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ENDOPLASMIC RETICULUM
Submitted by Dr Madhurima Sharma
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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
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Smooth Endoplasmic Reticulum
• Without Ribosomes• Found often in ovary and testes• Continuous with rough endoplasmic reticulum• Tubular in shape
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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
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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
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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
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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
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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
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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)
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Rough Endoplasmic Reticulum
• 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)
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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
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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
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Rough Endoplasmic Reticulum
• N-Glycosylation• Formation of Core Oligosaccharide
– 2 GlcNAc units– 9 Mannose– 3 Glucose