Week 4 => Fatty acid biosynthesis, modification, and regulating oxidation Flashcards
Where are enzymes for fatty acid synthesis found?
The cytosol
How does acetyl-CoA generated in the mitochondrial matric get transported to the cytoplam?
Transported indirectly in the form of citrate and released int he cytoplasm by the enzyme ATP citrate lyase
What metabolite production would cause a futile cycle with fatty acid synthesis?
beta-oxidation of fatty acyl-carnitine to acetyl-CoA
How many Acetyl-CoA does one Acetate produce?
8
What are cellular functions C16 fatty acid-palmitate contribute to?
- Post-translational modifications
- Membrane biogenesis
- Cell signaling and second messengers
- Energy storage
Example of molecule what contributed to post-translational modifications?
Protein acylation
Example of molecule what contributed to membrane biogenesis?
Phospholipids and sphingolipids
Example of molecule what contributed to cell signaling & second messengers?
Eicosanoids
Example of molecule what contributed to energy storage?
Triacylglycerol
Function of ATP citrate lyase?
Releases acetyl-CoA (from citrate) for fatty acid and cholesterol synthesis
Due to fatty acyl groups have limited aqueous solubility what form are they found as to involve aqueous solubility and serve as reactive forms in biosynthetic reactions?
Thioesters or acyl carrier proteins (ACP) or of coenzyme A (CoA)
On phopshopantetheine groups what do the free SH- groups on the cysteamine segment form?
Free -SH groups from thioester linkages to FAs
What is the source of malonyl-CoA carboxyl group (COO-) on Malonyl-CoA?
Bicarbonate
Enzyme for synthesis of Malonyl-CoA
Catalyzed by acetyl-CoA carboxylase (ACC1 and ACC2 isoforms)
ACC1 location and predominant function?
ACC1 predominates in fatty acid synthesizing tissues (Adipose and liver) and catalyzes rate-limiting step
What does phosphorylation of ACC1 do?
Blocks polymerization and inactivates the enzyme
ACC2 location and predominant function?
ACC2 is present in fatty acid oxidative tissues (muscle) where it associates with mitochondria and inhibits beta-oxidation by producing malonyl-CoA
ACC 3 functionalities
a) biotin carrier
b) biotin carboxylase
c) transcarboxylase
How does biotin carboxylase work?
Activates CO2 (from bicarbonate) by attaching it to ring nitrogen of biotin on the carrier
How does Transcarboxylase work?
Activity transfers the CO2 from biotin to acetyl-CoA, generating malonyl-CoA
Fatty acid synthesis is catalyzed by ____
Multi-enzyme Fatty Acid Synthase (FAS) complexes
FASI structure
Mammalian FASI is single polypeptide containing all the enzymatic activities
FASII structure
Prokaryotic FASII is a dissociated complex of individual enzymes and ACP
Products of FASII
The 16-carbon fatty acid palmitate, but intermediates in synthesis can be released
Products of FASI
The 16-carbon fatty acid palmitate but no intermediates released
What regulates FASI
Substrate availability and transcriptional mechanisms involving sterol-regulatory element binding protein (SREBP)
Where does fatty acid synthesis occur?
Occurs in the cytoplasm in eukaryotic cells. Also a minor fatty acid synthesis pathway in mitochondria
Generally how does fatty acid synthesis occur?
Synthesis involves series of 2-carbon condensation reactions using malonyl-CoA to extend an acyl chain
What are the 4 repeating reactions in FASI?
1) Condensation
2) Reduction
3) Dehydration
4) Reduction
What is the enzyme in 1st reduction step of FASI reactions?
beta-ketoacyl-ACP reductase
What is the enzyme in dehydration step of FASI reactions?
beta-hydroxyacyl-ACP-dehydrogenase
What is the enzyme in 2nd reduction step of FASI reactions?
Enoyl-ACP-reductase
what is the product in the first round of the FASI cycle?
C4 Butyryl
How many cycles is required to elongate fatty acid to 16 carbon palmitate?
7 cycles
What occurs after the formation of palmitate?
The palmitate is cleaved by thioesterase (TR) activity in FASI. Palmitate is immediately converted to palmitoyl-CoA by a fatty acyl-CoA synthase
List parts of FASII loose complex of 6 different polypeptides with enzyme activities plus ACP
1) Malonyl-CoA-ACP transferase (MT/FabD)
2) beta-Ketoacyl-ACP synthase (KS/FabH, FABH+F)
3) beta-Ketoacyl-ACP reductase (FR/FabG)
4) beta-hydroxyacyl-ACP dehydrase (DH/FabZ+A)
5) Enoyl-ACP reductase (ER/FabI)
Sources of NADPH for fatty acid synthesis
- Oxidation of malate to pyruvate
- PPP oxidative phase (when producing P-phosphogluconate and Ribulose 5-phosphate)
Hormonal regulation of Fatty acids
- Insulin => promotes fatty acid synthesis
- Glucagon/epinephrine => decrease fatty acid synthesis
Allosteric regulation of fatty acid biosynthesis
- Citrate is an allosteric activator of ACC1, promoting malonyl-CoA synthesis
- Product (palmitoyl-CoA) promotes depolymerization and inactivation
- ACC1 is inhibited by phosphorylation by AMP-dependent Kinase (turn off ATP consuming biosynthesis pathways when ATO is scarce)
Δ Nomenclature of fatty acids
Position of the double bond is from the carboxyl end of the acyl chain
To fully describe fatty acids which nomenclature should be used?
Both nomenclatures
n- or ω- Nomenclature
Identifies only the first double bond from the methyl end of the acyl chain
What are the two genomic configuration of double bonds?
Cis or trans
What does increasing chain length effect?
Increases melting point (mp)
How do double bonds effect melting point?
For a given chain length, increasing number of double bonds decreases melting point (mp)
Beyond what carbon can animals not introduce double bonds?
C10
What are essential fatty acids series
n-3 and n-6 series (must be acquired in the diet)
What increments does fatty acid elongation occur?
2-carbon increments
What can fatty acid desaturation occur by?
The Δ9 desaturase, Δ6 desaturase, or Δ5 desaturase
What does fatty acids desaturation introduce?
cis-double bonds
What is Δ9 desaturase expression stimulated by?
Insulin
What does Δ9 desaturase require?
3 membrane proteins, O2 and NAD(P)H and uses fatty acyl-CoA as substrate
In plants where is the fatty acid desaturated?
The fatty acid at the sn-2 position is desaturated
What desaturases do plants use?
Δ6, Δ9, Δ12, and Δ15
What do plant desaturases use as substrates?
Phospholipids
What do animal desaturases use as subtrate?
Fatty acetyl-CoA
Where is the majority of elongation acitivity?
ER
First step of fatty acid elongation
Condensation of the long chain acyl-CoA substrate with malonyl-CoA, catalyzed by one of two condensing enzymes, specific for either saturated or unsaturated acyl substrates
Second step of fatty acid elongation
the beta-ketoacyl-CoA is reduced by NADPH, dehydrated and the enoyl-CoA reduced again by NADPH to give a fatty acid two carbons longer than the original substrate.
Mitochondrial fatty acid elongation
- uses acetyl-CoA instead of malonyl-CoA as source of 2C units
- Pathway for synthesis of lipoic acid from C8 fatty acid
Eicosanoids
- A class of lipid compounds with potent physiological activities (signaling lipids)
- include prostaglandins, thromboxane, and leukotrienes
- action confined to local environment, and catalyzed extremely rapidly
- all originate from C20 ‘eicosanoid acids’, particularly arachidonic acid
Eicosanoid biosynthesis
After release from phospholipids, arachidonic acid undergoes modification in the cyclo-oxygenase (cyclic) or the lipoxygenase (linear) pathways to form active prostaglandins or leukotrienes, respectively
What does Eicosanoids signal through?
G-protein coupled receptors (GPCRs)
Cyclo-oxygenase (COX) inhibitors
Aspirin (acetylsalicylic acid) or ibuprofen reduces prostaglandin levels (which mediate pain sensation) and thromboxane levels (which mediates blood blotting)
Non-steroidal anti-inflammatory drugs (NSAIDs)
- Mimic the substrate or a reaction intermediate for COX1 and 2
- COX1 is constitutive enzyme with homeostatic functions while COX2 is induced by inflammations
Fatty acids are ___ in aqueous environments
Aqueous
Why are fatty acids kept at low concentration in the cell?
Because FA are good detergents (Can break apart cell membranes)
What are FA converted to in cell to increase solubility and reactitivy?
CoA ester
Where are long chain ligases found?
Cytosol and outer mitochondrial membrane
Where are short-chain ligases found?
In the mito matrix
What does malonyl-CoA strongly inhibit?
Carnitine acyltransferase I (CATI)
How do FA enter the mitochondrial matrix since they cannot cross the inner mitochondrial membrane?
Transesterification to carnitine by CATI on the OMM then transported into matrix by an acylcarnitine/carnitine transporter in IMM. In the matric the transesterification reaction ins reverse by CATII to reaerate fatty acetyl-CoA and carnitine
Rate-limiting step for fatty acid oxidation?
Carnitine-medicated entry of fatty acyl-CoA into the mito matrix
3 enzymes part of the Trifunctional protein complex (3alpha3beta)
- enoyl-CoA hydratase
- beta-hydrocyacyl-CoA dehydrogenase
- acyl-CoA acetyltransferase (thiolase)
Which enzyme is stereospecific for the trans isomer?
enoyl-CoA hydratase
beta-Oxidation of saturated fatty acids general steps
1) Dehydrogenation - produce trans double bond between alpha and beta carbons (C2 and C3) of fatty acyl-CoA
2) Hydration
3) Dehydrogenation
4) Acyltransfer to CoA
Overall goal of beta-oxidation of saturated fatty acids?
To break a stable C-C bonds by making a alpha-beta unsaturated carbonyl group (good target for nucleophilic attack)
Where is beta-oxidation of odd numbered carbon saturated fatty acids found?
In plants and marine organisms
What is Propionyl-CoA converted to for entry into the TCA cycle?
Succinyl-CoA
Autosomal recessive mutations in methylmelonyl-CoA mutase
- Deficiency in metabolism of odd-chain fatty acid as well as methionine, threonine, isoleucine, and valine
- Accumulation of toxic methylmalonic acid
- Incidence of 1/50000 births but treatable
- B12 deficiency causes a similar disorder
What are most of the fatty acids in triacylglycerols and phoshpholipids?
Cis-unsaturated at one or more positions
What changes double bond from cis to trans un unsaturated fatty acids?
Isomerization by then enzyme Δ3, Δ2-enoyl-CoA isomerase
Where are the two possible places for fatty acid metabolism/oxidation?
Mitochondrial and Peroxisome
Mitochondrial FA oxidation
Medium and long chains FAs (<18C)
Peroxisomal FA Oxidation
- Very long-chain FAs (>C20)
- Branched chain fatty acids
- Polyunsaturated fatty acids
Transporters used in peroxisomal beta-oxidation
ABC class D transporter (And ATP-dependent transporter)
Transporter used in mitochondrial beta-oxidation
Carnitine-acylcarnitine translocase
What is a toxic by product of peroxisomal beta-oxidation?
H2O2 (hydrogen peroxide)
What breaks down hydrogen peroxide into water?
Catalase
When is FA exported from peroxisomes and imported into mitochondria for complete oxidation?
Once oxidized to a length of C8-CoA
What is phytanic acid a product of?
Breakdown product of chlorophyll found in milk and fatty tissues of ruminants and rish
Refsum’s Disease
Is a neurological disorder cause by accumulation of phytanic acid (dysfunctional phytanoyl-CoA hydroxylase). Can be attenuated by restricting dietary phytanic acid
What can the heart and brain use as fuel for energy generation?
Ketone bodies produced in the liver
What happens under conditions of starvation of hypoglycemia (when TCA cycle intermediates life oxaloacetate are depleted)?
Excess acetyl-CoA from fatty acids is used for ketone body synthesis in the mitochondria
Ketone bodies
- Acetoacetate
- Beta-hydroxybutyrate
- acetone
What can excess production of ketone bodies cause?
Ketoacidosis (seen in poorly controlled diabetes and alcohol abuse)
