, the endosymbiont origin theory, the proliferation of mtDNA
Mitochondrial reproduction refers to the process by which mitochondria replicate through the process of binary fission (dividing into two separate mitochondria).
- human mtDNA
1. The size range of organelle DNA is similar to that of viral DNAs.
2. Genes in human mtDNA encode rRNAs (2), tRNAs (22), and some mitochondrial proteins (13 polypeptides).
3. mtDNA is a small, circular chromosome located within the mitochondria.
4. Exclusively passed from mother to child as the mitochondria in sperm are typically degraded during fertilization - semi-autonomous organelle
1. Mitochondria are partially self-replicating organelles because, while mitochondria have their own ribosomes, most of the proteins needed for mitochondrial function are still encoded by nuclear DNA (in the nucleus of the cell) and synthesized in the cytoplasm. - endosymbiont origin theory
1. Compares ribosomal RNA with the base sequence of various bacterial rRNAs.
2. Suggests that mitochondria originated from prokaryotes that were engulfed by a host eukaryotic cell and eventually evolved into organelles.
3. The ability of mitochondria to replicate themselves is therefore as a result of having their own small, circular DNA and a division mechanism (simple/binary fission) where they simply split into two identical copies (mirroring how bacteria reproduce). - proliferation of mtDNA
1. When a cell requires more energy, the proliferation of mtDNA is crucial to increase the number of functional mitochondria.
2. Mitochondria have both fission (division) and fusion (separate mitochondria coming together and mixing genetic material) capabilities to help maintain a healthy mitochondrial population in the cell.
3. When mitochondria divide through binary fission, they elongate and then split into two separate organelles.
Mitochondria and intrinsic apoptosis
Mitochondria play a vital role in intrinsic apoptosis, which is the programmed cell death pathway, crucial for tissue homeostasis and development.
The pathway involves:
1. Mitochondrial outer membrane permeabilization - triggers the release of pro-apoptotic proteins from the mitochondrial intermembrane space into the cytoplasm; often coupled with the release of cytochrome c which promotes caspase activation and the execution of apoptosis
2. Cell death modulators - released from the mitochondria to disrupt mitochondrial functions and lead to cell death
3. Cell death - cell shrinks, chromatin condenses, there is nuclear fragmentation, and cell breakage into small vesicle bound fragments
4. Activation - caused by a range of stimuli including DNA damage, etc.
5. Caspase cascade - triggered at the mitochondria and modulated by virus-encoded anti-apoptotic B cell leukemia (BCL) 2 - like receptors
6. Other proteins such as Endonuclease G (EndoG) and Apoptosis Inducing Factor (AIF) also aid apoptosis by cleaving (process involving the activation of caspases to dismantle cell contents) chromatin DNA and initiating chromatin condensation/large scale DNA fragmentation respectively in apoptosis.
