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Chapter 25: Fatty Acid Biosynthesis

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Only one stereoisomer is produced: why? NADPH functions similar to NaBH4 in this reaction. * NADPH NADPH is similar to NADH. It has an extra phosphate, ... – PowerPoint PPT presentation

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Title: Chapter 25: Fatty Acid Biosynthesis


1
Chapter 25 Fatty Acid Biosynthesis
  • CH234
  • Bruce A. Hathaway

2
Acetyl Coenzyme A
  • Acetyl Coenzyme A is the starting material for
    fatty acid biosynthesis.
  • It is derived from metabolism of carbohydrates,
    fats and some amino acids.
  • The two-carbon piece provides the carbons for the
    new fatty acid.

Acetyl Coenzyme A
3
Acetyl Coenzyme A (AcCoA)
4
Overview of Metabolism And Energy Production
5
Step 1 Preparation of Malonyl CoA
  • Acetyl CoA reacts with bicarbonate to form
    Malonyl CoA.
  • This is energy-requiring, so hydrolysis of an ATP
    is used to drive the reaction.
  • Malonyl CoA is more reactive than Acetyl CoA.

6
Step 2 Transthioesterifaction
  • Both Acetyl CoA and Malonyl CoA are connected to
    Acetyl Carrier Protein (ACP), which is another
    thiol.
  • One thiol replaces another to make a different
    thioester.
  • ACP shuttles the molecules to the different
    enzymes needed to do the following reactions.

7
Step 3 C-C Bond Formation
  • Malonyl ACP loses CO2 to make a carbon anion.
  • An acetyl ACP reacts with the carbon anion to
    give a four-carbon ß-ketoester.

8
Step 4 Reduction of the Ketone
  • The ketone is reduced to an alcohol.
  • Only one stereoisomer is produced why?
  • NADPH functions similar to NaBH4 in this reaction.

9
NADPH
  • NADPH is similar to NADH.
  • It has an extra phosphate, while NADH has an H.
  • Metabolism uses NADH.
  • Biosynthesis uses NADPH.

10
Reduction of the Ketone with NADPH
  • NADPH donates a hydride H- to reduce the ketone
    the H comes from water, ultimately.
  • Only one enantiomer is formed.

11
Step 5 Dehydration of the Alcohol
  • The alcohol is specifically dehydrated to the
    trans alkene.
  • Again, only one isomer is formed.

12
Step 6 Reduction of the Alkene
  • NADPH H reduces the CC to an alkane.
  • We have now made a four-carbon fatty acid.

13
Elongation of Fatty Acids
  • The Butyroyl-ACP replaces the Acetyl-ACP in Step
    3, to make a 6-carbon ketoacid.

14
Elongation of Fatty Acids
  • The 6-carbon keto-acid is reduced, dehydrated,
    and reduced again (steps 4-6), to make the
    six-carbon fatty acid.
  • This can react with more malonyl-ACP

15
Elongation of Fatty Acids
  • The cycle of steps 3-6 is repeated five more
    times until the 16-carbon fatty acid is made.
  • For some reason, this enzyme complex only makes
    palmitic acid.
  • Longer fatty acids and unsaturated fatty acids
    are synthesized in other locations.

16
Energy Requirements
  • To add two carbons to Acetyl CoA, it requires 1
    ATP (to make a malonyl CoA) and 2 NADPHs.
  • Each NADPH is the equivalent to 2.5 ATPs.
  • Therefore, it takes the equivalent of 6 ATPs to
    add two carbons to Acetyl CoA.
  • To make a 12-carbon fatty acid, lauric acid,
    requires five cycles, or 30 ATPs.
  • This compares to the 20 ATPs we get from
    metabolizing lauric acid to Acetyl CoA.
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