17 | Fatty Acid Catabolism

© 2009 W. H. Freeman and Company

Oxidation of Fatty Acids is a Major Energy-Yielding Pathway in

Many Organisms

• About one third of our energy needs comes from dietary triacylglycerols

• About 80% of energy needs of mammalian heart and liver are met by oxidation of fatty acids

• Many hibernating animals, such as grizzly bears relay almost exclusively on fats as their source of energy

Hydrolysis of Fats Yields Fatty Acids and Glycerol

• hydrolysis of triacylglycerols is catalyzed by lipases

• some lipases are regulated by hormones glucagon

and epinephrine

• epinephrine means:

“we need energy now”

• glucagon means:

“we are out of glucose”

Mobilization of triacylglycerols stored in adipose tissue. Low glucose triggers glucagon release, initiating the cascade.

PKA phosphorylates 3 the hormone-sensitive lipase and 4 perilipin molecules on the surface of the lipid droplet. Phosphorylation of perilipin permits hormone-sensitive lipase access to the surface of the lipid droplet, where 5 it hydrolyzes triacylglycerols. 6 Fatty acids leave the adipocyte, bind serum albumin and are carried in the blood, to myocytes where they are released from the albumin and 7 enter a myocyte via a specific fatty acid transporter. 8

Fatty Acid Transport into Mitochondria

• Fats are degraded into fatty acids and glycerol in the cytoplasm

-oxidation of fatty acids occurs in mitochondria

• Small (< 12 carbons) fatty acids diffuse freely across mitochondrial membranes

• Larger fatty acids are transported via acyl-carnitine / carnitine transporter

Glycerol from Fats Enters Glycolysis

• Glycerol kinase prepares glycerol for glycolysis at

the expense of ATP

• Subsequent reactions recover more than enough

ATP to cover this cost

• Allows limited anaerobic catabolism of fats

Fatty Acids are Converted into

Fatty Acyl-CoA

Step: 1Dehydrogenation of Alkane to Alkene

• Catalyzed by isoforms of acyl-CoA dehydrogenase (AD) on the inner mitochondrial membrane– Very-long-chain AD (12-18 carbons)– Medium-chain AD (4-14 carbons)– Short-chain AD (4-8 carbons)

• Analogous to – succinate dehydrogenase reaction in the citric acid cycle

Note that this is the L-isomer.

Step: 2Hydration of Alkene

• Catalyzed by two isoforms of enoyl-CoA hydratase:– Soluble short-chain hydratase (crotonase)– Membrane-bound long-chain hydratase, part of

the trifunctional complex• Water adds across the double bond yielding

alcohol• Analogous to

– fumarase reaction in the citric acid cycle

Note that this is the TRANS isomer

Step: 3 Dehydrogenation of Alcohol

• Catalyzed by -hydroxyacyl-CoA dehydrogenase• The enzyme uses NAD cofactor as the hydride

acceptor• Only L-isomers of hydroxyacyl CoA act as

substrates• Analogous to

malate dehydrogenase reaction in the citric acid cycle

Step: 4 Transfer of Fatty Acid Chain

• Catalyzed by acyl-CoA acetyltransferase (thiolase) via a covalent mechanism– The carbonyl carbon in -ketoacyl-CoA is

electrophilic– Active site thiolate acts as nucleophile and

releases acetyl-CoA– Terminal sulfur in CoA-SH acts as nucleophile

and picks up the fatty acid chain from the enzyme

• The net reaction is thiolysis of carbon-carbon bond facilitated by making a very stable bond more susceptible to cleavage!

Trifunctional Protein• Hetero-octamer

– Four subunits• enoyl-CoA hydratase activity -hydroxyacyl-CoA dehydrogenase activity• Responsible for binding to membrane

– Four subunits• long-chain thiolase activity

• May allow substrate channeling• Associated with inner mitochondrial membrane• Processes fatty acid chains with 12 or more

carbons; shorter ones by enzymes in the matrix

Fatty Acid Catabolism for Energy

• Repeating the above four-step process six more

times results in the complete oxidation of palmitic

acid into eight molecules of acetyl-CoA

– FADH2 is formed in each cycle

– NADH is formed in each cycle

• Acetyl-CoA enters citric acid cycle and is further

oxidizes into CO2

– This makes more GTP, NADH and FADH2

Oxidation of Monounsaturated Fatty Acids

http://www.csulb.edu/~cohlberg/Songs/betaox.mp3

What’s wrong here??

Oxidation of Polyunsaturated Fatty Acids

Oxidation of Propionyl-CoA

• Most dietary fatty acids are even-numbered

• Many plants and some marine organisms also synthesize odd-numbered fatty acids

• Propionyl-CoA forms from -oxidation of odd-numbered fatty acids

• Bacterial metabolism in the rumen of ruminants also produces propionyl-CoA

Oxidation of propionyl-CoA produced by β oxidation of odd-number fatty acids.

Conversion to succinate requires epimerization of D- to L-methylmalonyl-CoA followed by some really strange chemistry!,

Intramolecular Rearrangement in Propionate Oxidation Requires Coenzyme B12

Complex Cobalt-Containing Compound: Coenzyme B12

Pag

e 92

6Figure 25-23Proposed mechanism of methylmalonyl-CoA mutase.

Regulation of Fatty Acid Synthesis and Breakdown

-Oxidation in Plants Occurs in Mainly in Peroxisomes

• Mitochondrial acyl-CoA dehydrogenase passes

electrons into respiratory chain via electron-

transferring flavoprotein

– Energy captured as ATP

• Peroxisomal acyl-CoA dehydrogenase passes

electrons directly to molecular oxygen

– Energy released as heat

– Hydrogen peroxide eliminated by catalase

Formation of Ketone Bodies

• Entry of acetyl-CoA into citric acid cycle requires oxaloacetate

• When oxaloacetate is depleted, acetyl-CoA is converted into ketone bodies

• The first step is reverse of the last step in the -oxidation: thiolase reaction joins two acetate units

Liver as the Source of Ketone Bodies

• Production of ketone bodies increases during starvation

• Ketone bodies are released by liver to bloodstream

• Organs other than liver can use ketone bodies as fuels

• Too high levels of acetoacetate and -hydroxybutyrate lower blood pH dangerously

19 | Oxidative Phosphorylation and Photophosphorylation

© 2009 W. H. Freeman and Company

Energy from Reduced Fuels is Used to Synthesize ATP in

Animals• Carbohydrates, lipids, and amino acids are the

main reduced fuels for the cell

• Electrons from reduced fuels used to reduce NAD+ to NADH or FAD to FADH2.

• In oxidative phosphorylation, energy from NADH and FADH2 are used to make ATP

Oxidative Phosphorylation

• Electrons from the reduced cofactors NADH and FADH2 are passed to proteins in the respiratory

chain

• In eukaryotes, oxygen is the ultimate electron acceptor for these electrons

• Energy of oxidation is used to phosphorylate ADP

Energy of Light is Used to Synthesize ATP in

Photosynthetic Organisms

• Light causes charge separation between a pair chlorophyll molecules

• Energy of the oxidized and reduced chlorophyll molecules is used drive synthesis of ATP

• Water is the source of electrons that are passed via a chain of transporters to the ultimate electron acceptor, NADP+

• Oxygen is the byproduct of water oxidation

Chemiosmotic Theory

• How to make an unfavorable

ADP + Pi = ATP

possible?• Phosphorylation of ADP is not a result of a direct

reaction between ADP and some high energy phosphate carrier

• Energy needed to phosphorylate ADP is provided by the flow of protons down the electrochemical gradient

• The electrochemical gradient is established by transporting protons against the electrochemical gradient during the electron transport

Chemiosmotic Energy Coupling Requires

Membranes• The proton gradient needed for ATP synthesis can be

stably established across a topologically closed membrane– Plasma membrane in bacteria– Cristae membrane in mitochondria– Thylakoid membrane in chloroplasts

• Membrane must contain proteins that couple the “downhill” flow of electrons in the electron transfer chain with the “uphill” flow of protons across the membrane

• Membrane must contain a protein that couples the “downhill” flow of proton to the phosphorylation of ADP

Structure of a Mitochondrion

• The proton gradient is established across the inner membrane

• The cristae are convolutions of the inner membrane and serve to increase the surface area

Metabolic Sources of Reducing Power

“Alfonse, Biochemistry makes my head hurt!!”\