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ENDOPLASMIC RETICULUM

ENDOPLASMIC RETICULUM. Submitted by Dr Madhurima Sharma. Endoplasmic Reticulum. Endoplasmic- within the cytoplasm Reticulum – network Function Biosynthesis of proteins destined for incorporation into plasma membrane Synthesis of proteins destined for export from cell

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ENDOPLASMIC RETICULUM

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  1. ENDOPLASMIC RETICULUM Submitted by Dr Madhurima Sharma

  2. Endoplasmic Reticulum • Endoplasmic- within the cytoplasm • Reticulum – network • Function • Biosynthesis of proteins destined for incorporation into plasma membrane • Synthesis of proteins destined for export from cell • Biosynthesis of lipids a) cholesterol b) plasma membrane • Incorporation into organelles of endomembrane system

  3. Smooth Endoplasmic Reticulum • Without Ribosomes • Found often in ovary and testes • Continuous with rough endoplasmic reticulum • Tubular in shape

  4. Smooth Endoplasmic Reticulum • Drug Detoxification involves hydroxylation • Addition of OH groups are more soluble and easier to excrete • If not soluble, hydrophobic toxins may stay in membrane of cells • Reduced form of Cytochrome P-450 hydroxylates organic hydroxyl acceptor • Class of enzymes called mixed-function oxidases

  5. Smooth Endoplasmic Reticulum • Carbohydrate Metabolism • Membrane of smooth ER of hepatocytes contain glucose-6-phosphase (breaks down glucose-6-phosphate into glucose) • Glycogen is stored in liver • Allows glucose to leave cell and travel into blood system

  6. Smooth Endoplasmic Reticulum • Calcium Storage • Sarcoplasmic Reticulum in muscle cells is an example of smooth ER that specializes in storage of calcium • Lumen has ATP dependent calcium ATPases which aids in muscle contraction

  7. Smooth Endoplasmic Reticulum • Steroid Biosynthesis • Cholesterol, cortisol, testosterone, estrogen • All share a 4-ring structure but differ by hydroxyl groups & carbons side chains • P-450 monooxygenases are important in steroid hormones by hydroxylation

  8. Rough Endoplasmic Reticulum • Transitional Elements (TE) – plays role in formation of vesicles • Transition Vesicles – shuttle lipids and/or proteins • Flattened sacs • Proteins enter lumen cotranslationally • Proteins are anchored by hydrophobic interactions or covalent attachments • Soluble proteins are released into lumen

  9. Rough Endoplasmic Reticulum • Phospholipid translocastors (flippases) move phospholipids from ER to their destination by fusing of membrane • In cases of no fusion, phospholipid exchange proteins recognize specific phospholipids and move them to destination (mitochondria, chloroplast, peroxisome)

  10. Responsible for addition of carbohydrates to proteins Folding of proteins Recognition and removal of misfolded proteins Assembly of multimeric proteins Contains enzymes for posttranslational and cotranslational modification such as disulfide bonds Quality control – ER associated degradation proteins ERAD (cytoplasmic proteases) Rough Endoplasmic Reticulum

  11. Rough Endoplasmic Reticulum • Protein Glycosylation • N-linked glycosylation –oligosaccharide unit to attach to nitrogen on asparagine • O-linked glycosylation – oligosaccharide unit to attach to oxygen on hydroxyl group of serine or threonine

  12. Rough Endoplasmic Reticulum • Formation of Core Oligosaccharide occurs in the Cytoplasm • Dolichol Phosphate inserted into ER membrane • 2 N-acetylglucosime (GlcNAc) are added to dolichol • Addition of 5 mannose sugars

  13. Rough Endoplasmic Reticulum • N-Glycosylation • Formation of Core Oligosaccharide • 2 GlcNAc units • 9 Mannose • 3 Glucose

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