Module 13: Endomembrane System PDF

MODULE 13 Endomembrane System A group of membranes and organelles that work together to modify, package, and transport lipids and proteins. Includes membranes of the nucleus, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vesicles, endosomes, and the plasma membrane. Excludes the mitoc...

MODULE 13 Endomembrane System A group of membranes and organelles that work together to modify, package, and transport lipids and proteins. Includes membranes of the nucleus, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vesicles, endosomes, and the plasma membrane. Excludes the mitochondrial and peroxisomal membranes. It is a functional unit due to direct connections or exchanged material (vesicles from donor membranes move to and fuse with acceptor membranes to deliver cargo and membranes). Subcellular Fractionation Cell homogenization fragments the cytoplasmic membranes; vesicles are formed. Vesicles from the endomembrane system form similar size vesicles (microsomes). Microsomes are separated into smooth and rough subfractions. Once isolated, the biochemical identity of lipid and protein fractions can be determined. Ribosomes Responsible for protein synthesis (translation). Composed of two subunits: the large subunit contains three ribosomal RNA (rRNA) molecules and ~50 proteins; the small subunit has one rRNA and ~30 proteins. Ribosomes assemble when needed for protein synthesis and disassemble after completing mRNA translation. Located in the cytosol, either free or bound to the ER. Mitochondria Double-membrane organelles Erythrocytes do not contain mitochondria, but liver and muscle cells contain thousands. The inner membrane forms folded structures called cristae that protrude into the mitochondrial lumen (matrix). Mitochondria contain circular DNA, which encodes tRNAs, rRNAs, and proteins; however, most mitochondrial proteins are encoded by genomic DNA. Mitochondria also contain ribosomes, but most mitochondrial proteins are translated on free cytosolic ribosomes and imported into the organelle by targeting signals. New mitochondria are produced by the growth of preexisting organelles followed by fission. Mitochondrial Functions: Energy Production ATP synthesis: uses energy derived from electron transport and oxidative phosphorylation. Pyruvate (the product of glycolysis in the cytosol) and fatty acids are imported into the mitochondria and converted to acetyl-CoA in the matrix. Acetyl-CoA is oxidized to CO2 via the citric acid cycle, the central pathway of oxidative metabolism. The oxidation of acetyl-CoA to CO2 is coupled to the reduction of NAD+ and FAD. High-energy electrons from NADH and FADH2 are transferred through carriers in the membrane to molecular oxygen, creating a proton gradient that is used for ATP synthesis. Mitochondrial Functions: Cell Death Programmed cell death (apoptosis): one pathway is mediated by mitochondria. Cell death in development eliminates organs/tissues useful only during the embryonic/larval stages. Death of abnormal cells (e.g., virally infected, cancerous) is beneficial. Proapoptotic proteins insert into the mitochondrial membrane, forming pores. Cytochrome C leaves the intermembrane space of the mitochondria through the pores, entering the cytosol. In the cytosol, cytochrome C stimulates a cascade of events resulting in apoptosis. MODULE 13 Protein Translocators More than 95% of mitochondrial proteins are synthesized on free ribosomes and imported into mitochondria. These proteins have a signal sequence at the N-terminus, which is removed after import into the mitochondrial matrix. The precursor proteins are imported at contact sites that join the inner and outer membranes. Protein translocators: TOM complex (outer membrane) and TIM complexes (inner membrane). Mitochondrial proteins are unfolded by hsp70 proteins in the cytosol, bind to TOM receptors, and enter the mitochondria. The Mitochondrial Compartments The composition of the intermembrane space is equivalent to the cytosol, with a slightly lower pH. The outer membrane contains porins, proteins that form aqueous channels allowing the free passage of molecules smaller than 5 kDa. The inner membrane forms cristae, with over 70% of proteins involved in oxidative phosphorylation and metabolite transport. The inner membrane is impermeable to most ions and small molecules to maintain the proton gradient. Electron transport pushes protons from the matrix into the intermembrane space, creating a concentration gradient that drives ATP synthesis. Mitochondrial Diseases mtDNA is susceptible to mutations because: 1. It does not have the DNA repair mechanisms common to nuclear DNA. 2. Mitochondria produce reactive oxygen species (ROS). 3. Metabolic intermediates accumulate in the mitochondria. Defects in oxidative phosphorylation mostly affect tissues with the highest ATP demands (e.g., brain, heart, liver). Almost all mitochondria in fertilized eggs come from the oocyte, so germline mutations in mtDNA are transmitted maternally. Peroxisomes Peroxisomes are single-membrane organelles found in many cell types. They contain a dense crystalline core of oxidative enzymes synthesized on free ribosomes. Peroxisomes participate in: - β-oxidation of very long-chain fatty acids (also occurs in mitochondria). - α-oxidation of branched-chain fatty acids (oxidative decarboxylation removes the terminal carboxyl group as CO2). - Synthesis of bile acids and ether-linked phospholipids. - Removal of reactive oxygen species (e.g., hydrogen peroxide). Patients with peroxisomal disorders may have increased serum levels of very long-chain fatty acids (>22 carbon atoms) and branched-chain fatty acids. MODULE 13 Putting It All Together Approximately half of the volume of a eukaryotic cell is in intracellular compartments (organelles). The main organelles include the endoplasmic reticulum (ER), Golgi apparatus, nucleus, mitochondria, lysosomes, endosomes, and peroxisomes. Ribosomes function in protein translation and may be free or bound to the ER. Mitochondria have double membranes that form folded cristae and surround a matrix to form distinct compartments. ATP is generated using an electrochemical gradient across the matrix. Mitochondria can self-replicate and contain their own DNA and ribosomes. Cell survival depends on the integrity of the mitochondrial membrane; release of cytochrome C from the mitochondria into the cytosol initiates a cascade of reactions that result in apoptosis. Peroxisomes contain hydrolytic enzymes, detoxify hydrogen peroxide, and participate in the breakdown of fatty acids. They are also involved in liver synthesis of cholesterol and the production of myelin sheaths that protect neurons.