Self Study: Fatty Acid Metabolism - Abali

Question 1

fatty acid structure

Answer 1

• 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)

Question 2

essential fatty acids

• why “essential”
• type (omega…)
• fx
• sources

Answer 2

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)

Question 3

fatty acid synthesis : role of the citrate shuttle

Answer 3

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)

Question 4

fatty acid synthesis: malonyl CoA formation

• enzyme/cofactor
• regulation of enzyme

Answer 4

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]

Question 5

fatty acid synthesis: palmitate formation

Answer 5

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

Question 6

fatty acid synthesis: fates of palmitate

Answer 6

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

Question 7

summary: FA synthesis

Answer 7

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]

Question 8

fatty acid synthesis: diabetics

Answer 8

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

Question 9

what happens to FAs in healthy individuals?

Answer 9

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:

1. DHAP → glycerol 3 P [glycerol3P DH]
2. esterification rxns
3. addition of acyl groups to glycerol backbone [3 acyltransferases]

Question 10

sites of TAG synthesis

• 2 pathways of TAG synthesis

Answer 10

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

Question 11

sites of TAG storage

• role of glycerol phosphate

Answer 11

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

Question 12

regulation of TAG synthesis

Answer 12

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!

Question 13

mobilization of stored fat

Answer 13

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

Question 14

mobilization of TAGs in adipose tissue : role of perilipins

Answer 14

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)

Question 15

beta oxidation : basics

Answer 15

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

Question 16

carnitine shuttle : components

Answer 16

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

Question 17

carnitine shuttle : mech of action

Answer 17

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

Question 18

carnitine shuttle nomenclature reminder

CPT I & II = CAT I & II

carnitine palmitoyl transferase = carnitine acyl transferase

CAT = CACT

carnitine acylcarnitine transferase

Question 19

beta oxidation : overview

Answer 19

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)

Question 20

beta oxidation compared to other metab

• energy yield

Answer 20

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

Question 21

what if a FA is not a long chain FA?

alternative oxidation pathways for…

• unsaturated FAs
• branched chain FAs
• medium and short chain FAs

Answer 21

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

Question 22

oxidation of FAs with odd numbers of C

Answer 22

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!

Question 23

B12 deficiency and methylmalonyl CoA

• identifying B12 def
• explaining symptoms of B12 def

Answer 23

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!

Question 24

carnitine shuttle defects

Answer 24

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

Question 25

CAT I/CPT I deficiency

Answer 25

* 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)

Question 26

CAT II/CPT II deficiency

Answer 26

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

Question 27

impaired medium chain FA oxidation

Answer 27

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**

Question 28

methylmalonyl CoA mutase deficiency

Answer 28

**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

Question 29

when and why does ketone body synthesis happen?

Answer 29

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

Question 30

ketone synthesis vs chol synthesis

Answer 30

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

Question 31

ketone synthesis: major keys * key enzymes * major ketone products

Answer 31

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\*_

Question 32

ketone bodies as alt fuel

Answer 32

_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)

Question 33

why ketones and not FAs for brain food?

Answer 33

FAs are bound to albumin in plasma : cant cross the blood/brain barrier! ketones are like transportable equivalents of FAs

Question 34

regulation of ketone body synth

Answer 34

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

Question 35

role of OAA in determining whether ketone synthesis happens

Answer 35

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

Question 36

plasma concentration as fast occurs

Answer 36

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!

Question 37

ketone bodies and DKA

Answer 37

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