Chapter 13: Lipids & Fatty Acid Catabolism

What are fatty acids?

Building blocks of phospholipids (required for cell membranes) & glycolipids

What does it mean for a fatty acid to be unsaturated?

At least 1 double bond

What does it mean for a fatty acid to be monounsaturated?

1 double bond

What does it mean for a fatty acid to be polyunsaturated?

More than 1 double bond

What else does a fatty acid contain?

• Glycerol backbone

• Ester bond (acyl bond)

• Polar head

What does it mean for a fatty acid to be saturated?

No double bonds

What are fatty acids typically?

Cis

What is the most stable fatty acid?

Saturated

• Very tight packing

How are fatty acids modifications of proteins to target them to appropriate membrane locations?

Normally covalent attachment

What can fatty acid derivatives do?

Serve as hormones & intracellular messengers

• Prostaglandins & leukotrines

• DAG: involved in PIP₂ signal pathway

How are fatty acids a source of energy?

• Stored as triglycerides (triacylglycerols)

  • Highly concentrated energy store due to they are reduced & anhydrous

    • Anhydrous allows for multiple ox/red reactions & generation of lots of NADH/FADH₂/ATP

How much do fatty acids produce compared to carbohydrates & proteins?

• Fatty acids: 9 kcal/gm

• Carbohydrates & proteins: 4 kcal/gm

What is 16:0 common name?

Palmitic acid (palmitate)

What is 18:0 common name?

Stearic acid (storage)

What does the delta followed by number mean?

The number is the carbon where double bond is (delta9 → b/w 9 & 10 carbons)

What do the most naturally occurring fatty acids have?

Even number of carbon atoms

What are double bonds designed by delta & are usually?

Cis

What do double ones normally occur in same positions & are?

Rarely conjugated & usually 3 carbon atoms apart

What are the 3 sources where cells can obtain fatty acids?

• Diet

• Triglycerides stored in cells as lipid droplets

• Fats synthesized in 1 organ for export to another

How many of the daily energy requirement of humans is supplied by dietary triglycerides?

40% or more

What do triglycerides provide more than ½ energy requirements of?

Liver, heart, & resting skeletal muscle

• Heart preferentially uses fatty acids as opposed to glucose

• Fatty acids generate more ATP/molecule than molecule of glucose

What are lipids in aqueous solutions?

Insoluble

What happens to the triglycerides & cholesterol in order to be transported w/in the blood?

Solubilized through association w/ macromolecular aggregates known as lipoproteins (chylomicrons, VLDL, LDL, & HDL)

What does lipase do?

• Cleaves fatty acids from TG

• Produces free fatty acids (FFA)

What are chylomicrons primary composed of?

TG w/ some cholesterol

What is adipose tissue primarily?

Storage (use these later)

• Stored as a TG

What was muscle stored or used for?

Energy

What makes VLDL, LDL, & HDL by endogenous pathway?

Liver

Where is the solubilization started?

Dietary lipids in intestine

• chylomicrons → lipase

  • → FFA → adipose tissue & muscle

• Remnants —> liver

What does glucagon act opposite of?

Insulin & factors mobilization of energy sources

What are the steps after glucagon triggers mobilization of stored TGs?

1. Low levels of glucose trigger glucagon release, which binds to adipocyte receptors

2. Stimulation of adenylate cyclase forms cAMP that activates protein kinase A

3. PKA phosphorlyaes hormone-sensitive lipids & perilipin molecules

4. Perilipins coat lipid droplets & restrict access usually; however phosphorylation alters conformation & allows access of hormone sensitive lipoate

5. TGs are hydrolyzed to FFAs via steps 5-8. FFAs are available for transport from adipose tissue to other body locations

6. FFAs leave adipocyte & bind to serum albumin

7. Albumin transports fatty acids to myocytes where they enter specific transporter

8. Fatty acids undergo beta-oxidation to produce needed ATP

What can glycerol enter glycolysis via conversion to?

Dihydroxyacetone phosphate

• glycerol → (glycerol kinase, ATP→ADP) → glycerol 3-phosphate → (glycerol phosphate dehydrogenase, NAD+→NADH) → dihydroxyacetone phosphate → (isomerase) → gly-3-P

  • Net yield is 18.5 ATP per molecule of glycerol

What does fatty acid activation require?

Coenzyme A

• Precedes any oxidation

• 2-step process that involves high-energy bonds & acyl adenylate intermediate

• Total cost 2 ATP→2 ADP + 2Pi

What is the role of carnitine?

Fatty acids are

• activated on outer mitochondrial membrane

• Oxidized in the inner mitochondria matrix

Why is transport required by carnitine?

Inner membrane is otherwise impermeable

What does the acyl group attach to?

Carnitine at the OH group

What are the 4 enzyme-catalyzed reactions of beta-oxidation of fatty acids?

1. Catalyzed by acyl-CoA dehydrogenase

• Ox/red reactions, NADH or FADH₂ formed

2. Catalyzed by enoyl-CoA hydratase

• Adds water

3. Catalyzed by beta-hydroxyacyl-CoA dehydrogenase

• Another dehydrogenase enzyme

4. Catalyzed by acyl-CoA transferase (thiolase)

• Breaks bond w/ a -SH group

• Hydrolysis (breaks bond w/ H₂O)

What happens in each step of beta-oxidation?

1. FAD is used to add a C-C bond reaction specific, decrease always produces a trans isomer

2. Simple hydration of a double bond, stereospecific→ only one isomer is formed

3. Beta carbon is oxidized to ketone (why beta-oxidation name), C-O bond is oxidized to get NADH

4. Fatty acids is “shortened” by 2 carbon atoms & we can enter another round, acetyl CoA enters CAC

What happens in each round of beta-oxidation?

• 1 FADH₂, 1 NADH, & 1 acetyl CoA is formed

• Fatty acid is shortened by 2 carbon atoms

What happens in final round w/ 4 carbon fatty acid?

4 carbon fatty acid → (thiolase & CoA-SH) → 2 acetyl CoA

How to calculate number of rounds?

(x/2 - 1) where x=number of carbon atoms in fatty acid

How much ATP formed per FADH₂, NADH, & Acetyl CoA for fatty acids?

• 1.5 ATP/FADH₂

• 2.5 ATP/NADH

• 10 ATP/Acetyl CoA

Must make sure to subtract 2 ATP as activation cost

What is the mechanism of unsaturated fatty acids?

• Occurs by beta-oxidation

• Involves 2 additional enzymes: isomerase + reductase

• Monounsaturated fatty acids (more common) require 1 or both while polyunsaturated require both

Are odd numbered double bonds metabolized the same as even?

No, differently

What does the isomerase do in fatty acid oxidation?

• moves the double bonds

• Replaces dehydrogenase

  • No FADH₂ produced in step 1

  • 1.g ATP produced in this round

What does isomerase moving double bonds allow?

Complete round of B-oxidation since want trans delta2 not cis delta3

When does a double bond need removed via reductase?

Conjugated system since 2 double bonds are close, not substrate

What needs to happen with odd number double bonds?

• Use isomerase enzyme instead of dehydrogenase in step 1

• No FADH₂ formed in round & 1.5 ATP not formed

What needs to happen with even numbered double bonds?

• Insert 2 enzymes b/w steps 1 & 2

• FADH₂ is formed in step 1 & then reductase & isomerase are used in this order

What happens when using linoleic acid?

• An 18.3 (delta9,12) fatty acid w/ 1 odd & 1 even double bonds

• Initially calculate if there were no double bonds

  • 8 rounds, 8 FADH₂, 8 NADH, 9 acetyl CoA

  • 122 ATP - 2 ATP (act. Cost) - 1.5 ATP (1 odd number double bond)= 118.5 ATP → 118 ATP

What is the final step of beta-oxidation with odd number carbons?

C5 → (thiolase) → propionyl CoA (converts to Succinyl CoA) + acetyl CoA

How are regulation of fatty acid synthesize & fatty acids oxidation coordinated in a manner so that?

Fatty acids are not synthesized & oxidized at the same time

What is the fatty acid oxidation pathway taken depend on?

Rate of transfer into mitochondrial matrix

What is the rate-limiting process for fatty acid metabolism?

3-step carnitine shuttle → moves acetyl CoA into matrix

What does Malonyl-CoA, first intermediate in cytosolic biosynthesis of fatty acids do?

• Increases when excess glucose is present

  • Ibhibhits carnitine acyltransferase I (insulin triggered storage of acetyl CoA as fatty acids)

What are 2 enzymes w/in beta-oxidation pathway regulated by?

Metabolites (feedback control)

• Beta-hydroxyacyl-CoA dehydrogenase is inhibited when [NADH]/[NAD+] ratio is high

• High NADH signals on higher capacity to make ATP & don’t need more beta-oxidation

• Thiolase is inhibited when increased [acetyl CoA] → product feedback

What does low blood glucose mean?

Everything is stored, indicates individual is hungry

What does acetyl CoA carboxylase do?

Adds carboxylic acid (requires biotin which carries CO₂)

• then converts acetyl CoA to malonyl CoA (similar to conversion of pyruvate to oxaloacetate

When does acetyl CoA formed in fatty acid oxidation enter CAC only if?

Oxaloacetate is present

What do fats provide?

Acetyl CoA

What do carbohydrates provide?

Oxaloacetate

During periods of fasting, starvation, or poorly controlled diabetes, what happens with carbohydrates?

Unavailable or underutilized

• In these cases, insufficient supply of oxaloacetate (since shunted to gluconeogenesis)

What happens in previous questions to acetyl CoA then?

Diverted to form ketone bodies

• ketone - acetoacetate which can become d-3-hydroxybutyrate (dead end) or acetone (can be detected in breath)

Where can acetoacetate & 3-hydroxybutyrate be transported?

In blood to extrahepatic tissues where they can be used as an energy source

• Both heart muscle & renal cortex use acetoacetate in preference to glucose

• In prolonged starvation, acetoacetate provides 75% of fuel needed by brain (replacing normally used glucose)

What is the equation for acetoacetate to acetyl CoA?

Acetoacetate → (activation w/o ATP, Succinyl CoA → Succinate) → acetoacetyl CoA (thiolase, CoA enters) → 2 acetyl CoA

• (Last step is last step of beta-oxidation)

What can Acetoacetate be viewed as?

Water soluble, transportable form of acetal CoA

How does Acetoacetate play a regulatory role?

Increase [acetoacetate] signifies abundance of acetyl units & decreases fatty acid metabolism

What happens to gluconeogenesis during starvation?

Depletes CAC intermediates, diverts acetyl CoA to ketone body production

What happens in untreated diabetes when insulin levels are insufficient?

• Extrahepatic tissues cannot efficiently take up glucose in blood (glucose required)

• Levels of malonyl-CoA decrease & any inhibition of carnitine acyltrasnferase I is removed (or significantly decreased)

• Increases metabolism of fatty acids by beta-oxidation, ability to enter CAC is compromised due to lack of oxaloacetate

• Causes acceleration of ketone body formation & increase concentration of ketone bodies in blood

  • Increased concentration of acetoacetate & D-3-hydroxybutyrate cause: high concentration of acidic molecules in blood: acidosis of ketoacidosis

What can extreme acidosis lead to?

Coma or death

-normal levels of ketone bodies are 3mg/dL or less, in uncontrolled diabetic can exceed 90mg/dL