26/29: LIPID METABOLISM Flashcards
lipids
- major components of cell membranes
- principal form of stored energy in most organisms
- some form hormones
- some anchor some proteins to membranes
- fats stored in triglycerides (3 FAs in ester linkage to glycerol)
- common FAs: palmitate C16:0; stereate C18:0; oleate C18:1
why store fats as energy?
- much better source of energy than carbohydrates; because FAs are more reduced
- v. non polar; stored in anhydrous form so can be stored at v.high density
- stored fat ~12 weeks energy
- FAs used as energy source by most tissues; not brain/reed blood cells
- triglycerides stored in adipose tissue; made of adipocytes, formed droplets in cytoplasm
Degrading triglycerides
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- Mobilisation - of FAs from triglycerides in adipose tissue
- Transport - to other tissues in blood by serum albumin
- Activation - of FAs by reaction w/CoA
- Transport - into mit.matrix (from cyt), requires conjugation to carnitine
- ß Oxidation - produces acetyl coa
- Final Oxidation - of acetyl CoA in TCA cycle
Mobilisation and Transport
- released of FAs from TGs in adipose tissue controlled by glucagon and adrenaline
- activates production of cAMP, activates PKA, activates triglycerol lipase which can hydrolysed off the first FA of the triglyceride by breaking ester bond
- other lipase take off other 2 FAs
- FAs can then diffuse through membrane and bind to serum albumin (which has several hydrophobic pockets)
Activation w/CoA and Carnitine Conjugation
-thioester forms CoA (reactive thiol) + carb.acid on FA
-acetyl CoA always used as carrier of acyl groups
-2 step reaction catalysed by acyl coA synthetase (requires ATP)
1. FA + ATP; carboxylate displaces outer 2-P of ATP; release Ppi; hydrolysed to 2 Pi; adds adenylate group on FA = acyl adenylate intermediate
2. SH Nu attack displaces adenylate group; end up w/acyl CoA + AMP (uses equivalent of 2ATP)
3. carnitine and acyl CoA transesterification reaction = acyl-carnitine; needed for transport into mit.matrix
-2 enzymes involved:
carnitine acyltransferase I (outer mit. member) swaps CoA for carnitine;
II catalyses reaction where carnitine is taken off and CoA is put back on; end up w/acyl CoA in mit.matrix; needed for transport of long chain FAs
ß oxidation
- starts w/acyl coA
- initial oxidation of ßC for reactions; releases acetyl coA and a shorter acyl coa (then whole process repeats)
- OXIDATION - by acyl coa DH; 3 isoenzymes specific for diff. chain lengths; creates double bond; uses FAD; FADH2 produced feeds into ETC to generate ATP (passes to ETF protein, then e go to Q then Q cycle)
- HYDRATION -add water across double bond to get OH
- OXIDATION - by NAD = NADH into ETC; oxidise OH on ßC to form keto group
- THIOLYTIC CLEAVAGE - CoA comes in and using SH removes 2C and adds CoA to them
- after first set of 4 reactions, have a C14 acyl Coa and release 2C as acetyl coa (further ox. in TCA)
- for palmitate, need another 6 cycles of 4 reactions (7 cycles total = 8 acetyl coA)
- same process just diff. # of cycles for diff. FA length
odd # ß oxidation
- animals can’t synthesize odd #C, but bacteria can
- in last cycle, end up w/ 1x acetyl coa and 1x propionyl CoA
- first carboxylation uses bicarbonate and ATP; enzyme is propionyl-coa carboxylase; uses biotin as cofactor
- gives D-methyl-malonyl CoA and epimerase enzyme converts to L-methyl-malonly-CoA then mutase to succinyl coa (into TCA)
where are FAs oxidised?
- main place is mitochondrial matrix
- peroxisomes take rare v.long chain FAs and can degrade them to smaller chains which then go to mitochondria (don’t do ß oxidation; use different pathway which don’t generate ATP)
control of FA degradation
- hormonal control of triglyceride lipase
- FAs only released if glu is low (stimulated by glucagon) or if there’s potential need for energy (adrenaline); insulin inhibits release - transport into mitochondria
- enzyme carnitine acyl transferase I is regulated
- inhibited by malonyl CoA
- lots of malonyl CoA indicates high [acetyl coA] so don’t need to produce more by oxidising FAs
lipid synthesis overview
- starts w/acetyl coa
- chemical reverse of FA degradation but diff. set of enzymes; occurs in cytoplasm
- start w/ reaction cat. by acetyl coa carboxylase; adds CO2 from bicarbonate to acetyl coa = malonyl coa (committed step)
- acetyl CoA can go into TCA cycle for oxidation, can be used to make AAs, cholesterol, ketone bodies, etc
fatty acid synthesis (carboxylase reaction)
1) ACETYL COA CARBOXYLASE REACTION
- same mech as pyruvate carboxylase reaction in gluconeogen
- uses biotin as cofactor (carrier of CO2)
- enzyme has 2 activities; 1. CO2 activated by attachment to N of biotin - needs ATP hydrolysis; 2. activated CO2 transferred onto acetyl CoA = malonyl CoA
- intermediates in FA synthesis are attached to acyl carrier protein (ACP)
- difference of ACP is that it does not have nucleotide; instead is attached to Ser residue on protein
fatty acid synthesis (elongation phase)
2) ELONGATION PHASE OF FA SYNTHESIS
- starts w/formation of acetyl-ACP and malonyl-ACP
- mammals: ACP part of FA synthase complex (big multifunctional enzyme complex that carries out all FA synth reactions)
- malonyl CoA-ACP and acetyl-CoA-ACP transferase swap CoA for ACP
- condensation (and decarboxylation)
- gives acetoacetyl-ACP
- CO2 released is the one that came from bicarbonate in acetyl coA carboxylase reaction; all C in FA comes from acetyl coA
- malonyl coa acts as activated donor of 2C units; decarboxylation reaction drives condensation reaction w/acetyl-ACP - reduction
- use NADPH as e donor for reductive biosynthesis
- keto group reduced to OH - dehydration
- release water creating double bond - reduction
- use NADPH
- get rid of C=C as 2H are added to fully saturate
- have 4C FA = butyryl-ACP
cycle repeats:
-butyryl-COA + malonyl COA = 6C; add 2C in every round of 4 reactions
- longest FA can synthesise is palmitate C16; then hydrolysis reaction which releases palmitate from ACP
- to get shorter FAs, hydrolysis occurs earlier
- add 2C so mammals cannot synthesise odd # C
- bacteria use propionyl-CoA as substrate in first reaction
synthesis of longer C16 FAs
- C16 palmitate can be acted on by elongase enzyme to give stereate; that can be elongated further
- elongase enzymes are on cytoplasmic face of ER
- desaturase enzyme introduces double bonds = monounsaturated FAs
- mammals can’t synthesise polyunsaturated FAs (have to come from diet); plants can synthesise them; linoleate is important (C18:2) to synthesise prostaglandins and thromboxins important in inflammatory response of immune system (essential FAs)
citrate cycle
- citrate acts as carrier of acetyl groups out of the mit. membr
- citrate exchanges for malate
- citrate can be broken down by citrate lyase in the cytoplasm; using hydrolysis of ATP
- in mitochondria, have citrate synthase
- malic enzyme oxidises malate to pyruvate using NADP to make NADPH (can be used in FA synthesis; rest comes from pentose-P pathway); releases CO2
control of FA synthesis by acetyl coA carboxylase
Control by phosphorylation:
- P is inactive (by protein kinase); AMP-K; AMP-dependent kinase; activate AMP; inhibited ATP; responds to energy charge; glucagon/adrenaline activate P
- deP is active (by phosphoprotein phosphatase, PP2); insulin activates deP
Allosteric control:
- phosphorylated form can be allosterically activated by citrate = partially active so can start to catalyse formation of malonyl coA to allow FA synthesis to start
- [citrate] high = TCA cycle slow = [ATP] high so use acetyl CoA for FA synthesis rather than oxidising it; effect of citrate inhibited by palmityl CoA
