Lipid Metabolism 5 and 6

What is the carrier for free FA in the blood?

Serum Albumin

What is the carrier for TG in the blood?

Lipoproteins such as VLDL and Chylomicrons

Which apolipoprotiens mature chylomicrons?

ApoE and ApoC-II

What happens to TG levels if ApoC-II gets knocked out?

TG levels get >1000 mg/dL, which leads to pancreatitis.

What causes the pancreatitis found when TG levels exceed 1000 mg/dL?

Usually, post-prandially (after a meal), chylomicrons appear and then are cleared away. With extremely high TG levels, chylomicrons are basically always present. Because of their large size, they can obstruct capillaries and lead to tissue damage. This damage can expose TG to pancreatic lipases, leading to free FA in the blood, which leads to cytotoxic injury and further local injury that increases inflammatory mediators and free radicals, eventually manifesting as pancreatitis.

TL;DR chylomicrons cause damage that exposes TG to pancreatic lipases. The free FA in blood causes injury and increases inflammatory mediators and free radicals, manifesting as pancreatitis.

What are the two steps in beta-oxidation relating to mobilization of stored FA

To have both hydrophilic and hydropobic properties

Name two essential fatty acids

Linoleic acid and alpha-linoleic acid

How large must an FA be in order to require active transport to get through the membrane?

Larger than 12 carbons

What is the most common FA found in animals, plants, and microorganisms?

Palmitic Acid

Acetyl CoA and Acetyl CoC make...

Malonyl CoA

What are the 4 steps of De novo FA synthesis?

1. Transfer of Acetyl CoA from mitochondria to cytosol
2. Carboxylation of Acetyl CoA to form malonyl CoA (by means of Acetyl CoA carboxylase)
3. FA (palmitate C16) synethesis by FA synthase (FAS)
4. Elongation, desaturation of FA and forming storage form (TG)

In which tissues does Acetyl CoA become Palmitate?

In liver, lactating mammary glands, and in adipose

What is the rate limiting step of De novo Synthesis?

The carboxylation of Acetyl CoA to form malonyl CoA by means of Acetyl CoA carboxylase

Where does lipogenesis mainly occur?

In the liver

What is the rate limiting step of lipogenesis?

Acetyl CoA going to Malonyl CoA by means of Acetyl CoA carboxylase

Which step of lipogenesis requires NADPH?

NADPH is required in order to form FA palmitate by means of FAS

Name the two types of regulation in De Novo FA synthesis / Lipogenesis

1. Allosteric inhibition
2. Phosphorylatoin by insulin (activation), AMPK and others (inactivation)

How does insulin affect de novo FA synthesis / lipogenesis?

Activates Acetyl CoA Carboxylase and increases FA synthesis

How does metformin affect de novo FA synthesis / lipogenesis?

Reduces FA synthesis by activation of AMPK (ACC inactivation) and reduces hepatic glucose output by inhibiting gluconeogenesis by targeting mitochondrial glycerophosphate dehydrogenase (mGPD)

slide 17 diagram

What FA does FAS build up?

Palmitate (C16)

What components does FAS need to function?

8 AcetylCoA, 7 ATP, and 14 NADPH (mainly from HMP shunt)

What enzyme is used for FA elongation?

FA elongase

How many carbons can FA elongase add?

Up to 10 carbons

Where is FA elongase found?

Found in smooth ER

What two things are used in FA elongation?

Malonyl CoA and NADPH (reverse Beta-oxidation)

What is the purpose of FA desaturation?

To make mono and poly-unsaturated FA needed for the body through the desaturase system

What 3 components are required for the desaturation process?

Oxygen, NADH, and Cyto b3

What bonds cannot be desaturated?

The bonds between omega carbons for essential FA (linoleic acid and alpha linoleic FA)

For FA storage, what is the purpose of glycerol phosphate and where is it found?

Glycerol phosphate serves as a backbone for FA storage and it is found in the liver and adipose tissue

What enzyme is required in both the liver and adipose to create glycerol phosphate?

Glycerol-P dehydrogenase

Where do we get the largest portion of our FA?

From our diet

True/False: Excess proteins/carbs cannot be converted to FA and stored as TG

False. Excess proteins/carbs CAN be converted to FA and stored as TG

What gives the FA its amphipathic nature?

The anionic COO- group formed by ionization of the terminal carboxyl group at physiologic pH

How is mitochondrial acetyl CoA produced?

By oxidation of pyruvate, as well as catabolism of FAs, ketone bodies, and certain AA's

How is Acetyl CoA Carboxylase regulated?

Citrate positively regulates ACC
LCFA negatively regulates ACC
Insulin positively regulates ACC through phosphorylation

What are the two major steps needed for mobilization of stored fats and oxidation?

Release from TG and Beta-oxidation

What are the 3 steps needed in TG release? What hormones are utilized?

1. Homone-sensitive Lipase is activated
2. Glycerol backbone is recycled
3. Free FA is released

Epinephrine, glucagon, etc are used

What is the function of the carnitine shuttle?

To transport long-chain FA to mitochondria

What are the two steps in beta-oxidation relating to mobilization of stored FA?

Activation of carnitine shuttle
Beta-oxidation to ketogenesis

What does carnitine palmitoyl transferase 1 deficiency affect?

Affects the liver, leading to hypoglycemia, coma, and death

What does carnitine palmitoyl transferase 2 deficiency affect?

Affects muscle, leading to muscle weakness

What is the function of translocase in the carnitine shuttle?

Translocase (known as carnitine-acylcarnitine translocase) functions to push carnitine out of the inner mitochondrial matrix and pull acyl-carnitine into the inner mitchondrial matrix

What two defects are suspected in a patient with hypoketotic hypoglycemia or in an infant with acute fatty liver?

FA-oxidation and carnitine transport defects

How are long chain FA transferred across the outer mitchondrial membrane?

Carnitine palmitoyl transferase 1 (CPT1)

How are long chain FA transferred across the inner mitchondrial membrane?

Carnitine palmitoyl transferase 2 (CPT2)

How are long chain FA transferred from the inner to outer mitchondrial membrane?

Carnitine acyl-carnitine transferase (CACT)

Defects in Carnitine palmitoyl transferase 1 (CPT1), Carnitine palmitoyl transferase 2 (CPT2), or Carnitine acyl-carnitine transferase (CACT) can result in...

Defects in any of the enzymes can result in accumulation of unoxidized fatty acids, resulting in intracellular accumulation of fat and a variety of organ dysfunctions.

Which has the most sever clinical symptoms: long, medium, or short chain FA defects?

Long chain defects

Name 5 symptoms of Long Chain FA defects

Hypoketotic hypoglycemia, liver disease, skeletal myopathy with or without cardiomyopathy, sudden death, and pregnancy complications

True/False: Decreased Malonyl CoA inhibits CPT1

False! INCREASED Malonyl CoA inhibits CPT1

How does increased Malonyl CoA affect FA degradation?

Inhibits CPT1, thus FA degradation pathway is inhibited

What is the first step of FA shortening / Beta-oxidation?

Dehydrogenation by Acyl-CoA dehydrogenase

What carries out the last 3 steps of FA shortening / beta-oxidation?

A large trifunctional protein
(ie. hydratase, dehydrogenase, thiolase)

What are the effects of Medium-chain fatty acyl CoA dehydrogenase (MCAD) deficiency?

Hypoketotic hypoglycemia with accumulation of medium-chain FA to dicarboxylic acids in infancy

Alcohol detoxification needs...

NAD+

True/False: Too much alcohol stops...

Beta-oxidation

When do signs/symptoms appear in MCAD deficiency?

Infancy and early childhood

Name 3 symptoms of MCAD deficiency

Vomiting
Lack of energy (lethargy)
Low blood sugar (hypoglycemia)

How can problems related to MCAD deficiency be triggered?

Can be triggered by fasting or illnesses like viral infections

True/False: MCAD is not responsible for the initial dehydrogenation of acyl-CoAs with a chain length between four and 12 carbon atoms

False! MCAD is responsible for the initial dehydrogenation of acyl-CoAs with a chain length between four and 12 carbon atoms

Impaired Beta-oxidation in the liver leads to...

hypoglycemia

What kind of pathway is beta-oxidation?

Mitochondrial pathway

FA beta-oxidation breaks FA CoA into what?

Acetyl CoA, NADH, and FADH

Give a very basic description of ketogenesis

Low or no insulin
Hormone sensitive lipase (HPL) releases FA from adipose tissue
Adipose releases FA to Liver
Liver produces ketone bodies
Ketone bodies travel to brain, muscle, and urine

What starts off ketogenesis?

Occurs when there's a high rate of FA oxidation in the liver (high circulating FFA)

What 3 substances are produced during ketogenesis?

Ketone bodies
Acetoacetate (becomes acetone)
Hydroxybutyrate

Ketone bodies are produced in the liver and exported to...

Extra-hepatic tissues

What are the 3 regulatory steps of ketogenesis

1. FFA mobilization
2. Transport of FA in mitochondria (CPT1)
3. Partition of acetyl-CoA between TCA and Ketone

How does acetoacetate go to acetone?

It spontaneously goes to it and gives off CO2 in the process

How does acetoacetate go to 3-Hydroxybutyrate?

3-Hydroxybutyrate dehydrogenase and NADH

Look at slide 43

...

What does ketonuria urine smell like?

Fruity smell

True/False: Ketone accumulation leads to acidosis leads to coma

True

How does ketoacidosis come about in type I diabetes? (5 steps)

1. Low insulin and high glucagon
2. Increased lipolysis
3. Free FA in plasma
4. Hepatic output of ketone bodies
5. Ketoacidosis

In Diabetic Ketoacidosis and prolonged starvation, how is Oxaloacetic Acid affected

Depleted for gluconeogenesis

In alcoholism, excess NADH promotes conversion of...

OAA to malate

How does Type 1 Diabetes lead to hyperglycemia, ketonemia, and eventually Diabetic Ketoacidosis?

B/c insulin is not produced, insulin cannot inhibit lipolysis (by HSL) or gluconeogenesis. It also cannot stimulate FA synthesis. Thus, body is in prolonged starvation mode, pushing ketone body production into excessive levels. Prolonged ketonemia leads to acidic pH and thus diabetic ketoacidosis

What are 5 ways for ketosis to start?

Diabetes, diets high in fat, starvation, leads to ketone bodies accumulation, and then acidic ketone bodies lower blood pH below 7.4

How does FA oxidation in mitochondria lead to large quantities of ATP?

Beta-oxidation cleaves acetyl CoA units off fatty acyl chains. The acetyl-CoA is oxidized in the citric acid cycle, generating more ATP

Where are ketone bodies formed?

Ketone bodies are formed in hepatic mitochondria where there is a high rate of fatty acid oxidation

What are the 2 key enzymes in ketogenesis and what do they synthesize / break down

HMG-CoA synthase and HMG-CoA Lyase
They synthesize and breakdown 3-hydroxy-3-methyl- glutaryl-CoA (HMG-CoA)

True/False: Ketone bodies are important secondary energy sources in hepatic tissues

False! Ketone bodies are important secondary energy sources in EXTRA-hepatic tissues

What are the 3 steps at which ketogenesis is regulated?

1. Control of FA mobilization in adipose tissues
2. Activity of carnitine palmitoyl transferase 1 (CPT1) (RLE) that determines ration of FA flux that is oxidized
3. Partition of Acetyl CoA between pathways of ketogenesis and citric acid cycle

Diseases associated with impairment of fatty acid oxidation lead to what 3 things?

Hypoglycemia, fatty infiltration of organs, and hypoketonemia

True/False: Ketosis is severe in starvation but mild in diabetes mellitus

False! Ketosis is mild in starvation but severe in diabetes mellitus