L15: Fatty Acid Synthesis Flashcards
what do cells need to synthesise first before triacylglycerols
need to synthesise fatty acids (e.g: palmitic acid 16C)
what does process begin with
Acetyl-CoA
what kind of process is fatty acid biosynthesis
endergonic & reductive
needs ATP & H: e- reducing power of NADPH
where does f.a synthesis occur
cell cytosol
f.a synthesis: overall
- f.a synthesis activated in times of plenty - when cells have surplus energy. citrate builds up in mitochon
- citrate exits via citrate transporter in inner mito memb & enters cytosol
- citrate in cytosol reconverts to acetyl-CoA & oxaloactetate using ATP citrate lyase (ACL) enz
- acetyl-CoA used for lipid/f.a synthesis
what enzyme catalyses the carboxylation of acetyl-CoA
acetyl-CoA carboxylase
what does acetyl-CoA form when carboxylated with Acetyl-CoA carboxylase
forms 3C malonyl-CoA (the strating point for f.a synthesis)
what does malonyl-CoA do for f.a synthesis
donates 2C acetyl grps to fatty acid synthase enzyme complex, which builds up fatty acid chain 2C at a time in a 4 step process
step 1 fatty acid synthesis
- condensation of activated acyl grp (acetyl grp from acetyl-coa is first acyl grp) and 2 carbons from malonyl-coa
- with elimination of CO2 from malonyl grp
- this extends the acyl chain by 2 carbons
- uses synthase enzyme
step 2 f.a synthesis
- the beta-keto grp reduced to an alcohol - reductase enzyme
step 3 f.a synthesis
- elimination of H2O creates a double bond - dehydratase enzyme
step 4 f.a synthesis
- double bond reduced to form corresponding saturated fatty acyl grp - reductase enzyme
when does chain elongation stop
stops at 16C palmitate
how do the f.as synthesised convert to glycerol
they esterify to glycerol -> TAGs for storage & energy reserve
what is the key regulated enz in f.a synthesis (also rate limiting step )
acetyl-CoA carboxylase (ACC)
what allosterically activates acetyl-CoA carboxylase
high [acetyl-CoA] and [citrate], high energy indicators
what allosterically inhibits acetyl-CoA carboxylase
high [AMP] & [palmitoyl-CoA]
what triggers ACC phosphorylation & inactivation
glucagon/adrenaline - inactivation via PKA
also AMPK inactivates
what triggers ACC dephophorylation & activation
insulin
