Fatty Acid Oxidation And Ketones Flashcards
Sources of energy in the body
Carbohydrates
We have enough glycogen to sustain energy levels for 12 hours.
Fats
Lipid energy reserves provide energy for up to 12 weeks
Protein
Used when muscle glycogen stores fail.
Structure of fatty acids
Carboxylic head group with aliphatic tail
Head: hydrophilic (choline, phosphate and glycerol)
Tail: hydrophobic (2 fatty acid chains)
Saturated and unsaturated
In the body the two major locations that fatty acids are stored/utilised in are:
Triglycerides
Phospholipids
Fatty acid activation and b-oxidation
• Fatty acids must be activated in the cytoplasm before they can be oxidised in the mitochondria to then produce Acetyl-CoA
ACTIVATION:
Fatty acid + ATP + CoA —> Acyl-CoA + PPi (pyrophosphate) + AMP
The adenosine is taken from ATP and used to make the Acyl-Coenzyme A (Acyl-CoA).
• If the Acyl-CoA has < 12 carbons – can diffuse through outer mitochondrial membrane
• Most dietary fatty acids have > 14 carbons – Taken through mitochondrial membrane using the carnitine shuttle.
In order to get through, the Acyl CoA must be converted: Acyl CoA > (enzyme Carnitine acyltransferase 1 (resides in the outer mitochondrial membrane))> Acyl Carnitine.
In this process the Coenzyme A is removed from Acyl CoA and is recycled, as you can see, the molecule Carnitine is added. The Acyl Carnitine can then be transported into the mitochondria through the outer mitochondrial membrane.
Once inside the mitochondria another enzyme Carnitine acyltransferase 2 converts Acyl carnitine back to Acyl CoA : Acyl Carnitine > (enzyme Carnitine acyltransferase 2) > Acyl CoA. In this process a Coenzyme A is re-added and the carnitine ripped off to regenerate Acyl CoA. The carnitine can then diffuse through the outer mitochondrial membrane to be used again to covert Acyl CoA to Acyl Carnitine (THIS IS KNOWN AS THE CARNITINE SHUTTLE)
Now the fatty Acyl CoA can be oxidised. It is termed beta-oxidation (b-oxidation) since it occurs through the sequential removal of 2-carbon units by oxidation at the beta-carbon position of the fatty Acyl-CoA molecule.
Each round of b-oxidation produces 1 mol of NADH, 1 mol of FADH2 & 1 mol of Acetyl CoA.
The Acetyl CoA can then be used in the Kreb’s cycle
FATTY ACID B-OXIDATION takes place in the mitochondria matrix
1) Dehydrogenation (oxidation) of Acyl-CoA by enzyme acyl-CoA dehydrogenase producing a double bond in the acyl-CoA molecule. This reduces FAD to FADH2.
2) Hydration of the double bond by enzyme enol-CoA hydrate, producing a new hydroxyl group in the molecule. This requires the supply of water- hydration process
3) NAD+ dependant dehydrogenation (oxidation) of the Hydroxy group by the enzyme Hydroxyacyl-CoA dehydrogenase. This produces a new carbonyl group and NADH and H+
4) Cleavage of the bond by HS-CoA in a thiolysis reaction catalysed by the thiolase enzyme, producing molecule of Acetly-CoA and acyl-CoA
Slide 7 and 8
Utilisation of Acetyl-CoA
Under normal metabolic conditions most Acetyl-CoA is utilised via the TCA acid cycle to produce glucose
A small proportion of Acetyl-CoA is converted into ketones
During high rates of fatty acid oxidation, large amounts of acetyl-CoA are generated
This exceeds the capacity of the TCA cycle which results in ketogenesis.
What are Ketones
• Molecules produced by the liver from acetyl-CoA
• Have a characteristic fruity/nail polish remover-like smell
What reactions can acetoacetate undergo? Ketogenesis
FIRST Go to flashcard 17
Acetoacetate can undergo spontaneous decarboxylation to acetone, or be enzymatically converted to beta-hydroxybutyrate.
Ketone bodies utilised by extrahepatic tissues through conversion of beta- hydroxybutyrate and acetoacetate to acetoacetyl-CoA
• This requires the enzyme acetoacetate:succinyl-CoA transferase, which is found in all tissues but hepatic tissue (liver)
Ketogenesis is affected by several factors
Release of free fatty acids from adipose tissue
A high concentration of glycerol-3-phosphate in the liver results in triglyceride production, whilst a low level results in increased ketone body production
When demand for ATP is high, acetyl-CoA is likely to be further oxidised via the TCA cycle to carbon dioxide
Fat oxidation is dependent upon the amount of glucagon (activation) or insulin (inhibition) present as insulin inhibits Lipolysis of fats
Why is ketogenesis significant
During normal physiological conditions the production of ketones occurs at a low rate
Carbohydrate shortages cause the liver to increase ketone body production from acetyl-CoA
The heart and skeletal muscles preferentially utilise ketone bodies for energy preserving glucose for the brain
Who does ketoacidosis occur in?
Occurs in insulin-dependent diabetics when dose is inadequate or because of increased insulin requirement (infection, trauma, acute illness)
Is often the presenting feature in newly diagnosed type 1 diabetics
Also occurs in chronic alcohol abuse
Slide 16 and 17
What do patients with ketoacidosis show/do
Hyperventilation
Vomiting
Breath smells of pear drops
Consequences of ketoacidosis
• Ketones are relatively strong acids (pKa ~ 3.5)
• Excessive ketones lower the pH of the blood
• This impairs the ability of haemoglobin to bind oxygen
Beta-oxidation
Strictly aerobic, dependant on oxygen
Good blood supply
Adequate numbers of mitochondria
What happens to the products of beta oxidation?
The NADH & FADH2 produced from the beta-oxidation and from the Kreb’s cycle can then be used in oxidative phosphorylation
Oxidation of fatty acids compared to the oxidation of carbohydrates
THE OXIDATION OF FATTY ACIDS YIELDS SIGNIFICANTLY MORE ENERGY PER CARBON THAN DOES THE OXIDATION OF CARBOHYDRATES (I.E GLUCOSE). The net result of the oxidation of one mole of oleic acid (an 18-carbon fatty acid) will be 146 moles of ATP as compared to 38 moles of ATP produced from 1 mol of glucose
Fatty acids as fuels
FATTY ACIDS DO NOT ACT AS A FUEL SOURCE FOR THE NERVOUS SYSTEM SINCE FATTY ACIDS CANNOT GET THROUGH THE BLOOD-BRAIN BARRIER
Fatty acids are principally used as fuel when hormones signal fasting or increased demand
• Example of fatty acids; Linoleic acid (18 carbons), Oleic acid (18 carbons), Palmitic acid (16 carbons) & Arachidonic acid (20 carbons)
KETOGENESIS: IN LIVER - HEPATOCYTES
SLIDE 12
• During high rates of fatty acids oxidation, primarily in the liver, large amounts of acetyl-CoA are generated. These exceed the capacity of the Kreb’s/TCA cycle, and one result is the synthesis of ketone bodies - known as ketogenesis
• Ketone bodies (acetone, acetoacetate & B-hydroxybutyrate) are synthesised in the mitochondrial matrix from Acetyl CoA generated from b-oxidation
Two Acetyl CoA ’s are converted by the enzyme thiolase (used in b-oxidation), back to Acetoacetyl CoA releasing CoA-SH
Under the action of two further enzymes the Acetoacetyl CoA can be converted to Acetoacetate, acetoacetate can then enter the blood or it can be converted to b-hydroxybutyrate under the action of another enzyme, which can then enter the blood.
When the level of glycogen in the liver is high the production of b-hydroxybutyrate increases.
Another fate of the acetoacetate is that it can spontaneously be converted to acetone. Since acetone is volatile it is rapidly expired by the lungs
NOTE: when carbohydrate utilisation is low or deficient, the level of oxaloacetate will also be low resulting in a reduced flux through the Kreb’s cycle - leading to an increased release of ketone bodies from the liver to be used as fuel by other tissues
Acetoacetate & b-hydroxybutyrate
Can both be oxidised as fuels in most tissues, including skeletal muscle
Cells transport the acetoacetate and b-hydroxybutyrate from the blood into the cytosol then into the mitochondrial matrix. Here b-hydroxybutyrate is oxidised back to acetoacetate.
Acetoacetate can then be activated to Acetoacetyl CoA which can then be cleaved into two molecules of Acetyl CoA by the thiolase enzyme (same enzyme involved in b-oxidation)
Then the Acetyl CoA can be used in the Kreb’s cycle to produce ATP
THE LIVER DOES NOT HAVE THE ENZYME SUCCINYL CoA: ACETOACETATE CoA (enzyme used to convert acetoacetate to acetoacetyl CoA) IN SUFFICIENT CONCENTRATIONS
THUS CANNOT UTILISE KETONE BODIES AS FUEL since they cannot be converted to Acetyl CoA in the liver - this ensure that extrahepatic tissues have access to ketone bodies as a fuel source during prolonged starvation
Starvation
In the early stages of starvation, the last remnants of fat are oxidised, the heart and skeletal muscle will consume primarily ketone bodies in order to PRESERVE GLUCOSE FOR USE BY THE BRAIN - when glucose in the brain decreases then the brain CAN use ketone bodies for energy
Regulation of ketone body synthesis:
Control in the releases of free fatty acids from adipose tissue directly affects the level of ketogenesis in the liver
CLINICAL SIGNIFICANCE:
- Production of ketone bodies occurs at a relatively slow rate during normal feeding and under normal physiological status
Normal physiological responses to carbohydrate shortages
cause the liver to increase production of ketone bodies from the acetyl-CoA generated from fatty acid oxidation.
This allows the heart and skeletal muscles primarily to use ketone bodies for energy, thereby preserving the limited glucose for use by the brain.
- The most significant disruption in the level of ketosis occurs in untreated insulin- dependent diabetes mellitus
DIABETIC KETOACIDOSIS
can occur, it results from a reduced supply of glucose
(since there will be a significant decline in circulating insulin) and an increase in fatty acid oxidation (due to an increase in circulating glucagon)
increased production of Acetyl-CoA
leads to ketone body production that exceeds the ability of peripheral tissues to oxidise them.
Ketone bodies are relatively strong acids (pH 3.5), and their increase lowers the pH of blood. This acidification of the blood can have many consequences but most critical is the fact that it IMPAIRS THE ABILITY OF HAEMOGLOBIN TO BIND TO OXYGEN
Effect on patient
If a patient is in diabetic ketoacidosis, the excess ketones in the blood will result in their BREATH SMELLING OF PEAR DROPS (KETONES).
