FA Synthesis

Question 1

Acetyl CoA Carboxylase consists of?

Answer 1

• biotin carboxlyase
• transcarboxylase
• Biotin-carboxyl Carrier Protein (BCCP)

Question 2

Acetyl transferred from mitochondrial pool into cytosol during fed-state by means of a malate-citrate shuttle, a process that involves?

Answer 2

• Acetyl-CoA condensing with OAA to form citrate (TCA)
• Citrate crossing mitochondrial membrane into cytosol
• Exported citrate undergoing the reverse rxn forming Acetyl-CoA & OAA, a rxn catabolised by ATP-citrate lyase

Acetyl-CoA cannot be converted into pyruvate or OAA in animals, hence animals cannot convert fat into sugars.

Question 3

FAS Synthase rxn

Answer 3

• located in cytosol
• End-product is palmitic acid C16:0
• pathway essentially same in most organisms
• FAS enzyme has phosphopantetheine-binding domain, referred to as ACP
• Before 1st condensation rxm, 2 thiol groups must be loaded.
• Acetyl CoA is transferred initially to ACP and then to thiol group of ketoacyl-ACP synthase
• Malonyl group from malonyl-CoA transferred to a thiol group of ACP.
• ACP acts as flexible arm involved in building an acyl chain by moving the substrate from one active site to the next in a sequential manner
  
  • Condensation rxn
  • Reduction
  • Dehydration
  • Reduction
• Butyryl-ACP (4C) then enters 2nd round of FA synthesis
• Cycle continues until palmitoyl-ACP is produced
• Thioesterase releases palmitic acid into cytosolic pool
• All C in palmitic acid derived from malonyl-CoA except 2C at methyl-end from Acetyl-CoA

Question 4

CPT-1 inhibition

Answer 4

• Malonyl-CoA has inhibitory effect on CPT-1
• prevents newly synthesized acyl-CoA from undergoing immediate Beta-oxidation

Question 5

Lactating mammary gland FA synthesis?

Answer 5

• MCFA: end-product of FA synthesis in mammary gland, usually C8:0 & C10:0
• mammary thioesterase isoforms specific to MCFA
• secreted in esterified (TAG) as milk fat

Question 6

FA Elongation rxn

Answer 6

• 2 different FA elongation pathways:
  
  • Microsomal pathyway: uses malonyl-CoA
  • Mitochondrial pathway uses Acetyl-CoA

Question 7

Desaturation of FA

Answer 7

• Desaturation of FA in animals & plants occurs by means of aerobic desaturation, whereby 2 H atoms removed from an acyl chain are subsequently used to reduce O2 to H2O via NADPH & other reductants.
• Humans have C9, 6, 5, &4 desaturases but lack enzymes to desaturate beyond delta9 position in the hydrocarbon chain.
  
  • mammals cannot synthesize essential FA linoleate (C18:2) or alpha-linolenate (C18:3)
  • mammals capable of introducing double bonds b/w delta9 position & carboxyl end of hydrocarbon chain.
  • therefore, possible to desaturate oleate further at delta6 position, forming an C18:2 delta6,9 FA
  • All desaturases associated with microsomes
• Stearoyl-CoA (C18:0) can be desaturated to form oleoyl-CoA (C18:1delta9) by means of a complex rxn involving the following microsomal enzymes
  
  • requires delta9 desaturase to make stearic acid
• Stearoyl-CoA desaturase cytochrome b5 reductase
  *

Question 8

Essential FA (EFA) families

Answer 8

• EFA are either membres of n-3 or n6 families
• n-3 families
  
  • alpha-linolenic C18:3delta9,12,15
  • EPA C20:5
  • DHA C22:6
• n-6 family
  
  • Linoleic acid (18:2delta9,12)
  • gamma-Linolenic (18:3delta6,9,12)
  • Arachidonic acid (20:4delta5,8,11,14)

Question 9

Mead acid

Answer 9

• Redundant eicosanoids
• biomarker for EFA deficiency
• when the body is deficient in EFAs. it responds by trying to synthesize “next best thing”, i.e. Mead acid, C20:3

Question 10

DHA C22:6 is a key PUFA in brain tissue

Answer 10

• Not all PUFAs are equally converted to VLCFAs because of the limiting activities of some of the enzymes (i.e. elongase, delta5 & delta6 desaturase)
• humans rely on exogenous sources of EPA & DHA in their diet to supplement the endogenously synthesized FA

Question 11

Regulation of FA synthesis

Answer 11

• Acetyl CoA Carboxylase key regulatory enzymes
  
  • short-term
    
    • Citrtate activates
    • Palmitoyl-CoA inhibits
    • Insulin stimulates phosphatase to activate
    • Glucagon/Epi stimulates PKA to inactivate
  • Long-term
    
    • high carb diet increases expression of Acetyl-CoA Carboxylase (& FAS)