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BIOC13H3 (52)
Daman Bawa (12)
Lecture 11

BIOC13Winter2013 Lecture 11 Notes.docx

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Biological Sciences
Daman Bawa

BIOC13Winter2013 Lecture 11: FA Biosynthesis, Cholesterol and Eicosanoid Metabolism Fatty Acid Biosynthesis o Liver is the major site for FA biosynthesis o FAs are synthesized when o Diet is low in fat o High carb, protein diet o Formed when citrate levels are high (high glucose, high insulin) o Requires NADPH (pentose phosphate pathway) and ATP o Reactions occur in reverse of b-oxidation pathway by addition2of C but there are differences in the required enzymes, subcellular location and source of redox energy. o Cytoplasm o Transfer of acetyl coA to cytoplasm via ATP citrate lyase: citrate + ATP + COA  OAA + AcCoA + ADP + Pi Fatty Acid Synthesis o Activation o Carboxylation of acetyl CoA to malonyl CoA o Transfer from CoA to Acyl Carrier Protein (ACP) o Condensation o Acetyl ACP + malonyl ACP → acetoacetyl ACP + CO 2 o Elongation o Reduction, dehydration, reduction o Acetyl-CoA carboxylase catalyzes the first commited step in fatty acid biosynthesis by converting Acetyl-CoA to malonyl CoA Regulation of acetyl-CoA carboxylase o Hormonal control via phosphorylation o inhibited by AMP kinase (AMPK) o inhibited by PKA o glucagon (liver) o adrenalin (adipocytes) o enhanced by insulin via activation of Protein Phosphatase 2A o Allosteric regulation o citrate activates phosphophorylated form o inhibited by palmitoyl CoA Fatty Acid Synthase o KS = b-ketoacyl-ACP Synthase o MAT = Malonyl/Acetyl-CoA-ACP Transacylase o DH = b-hydroxyacyl-ACP dehydrase o ER = Enoyl-ACP reductase o KR = b-ketoacyl-ACP reductase o TE = Palmitoyl Thioesterase Fatty acid elongation and desaturation o FA synthase can only make 16 – 18 carbon chains o Elongation in mitochondria or ER (elongases) o Addition of 2 carbon units from acetyl CoA (Mito.) or malonyl CoA (ER) o Palmitate (16C) or Stearate (18C) o Desaturation at ER membrane o Accessory enzymes (desaturases) o Eukaryotes add double bond to middle of the chain - 2eed O to do it o Eukaryotes – unsaturated bond up to C15 from carboxyl end o Mammals – only up to C9 o Linoleate (18:2), linolenate (18:3) essential in diet Clinical conditions releated to FA Biosynthesis o No known diseases resulting from deficiencies of enzymes o Essential FA Deficiency o Deficiency of w6 and w3 FA in diet  required for synthesis of 20 and 22 carbon polyunsaturated FA o w6 and w3 are abundant in fish and fish oils, vegetable oils, nuts and seeds o Symptoms include skin rash, hair loss (alopecia), disturbance in sleeping patterns, poor night vision, sensitivity to bright light etc. o Rare because of widespread availability and use of appropriate supplements Synthesis of Complex Lipids o Very little free FA in the cell o Activated by addition of CoA o Esterified into diacyl & triacyl glycerols, glycerophospholipids, sphingolipids o Phosphatidic acid (from glycerol or DHAP) is precursor molecule o Sphingolipids and triacylglycerols only made in eukaryotes o Formation of Eicosanoids o Formation of Cholesterol Cholesterol o Large hydrophobic molecule o Controls membrane fluidity in eukaryotes o Metabolic precursor to steroid hormones. o The majority of serum-derived cholesterol comes from de novo biosynthesis in the liver, not from dietary cholesterol. Therefore, understanding cholesterol biosynthesis in relation to cardiovascular disease has been
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