BCH 3120 Lecture Notes - Lecture 10: Citric Acid, Malic Acid, Lyase
28 Apr 2018
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February 6, 2017
Fatty Acid Biosynthesis
Fatty Acid Metabolism
Acetyl-CoA is a key building block
Occurs outside the mitochondria - acetyl-CoA must be transported out
Typically makes C16:0 fatty acids
β-oxidation: fatty acid catabolism in mitochondria
-Makes acetyl-CoA from fatty acids
Major fatty acid source: diet
-However, any tissues can synthesize fatty aids (ex. Liver, adipose tissues, mammary glands)
Transport of Acetyl-CoA from Mitochondria
Acetyl-CoA will combine with oxaloacetate to form citrate
Citrate can be transported out of the cell by the citrate shuttle
-Antiport with malate
-Citrate has a -4 charge, malate has a -3 charge - citrate will be pumped out with an H+
Once in the cytosol, citrate will be broken back down to acetyl-CoA and oxaloacetate by citrate lyase
-Needs ATP and HSCoA
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February 6, 2017
-Will go through a CoA ester intermediate, which will then go through a retroaldol reaction
Oxaloacetate will be transported back into the mitochondria through the malate aspartate shuttle by producing malate
-Can also use malic enzyme to convert malate to pyruvate
•Pryuvate will be transported into the cell
•Generates NADPH to be used in fatty acid biosynthesis
Saturated Fatty Acid Biosynthesis
Net balanced equation:
-Makes C16 palmitate and requires 8 acetyl-CoA (16 C total)
-Requires lots of reducing power - lots of NADPH
Comprises stepwise additions of two-carbon units
Enzyme responsible: fatty acid synthase (FAS) - mainly uses malonyl-CoA (acetyl-CoA derivative)
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Malonyl-coA is generated by Acetyl-CoA carboxylase (ACC)
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-Requires ATP
Acetyl-CoA Carboxylase (ACC)
Cytosolic enzyme
First dedicated step, key limiting step
Requires biotin co-factor, HCO3-, and ATP
Very exergonic, essentially irreversible
Allosterically regulated:
-Inhibited by long chain fatty acids - feedback inhibition
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-Activated by citrate - being pumped out of the mitochondria to form acetyl-CoA (feedforward control)
•Want to store the extra acetyl-CoA as fatty acids
Covalent control: inactivated by phosphorylation
-Regulated by AMPK: cAMP-independent kinase
-However, glucagon can still control this process a little via phosphatase inhibitor-1 - stimulates phosphorylation of
ACC
-Can still be regulated by insulin: stimulates dephosphorylation of ACC
Fatty Acid Synthase (FAS)
Massive protein with multiple domains
-KS, MAT catalytic domains: elongate fatty acid
-KR, DH, ER catalytic domains: reduce fatty acids
-Also has LD, ΨME, 4KR structural domains
-ACP catalytic domain moves throughout the protein to present the fatty acid to the various catalytic domain
Dimerizes
~2.5 million Da
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Document Summary
Occurs outside the mitochondria - acetyl-coa must be transported out. However, any tissues can synthesize fatty aids (ex. Acetyl-coa will combine with oxaloacetate to form citrate. Citrate can be transported out of the cell by the citrate shuttle. Citrate has a -4 charge, malate has a -3 charge - citrate will be pumped out with an h+ Once in the cytosol, citrate will be broken back down to acetyl-coa and oxaloacetate by citrate lyase. Will go through a coa ester intermediate, which will then go through a retroaldol reaction. Oxaloacetate will be transported back into the mitochondria through the malate aspartate shuttle by producing malate. Can also use malic enzyme to convert malate to pyruvate: pryuvate will be transported into the cell, generates nadph to be used in fatty acid biosynthesis. Makes c16 palmitate and requires 8 acetyl-coa (16 c total) Requires lots of reducing power - lots of nadph.
