BE230_Lecture_10 - BE230 Lecture 12 ENDOCYTOSIS • Bulk...

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Unformatted text preview: BE230 Lecture 12 ENDOCYTOSIS • Bulk –phase endocytosis – Non specific uptake of extracellular fluids • Receptor-mediated endocytosis – Uptake of specific extracellular macromolecules that bind to receptors on external surface of plasma membrane • 50% surface membrane can be internalised in 30minutes! The endocytic pathway • Involves early endosomes (nearer plasma membrane) and late endosomes (nearer nucleus) • Acidic due to H + ATPase proton pump in membrane • Materials get sorted in early endosomes – i.e receptors sent back to plasma membrane – Materials within sent to late endosomes • Directed to lysosome Receptor-mediated endocytosis • Cells have receptors for uptake – hormones – Growth factors – Enzymes – Plasma proteins – Receptors collect in speed areas of plasma membrane called coated pits LDLs and cholesterol metabolism • Best understood example of receptor mediated endocytosis LDL cholesterol • 1500 cholesteryl ester molecules • Single layer of phospholipids, free cholesterol • Single molecule of apolipoprotein B-100 – Binds specifically to LDL receptors on surfaces of cells Composition & Density of Human Lipoproteins Chylomicron VLDL IDL LDL HDL Density (g/ml) <0.95 0.95-1.006 1.006- 1.019 1.019- 1.063 1.063 1.210 Diam (nm) 75-1200 30-80 25-35 18-25 5-12 Protein (%) 1-2 10 18 25 33 TG (%) 83 50 31 10 8 C & CE (%) 8 22 29 46 30 PL (%) 7 18 22 22 29 Apo Chylo: AI, AII, B48, CI, CII, CIII VLDL:B100, CI,CII,CIII, E IDL:B100, CI,CII,CIII, E LDL: B100 HDL:AI,AII, CI, CII, CIII, D, E Apoproteins Generalised Structure • Spherical • 10-1000nm diameter • neutral lipid core of TG and or CE • Around core is layer of protein, PL, C oriented with polar portions exposed to surface of lipoprotein • 9 apoproteins • several enzymes • cholesteryl ester transfer ptotein LDL particle LPs made in Endoplasmic Reticulum of Liver & Intestine Cholesterol + acyl Co A------->Cholesteryl ester storage form of cholesterol ENZYME: acyl-Co: cholesterol acyltransferase (ACAT) LOCATED on cytosolic surface of liver ER apoproteins synthesised on ribosomes Chylomicrons • Produced by intestinal cells • secreted into lymphatic capillaries • enter bloodstream at large subclavian vein • bypass liver VLDL & HDL • Produced in liver cell CMs & VLDLs transport C & TG to other tissues • Triglyceride-rich particles • degraded by lipoprotein lipase • extracellular enzyme that is most active within capillaries of adipose tissue, heart muscle, skeletal muscle, lactating mammary gland • LPL activated by CII (CMs, VLDL) – supplies heart and adipose tissue with fatty acids derived from these LPs • used for energy or stored •...
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BE230_Lecture_10 - BE230 Lecture 12 ENDOCYTOSIS • Bulk...

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