Self Study: Fatty Acid Metabolism - Abali Flashcards
fatty acid structure
- hydrocarbon chain with terminal carboxylyl group (-COOH, ionized at pH 7)
- bonds determine saturation
- all single bonds = saturated
- 1 or more double bonds = unsaturated (usually cis)
essential fatty acids
- why “essential”
- type (omega…)
- fx
- sources
WHY ESSENTIAL?
mammals can’t introduce double bonds beyond C9, so can’t make either one
linoleic acid 18:2(9,12)
- omega 6
- pro-inflammatory
linolenic acid 18:3(9,12,15)
- omega 3
- anti-inflammatory
arachidonate acid 20:4(5,8,11,14)
- omega 6
- synth’d from linolenic
- prostaglandin precursor
sources: SMASH (salmon, mackerel, albacore, sardines, halibut)
fatty acid synthesis : role of the citrate shuttle
FA synthesis takes place in cytosol of liver and adipose cells
problem: main ingredient (acetyl CoA, from glycolysis or alcohol metab) is in the mito matrix, and the inner mito membrane is v selective
solution: citrate shuttle!
- in mito matrix: acetyl CoA + OAA → citrate [citrate synthase]
- transport across inner membrane via citrate shuttle
- in cytosol: citrate → acetyl CoA + OAA [ATP-citrate lyase]
summary: citrate shuttle allows for acetyl CoA to get from mito matrix (where it’s synthesized) to cytosol (where it’s needed for FA synthesis)
fatty acid synthesis: malonyl CoA formation
- enzyme/cofactor
- regulation of enzyme
acetyl CoA → malonyl CoA [acetyl CoA carboxylase; biotin cofactor]
COMMITTED RXN
acetyl CoA carboxylase is an ABC carboxylase
- ATP (plenty in fed state), biotin required
- dimer when inactive, polymer when active
allosteric regulation
+ : citrate
- : long chain fatty acyl CoA
hormonal regulation
+ : insulin [dephos via protein phosphatase]
- : glucagon, epi [phos via AMP-dep kinase]
fatty acid synthesis: palmitate formation
chain elongation via fatty acid synthase, eventual synth of palmitate
- multi-enzyme complex : condensation, reduction, dehydration, reduction activity
- 2 reductions = 2 NADPH consumed as chain extended by 2 Cs
- also need pantothenic acid/B5 for fatty acid synthase
final pdt: 16C palmitate/palmitoyl CoA/palmitic acid
fatty acid synthesis: fates of palmitate
can be either…
elongated (mitochondria, ER)
- 2 C elongation
- stearate 18:0 is most common pdt
desaturated (ER)
- via fatty acyl CoA desaturase, using NADPH as reducing agent
- reduces bond b/w C9 and C10
summary: FA synthesis
acetyl CoA [transported from mito matrix to cytosol via citrate shuttle]
→ malonyl CoA [via acetyl CoA carboxylase; requires ATP, biotin]
→ palmitate [via fatty acid synthase; requires NADPH]
fatty acid synthesis: diabetics
lack of insulin or insulin-resistance means no activation of acetyl CoA carboxylase
[insulin also upregs malonyl CoA → palmitate]
can’t turn acetyl CoA → malonyl CoA!
- diminished FA synth
- acetyl CoA → ketone body production
what happens to FAs in healthy individuals?
triacylglycerol synthesis
TAG : glycerol + 3 FAs (can be diff lengths, sat)
- need glycerol phosphate, derived from DHAP made in glycolysis
- need FAs made in liver, adipose tissue
steps of synthesis:
- DHAP → glycerol 3 P [glycerol3P DH]
- esterification rxns
- addition of acyl groups to glycerol backbone [3 acyltransferases]
sites of TAG synthesis
- 2 pathways of TAG synthesis
liver is main site of TAG synthesis
adipose tissue also contributes
2 pathways of TAG synthesis boil down to two ways to make glycerol3P
1. glycolysis intermeds: glucose → DHAP → glycerol3P [glycerolP DH]
- liver
- adipose tissue (regulated by glucose availability, mediated by GLUT4, which is insulin dep - no glucose, no insulin → no glycerol3P, no TAG synth in adipose tissue]
2. free glycerol → glycerol3P [glycerol kinase}
- liver only
sites of TAG storage
- role of glycerol phosphate
only adipose tissue can store TAGs
- most TAG synth happens in liver → packaged and shipped into circ in VLDLs → TAGs degraded into glycerol and FAs by endothelial cell lipoprotein lipase (LPL)
- adipose tissue picks up FAs, does not pick up glycerol (bc it doesn’t have glycerol kinase)
- FAs packaged back into TAGs in adipose cells
- glycerol that was not picked up heads back to liver and is re-P’d by glycerol kinase to recycle into TAG synth
regulation of TAG synthesis
fed state: insulin upregs glycolysis and LPL
- glycolysis: accumulation of acetyl CoA and glycerol3P
- acetyl CoA → FAs : FAs + glycerol3P → TAGs
- LPL: efficient release/uptake of free FAs by adipose tissue
alcohol : impairs VLDL secretion
- alcoholic fatty liver disease!
mobilization of stored fat
adipose tissue
+ : stress hormones (glucagon, epi, cortisol) trigger hormone-sensitive lipase : TAG → glycerol + FAs, both released into bloodstream
liver
+ : glucagon, cortisol upregulate…
- gluconeogenesis
- beta ox FA degradation
- ketogenesis → can’t be used by liver! transported out for use by extrahep tissues
mobilization of TAGs in adipose tissue : role of perilipins
TAGs are coated with perilipins (protein fam)
fx in regulation of basal and hormonally stimulated lipolysis
- basal: restricts access of cytosolic lipases to TAGs → promotes TAG storage
- energy deficit/hormone stimulation: perilipin P’d by PKA → facilitates max lipolysis via HSL (hormone sensitive lipase) and ATGL (adipose triglyceride lipase)
beta oxidation : basics
each cyle of beta ox generates…
- 1 FADH2
- 1 NADH
- 1 acetyl CoA
FAs arrive in cytosol after mobilization from adipose tissue, but have to be transported into mitochondria for beta ox
- carnitine cycle : used for FAs 14C or longer
- carnitine has affinity for activated FAs (over free FAs) - CoA is the activating molecule in this case
carnitine shuttle : components
carnitine shuttle = CPT (carnitine palmitoyltransferase) = CAT (carnitine acyl transferase) is composed of two enzymes:
1. CPT I = CAT I : outer part of inner mito mem
fatty acyl CoA + carnitine → fatty acyl carnitine + free CoA
- fatty acyl carnitine moves into mito matrix through shuttle; CoA hangs out in cytosol
- inhibition: malonyl CoA (int of FA synth) inhibits CPT I → prevents synth/degrad cycling
2. CPT II = CAT II : inner part of inner mito mem
fatty acyl carnitine + CoA → fatty acyl CoA + free carnitine
- CoA is already present in mito matrix; freed carnitine moves out into cytosol via shuttle
- fatty acyl CoA moves on into beta oxidation
carnitine shuttle : mech of action
summary/point: need some way to get FAs into mitochondria for beta ox. carnitine shuttle will do the job, but only if FAs are “activated” by CoA [accomplished by fatty acyl CoA synthetase in cytosol]
problem here: CoA can’t get through the inner mito mem
solution: CPT I/CPT II system that strips CoA/adds carnitine and vice versa
outcome: fatty acyl CoA moved into mito matrix, where beta ox can take place
carnitine shuttle nomenclature reminder
CPT I & II = CAT I & II
carnitine palmitoyl transferase = carnitine acyl transferase
CAT = CACT
carnitine acylcarnitine transferase
beta oxidation : overview
series of rxns involving C3 (beta C), shortens FA chain by 2Cs
each set of rxns produces: 1 NADH, 1 FADH2, acetyl CoA (for TCA cycle) + shortened FA chain
each involves: oxidation, hydration, oxidation, 2C cleavage
- catalyzed by acyl CoA dehydrogenases (diff DHs for diff length chains: short, med, long, v long chain acyl CoA dehydrogenases)
beta oxidation compared to other metab
- energy yield
beta oxidation of palmityl CoA (16C) yields…
- 7 FADH2 = 14 ATP
- 7 NADH = 21 ATP
- 8 acetyl CoA x TCA cycle = 96 ATP
131 ATP
takeaway: beta ox >>> glycolysis
what if a FA is not a long chain FA?
alternative oxidation pathways for…
- unsaturated FAs
- branched chain FAs
- medium and short chain FAs
unsaturated FAs
- yield less FADH2 than saturated FAs
- already partially oxidized, so fewer ox rxns overall
- need addtl enzymes to process
branched chain FAs
- alpha ox → acetyl CoA, propionyl CoA
- clinical: defect in alpha ox can lead to nerve tissue deposits of phytanic acid (branched chain lipids in plant chlorophylls)
medium/short chain FAs
- can get into mito matrix without carnitine shuttle
- need specific DHs for beta ox
oxidation of FAs with odd numbers of C
beta ox of odd-numbered FAs goes on until you end up with a final 3C FA
- yields acetyl CoA, NADH, FADH2, and a propionyl CoA
propionyl CoA (3C) → methylmalonyl CoA (4C) [propionyl CoA carboxylase; ABC carboxylase]
methylmalonyl CoA → succinyl CoA [methylmalonyl CoA mutase; requires B12*** - links to signs of B12 def]
succinyl CoA → energy via TCA cycle or shuttled into gluconeogenesis!
- only odd chain FAs are glucuneogenic!
B12 deficiency and methylmalonyl CoA
- identifying B12 def
- explaining symptoms of B12 def
conversion of methylmalonyl CoA → succinyl CoA requires B12 (and only B12; not B9)
- B12 def → methylmalonic acid buildup!
- can be used to distinguish B12 def from folate def
methylmalonyl CoA is analogous to malonyl CoA (made in committed step of FA synth)
- in B12 def, built up methylmalonic acid begins subbing in for malonyl CoA → branched chain FAs
- if integrated into membranes of nervous tissue, interferes with tissue integrity : neuropathy!
carnitine shuttle defects
pathophysio
- 1: congenital CAT I deficiency
- 2: low dietary intake of carnitine
symptoms
- muscle pain/fatigue following exercise (inability to utilize FAs for energy after glycogen stores depleted)
- high FA conc in blood (inability to utilize, so mobilized FAs stay in blood)
- hypoketotic hypoglycemia (cant produce ketone bodies without FA metab!)
tx
- high carb diet supplemented with medium and short chain FAs
CAT I/CPT I deficiency
- relatively rare
- affects primarily liver
- leads to reduced FA oxidation and ketogenesis
symptoms
- most common: hypoketotic hypoglycemia
- elevation in blood carnitine
- hepatomegaly (liver), weakness (muscles)
CAT II/CPT II deficiency
3 main forms
1. adult myopathic form : muscle pain, fatigue, myoglobinuria after exercise
2. severe infantile multisystem form : first 6-24 months of life
- hypoketotic hypoglycemia → severe hepatomegaly, cardiomyopathy
3. neonatal lethal form : rarest, sx hrs-4d after birth
- resp failure, hepatomegaly, seizures, hypoglycemia, cardiomegaly → fatal arrythmia
impaired medium chain FA oxidation
MCAD (medium chain acyl CoA DH) deficiency due to auto recessive disease, presents in infancy
characteristic symptom: hypoketotic hypoglycemia
diagnosis:
- plasma/urine buildup of MC carboxylic acids, acyl carnitines, dicarboxylic acids due to omega ox
- possible hyperammonemia due to liver damage
tx : frequent feeding, avoiding fasting, carnitine supplementation
methylmalonyl CoA mutase deficiency
key enzyme in processing odd-number FAs
could present as a result of…
- B12 def
- IF def
- actual enzyme def (rarer than other causes)
results in methylmalonyl aciduria → peripheral neuropathy
tx: B12 supplementation
when and why does ketone body synthesis happen?
in fed state, production of ketone bodies is low
in fasted state, low blood sugar → more beta ox → more acetyl CoA
- acetyl CoA buildup in fasted and starvation state exceed capacity of TCA cycle → utilized in ketogenesis in mito
- heart and sk muscles use ketone bodies for energy; saves glucose for the brain
ketone synthesis vs chol synthesis
v similar to chol synthesis…to a point
site of synthesis
- ketones : mito
- chol : cytosol, ER
key enzyme
- ketones: HMG CoA lyase
- chol: HMG CoA reductase
ketone synthesis: major keys
- key enzymes
- major ketone products
occurs primarily in liver
makes use of mito isoform of HMG CoA synthase
- only found in liver, regulated at transc level
- : fasting, cAMP, FAs
- : feeding, insulin
HMG CoA then cleaved by HMG CoA lyase → acetoacetate, which can be modified into DBhydroxybutyrate, acetone*
ketone bodies as alt fuel
HBhydroxybutyrate → acetoacetate → energy production
*acetone can’t be converted back to acetoacetate; excreted via expiration due to volatility
pathway
acetoacetate → acetoacetyl CoA [CoA transferase/thiphorase*]
- CoA is donated from succinyl CoA
acetoacetyl CoA → 2 acetyl CoA [thiolase]
- acetyl CoA heads into TCA cycle!
*CoA transferase NOT present in liver. why?
- don’t want liver to use it’s own supply as fuel! makes more available to others (brain)
why ketones and not FAs for brain food?
FAs are bound to albumin in plasma : cant cross the blood/brain barrier!
ketones are like transportable equivalents of FAs
regulation of ketone body synth
blood glucose and availability of acetyl CoA dictate ketone body synth
fed state: glucose is broken down via glycolysis, TCA cycle, oxphos; excess is moved into glycogen, HMP, and FA synth
fasting/starvation: different story : no glucose!
- absence of carbs during fasting starvation leads to FA breakdown and buildup of acetyl CoA
- can’t shuttle to TCA cycle bc intermediates (like OAA) will already be tapped for gluconeogenesis
role of OAA in determining whether ketone synthesis happens
acetyl CoA only enters TCA cycle if OAA is available for formation of citrate
when you’re low on carbs, body shuttles OAA into gluconeogenesis
- carb starvation is required before you’ll take the acetyl CoA (that you now CANT combine with OAA to enter TCA cycle) and pump it into ketone synth
plasma concentration as fast occurs
FAs increase approx 3-4 hr post-meal, increase up to 2-3 d of fast
in liver, ketone synth rises with supply of FAs
in blood, ketones rise and continue to rise (prob bc utilization by sk muscle drops)
- after 2-3 d starvation, blood ketone reaches level that allows them to enter brain cells → use as fuel (up to 2/3 of brain egy supply in prolonged fast) reduces brain’s glucose req
- reduction in glucose req spares sk muscle, which is major source of a.a. precursors for gluconeogenesis!
ketone bodies and DKA
insulin deficiency/insensitivity (diabetes) leads to drop in cellular glucose levels
- inability to replenish TCA cycle ints (esp OAA)
- limits ability of mobilized FAs to be utilized by cells through TCA cycle
- acetyl CoA from FA metabolism is shunted into ketogenesis instead
accumulation of ketone bodies → low blood pH → DKA and pathology
