Unit 6 - FA Storage, Mobilization, Oxidation Flashcards
how is fat storage promoted by insulin?
- adipocytes release lipoprotein lipase in response to insulin
- FA unloaded from chylomicrons and VLDL, and taken up by adipocytes
- insulin promotes uptake of both glucose (GLUT4) and FA (hormone sensitive lipase) into adipocytes (via translocation of transport PRO from vesicles to plasma membrane)
- glucose generates, via glycolysis, the glycerol backbone needed to synthesize TG
thus, fat cells have everything they need to store TG
how is NEFA converted to fatty acyl-CoA?
thiokinase (acyl-CoA synthetase)
what happens to acyl groups for storage?
acyl groups transferred from fatty acyl-CoA to glycerol-3-phosphate catalyzed by acyl transferases
-makes phosphatidic acid, then dephosphorylated to diacylglycerol before another group makes TG
what 2 enzymes are needed to esterify an NEFA to a glycerol backbone?
thiokinase (acyl-CoA synthetase) and acyl transferase
what is de-esterification and what is it involved in? how is it achieved?
hydrolytic release of FA from TG (also called lipolysis)
- involved in fat mobilization
- done via esterases
what does hormone-sensitive lipase (HSL) do? what activates it? where is it found? what happens if deficient in HSL and what does this mean?
release FA preferentially from DG and MG (mainly responsible for 2nd and 3rd hydrolysis steps to make NEFA and glycerol)
- broad specificity releases FA more slowly from TG, retinyl esters, and cholesteryl esters
- activated by catecholamines and glucagon (becomes phosphorylated by cAMP-dependent PRO kinase)
- found in adipocytes and cells that make steroids from CE
- if deficient, are lean and can mobilize NEFA from fat stores, showing that another enzyme is rate-limiting
what does adipose triglyceride lipase do? where is it found? what happens if deficiency?
catalyzes rate-limiting step in lipolysis
-catalyzes first step in hidrolysis (TG –> DG)
-found in many tissues that accumulate TG, not just adipocytes
-if deficient, become mildly obese, accumulating TG in many tissues (including cardiac muscle)
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what NEFA do to HSL?
inhibit via product inhibition
-unclear significance, b/c ALBPrapidly shuttles mobilized FA to cell surface, where loaded onto serum albumin
what is the key regulatory event in lipolysis?
parallel phosphorylation of perilipid protein via cAMP-dependent PRO kinase
-more important than phopshorylation of HSL
what are the types of lipid storage droplets in mature adipocytes?
- prominent, centrally located, large droplet
- much smaller, peripheral droplets interposed between central droplet and plasma membrane
- -metabolically active in lipolysis
what do perilipins do?
prevent access of HSL and other lipases to peripheral fat droplets
-phosphorylated by cAMP-dependent PRO kinase disrupts the sheet, allowing lipases to get at TG
what does lipotransin do?
PRO on droplet surface with perilipin
-helps HSL “dock” to droplet surface
what happens to glycerol in TG?
not much glycerol kinase, so glycerol made by hydrolysis leaves the cell via AQPad (aquaporin adipose)
what happens in adipocytes under fasting conditions?
upregulate synthesis of GAP from lactate, pyruvate, and AA to enable appreciable resynthesis of TG as lipolysis occurs
-“energy-wasting” might modulate rate of FA release from adipocytes
what is the “delta” nomenclature used for? how does it work?
enzymes that desaturate, elongate, and oxidize FA
- A:B delta C,D
- A is number of C atoms, B is number of DB, and C and D are where DB are, counting carboxyl C as number 1
how does the “omega” nomenclature work?
A:B omega-C
-A is number of C atoms, B is number of DB, and C is the first DB numbered from methyl C (farthest C from carboxyl group)
are palmitoleic, oleic, linoleic, and linolenic essential or nonessential? what are their omega designations?
palmitoleic (16:1 omega-7) and oleic (18:1 omega-9) are nonessential b/c can be made in body
linoleic (18:2 omega-6) and linolenic (18:3 omega-3) are essential
what is the overall reaction of beta-oxidation? how much free energy is conserved as ATP?
CH3 - (CH2)n - COO- –> CO2 + H2O + ATP + heat
about 40% of free energy is conserved as ATP
what are the 5 phases of beta-oxidation that ultimately make ATP?
- activation of FA to fatty acyl-CoA
- entry of acyl-CoA into mitochondria
- beta-oxidation to make acetyl-CoA, FADH2, NADH
- oxidation of acetyl-CoA thru TCA
- utilization of ETC to make ATP
other than mitochondrial beta-oxidation pathways, what 3 other pathways of FA oxidation exist? what kind of roles dot hey have?
they have important regulatory roles, despite minor CO2 production
- beta-oxidation of VLCFA and PUFAs in peroxisomes
- shortened FA are further oxidized in mitochondria - alpha-oxidation of branched FA in mitochondria
- omega-oxidation in ER
what step is activation of fatty acids, and what happens during it? what is overall delta G?
first step
- E-dependent step catalyzed by thiokinase, with enzyme-bound acyl-adenylate (mixed anhydride) intermediate
- overall standard free energy is near zero, and driven to completion by coupled pyrophosphatases
- overall E cost is 2 ATP
what is the second step of beta-oxidation comprised of?
need to transfer activated FA to matrix
2a. transfer LCFA moiety from CoA to carnitine
- carnitine palmitoyl transferase makes acyl-carnitine
2b: acyl-carnitine is transported across IMM and acyl moiety is transfered back to CoA
- -ACoA is regenerated at inner face of IMM
what is the third step of beta-oxidation? why is this a major control point in metabolism?
ACoA enters beta-oxidation pathway
- MCFA diffuse thru mitochondrial membranes, and activated by distinct thiokinase in matrix
- major control point in metabolism b/c:
- -CPT1 is inhibited by malonyl-CoA (product of first committed step in FA synthesis)
- -mCoA production itself is carefully regulated
- -so when FA are synthesized, they aren’t being used as fuel, and vice-versa
what are the 4 enzymatic reactions in mitochondrial beta-oxidation? what enzymes are used?
- formation of trans-alpha,beta DB thru dehydrogenation by flavoenzyme acyl-CoA dehydrogenase
- e- from reduced flavin are passed down ETC to make 2 ATP per FADH2 - hydration of DB to make 3-hydroxyacyl-CoA (AKA beta-hydroxyacyl-CoA)
- dehydrogenation of beta-hydroxyacyl-CoA to make beta-ketoacyl-CoA
- each NADH makes 3 ATP - thiolysis with CoA-SH makes ACoA and new acyl-CoA with 2 fewer atoms than original
single trifunctional mitochondrial enzyme does the last 3 steps
what does deficiency in medium-chain acyl-CoA dehydrogenase do?
rare genetic disorder of fat metabolism that is fatal if unrecognized, but treatable if detected in time
- autosomal, more in Caucasians of north Europe (most common inborn error of fat metab, accounting for 1% of SIDS)
- octanoylcarnithine that accumulates in homozygotes is toxic
- no symptoms until hypoglycemia occurs, then rapid progression
- 90% of all individuals have same K304E mutation, so screening of newborns is feasible
- treat by avoiding fasting, and staying on high CHO, fat-restricted diets
how much ATP is made per C6 oxidized in glycolysis, and C16 oxidized in beta-oxidation?
36 ATP for glycolysis, meaning 6 ATP per C
129 ATP for B-oxidation, meaning 8 ATP per C
-this means FA yield about 15 kcal/mol/C of additional E
what problems do unsaturated FA pose to beta-oxidation?
at second step of beta-oxidation, must deal with 2 DB:
- beta-gamma DB and delta-4 DB
- handled by separate enzymes for complete oxidation to occur
how is the problem of a beta-gamma DB dealt with? what is the additional E cost?
enoyl-CoA isomerase converts the cis-delta-3 DB to a trans-delta-2 DB for hydratase to continue
-no requirements for ATP or reducing equivalents, so no additional E cost, but it “reduces” the net E yield by 1 FADH2 = 2 ATP
how is the problem of delta-4 DB dealt with? what is the additional E cost?
reduced by NADPH-dependent reductase to trans-delta 3 DB, then converted to trans-delta-2 DB by isomerase
-“reduces” E made by 1 NADPH = 1 NADH = 3 ATP
how does beta-oxidation of odd-chain FA occur?
proceeds normally until final step, making propionyl CoA (3C) instead of 2 ACoA, so requires 3-part process
- PCoA is carboxylated with PCoA carboxylase + biotin to make S-methylmalonyl-CoA, which is then isomerized to succinyl-CoA in 2 step reaction with B12
- SCoA converted to malate and exported to cytoplasm, then to pyruvate via malic enzyme
- pyruvate oxidized by pyruvate dehydrogenase and TCA cycle
how is ketone body metabolism?
made by liver during fasting and starvation
-production and export of ketone bodies from ACoA allows for continued beta-oxidation of FA by liver, with only minimal oxidation in liver itself for ACoA made
what are the compounds referred to as ketone bodies/acids?
- acetoacetate (primary product)
- beta-hydroxymutyrate (reduced acetoacetate)
- acetone (made by spontaneous decarboxylation of acetoacetate, and exhaled)
where does ketogenesis occur?
in liver mitochondria, and KB are released into plasma
- acetoacetate and beta-hydroxybutyrate used as fuel by many tissues, including brain
- during starvation, up to 75% of E requirements of brain are met by these KB
- KB are preferred substrate by heart and kidneys (water-soluble equivalents of FA)
when will ACoA enter TCA cycle?
ACoA made in FA metabolism enters TCA only if fat and CHO degradation are appropriately balanced
-high ACoA from beta-oxidation inhibits pyruvate dehydrogenase, and activates pyruvate carboxylase, increasing OAA levels
how is OAA diverted for glucose synthesis?
if glycolysis is deficient
- high levels of NADH in mitochondria from beta-oxidation inhibit isocitrate dehydrogenase, and citrate accumulates
- inhibits production of citrate from pyruvate and OAA
- high levels of NADH promote OAA –> malate
- malate leaves mitochondria for gluconeogenesis
what happens to excess ACoA from beta-oxidation?
not able to enter TCA cycle, so turned into ketone bodies
-happens during fasting, heavy alcohol consumption, high fat/low CHO diets, uncontrolled diabetes
why can’t liver use ketone bodies for fuel?
key enzyme (acetoacetate:succinyl-CoA transferase, or 3-ketoacyl CoA transferase) isn’t present
how are ketone bodies metabolized by extrahepatic tissues?
- beta-hydroxybutyrate converted to acetoacetate, which is transfered to CoA of SCoA
- acetoacetyl-CoA acted on by thiolase for beta-oxidation
- need 1 GTP to remake SCoA from succinate
- since OAA not diverted for gluconeogenesis in peripheral tissues, ACoA enters TCA cycle
why can’t extrahepatic tissues make ketone bodies?
- mitochondria lack high levels of HMG-CoA synthase in liver mitochondria
- non-liver mitochondria lack HMG-CoA lyase altogether
why does DM1 cause ketone overproduction?
disorder of FA mobilization and oxidation, b/c if no insulin and high stress hormones, runaway lipolysis occurs
- FA mobilized from TG stores in adipocytes are converted by liver to ketone bodies at rate that exceeds ability of other tissues to oxidize and excrete KB
- KBs accumulate in blood, causing metabolic acidosis (diabetic ketoacetosis)
why doesn’t DM2 cause ketone overproduction?
continued presence of insulin prevents uncontrolled lipolysis, so life-threatening ketoacidosis doesn’t occur
-KB blood levels elevated if starvation (ketosis), but still not life-threatening
