Biochem revision - Week 7 Lipid Metabolism in Exercise

Question 1

What are the different types of lipids?

Answer 1

1. Fatty acids and Triacylglycerols
2. Steroids - help in cell signaling and membrane function (in the form of sterols)
3. Phospholipids - form membranes (present at the interface btw lipid and water)

Question 2

How do lipids differ from carbs?

Answer 2

a) They are hydrophobic (polar heads of phospholipids)

b) They can be saturated or unsaturated (single or double bonds, respectively)

Question 3

What is the make-up of:

a) Fatty acids
b) Phospholipids
c) Triacylglcerols

Answer 3

a) Fatty acids are made up of:
A hydrocarbon chain
A acyl group

b) Phospholipids are made up of:
1 glycerol connected
2 fatty acids
1 phosphoric group
connected via ester bonds

c) Triacylglycerols consist of:
3 fatty acids
1 glycerol
connected via ester bonds

Question 4

What are the majority of dietary fats in the body?

Answer 4

Triacylglycerols make up 90-95% of lipids from dietary fats

They serve as E stores in adipose tissue and in muscle

They cannot pass through the cells of the gastrointestinal wall and therefore must be broken down

Question 5

How are triacylglycerols processed upon ingestion?

Answer 5

1. Upon ingestion, large lipid droplets are reduced to fine lipid droplets by bile acids
2. Upon reaching the intestinal lumen, pancreatic lipase hydrolyses ester bonds to yield;
   2 fatty acids and 2-monoacylglycerol
3. These products freely pass through into the intestinal cells and TAG is reformed
4. TAGs are packaged into chylomicrons and exported from the enterocytes into the lymph nodes and then the blood where they are distributed

Question 6

What is a chylomicron?

Answer 6

A class of lipoprotein with a single layer of phospholipids and a hydrophobic core

They are used to transport insoluble TAGs

Question 7

What is the basic structure and purpose of an adipocyte?

Answer 7

An adipocyte’s cytoplasm is 80% dominated by a large lipid droplet filled with triacylglycerol.

Adipocytes serve as a huge E reserve utilized by working muscle and can be present around internal organs or under the skin

Question 8

What process is responsible for fatty acid/TAG degradation during exercise in muscle and adipose tissue surrounding it?

Answer 8

Beta (B)-oxidation

Question 9

What are the three enzymes that conduct TAG lipolysis? Where are these enzymes located?

Answer 9

1. Adipose TAG Lipase (converts TAG into Diacylglycerol)
2. Hormone sensitive lipase (converts Diacylglycerol into monoacylglycerol)
3. Monoacylglycerol acyltransferase (Converts monoacylglycerol into glycerol)

*These enzymes are only all located in type-1 fibers, making lipolysis optimal during aerobic exercise

Question 10

What happens to lipolysis during exercise?

Answer 10

During the first 5-10 mins of exercise onset, lipolysis in adipose tissue increases. TAG lipolysis also increases in skeletal muscle

Question 11

What hormones influence lipolysis in adipose tissue and how?

Answer 11

1. Epinephrine: The B-adrenergic pathway increases lipolysis and overrides other factors
2. Epinephrine: The a-adrenergic pathway decreases lipolysis
3. Insulin reduces lipolysis

*Increases in lipolysis drive the cAMP pathway that favorably activates the 3 enzymes that breakdown TAGs

Question 12

What happens to lipolysis during high intensity exercise?

Answer 12

Lipolysis can be inhibited due to the binding of epinephrine to the a-adrenergic pathway and and the stable or increased [insulin].

These processes in turn suppress the cAMP pathway to inhibit the 3 enzymes involved in TAG hydrolysis

Question 13

What happens to the products of lipolysis during exercise?

Answer 13

FFA and glycerol in adipose tissue are transported via Albumin in the blood after leaving the tissue

Muscle takes up FFA and uses it in B-oxidation/ATP provision

Glycerol is taken up by the liver and used in gluconeogenesis and some FFA may be taken up to re-synthesize TAG

Question 14

Following lipolysis or arrival of FFAs in the muscle cell, what rctns take place in order for acetyl coA to be used in oxidation in the mitochondria?

Answer 14

1. FFAs (acyls) are activated by a rctn with CoA forming acyl CoA in the cytosol (by the action of carnitine acyltransferase
   I) and can then pass the outer mitochondrial membrane
2. Carnitine then binds to the acyl group forming Acylcarnitine (through the action of carnitine acylcarnitine translocase) and passes through the inner mitochondrial membrane into the mitochondrial matrix
3. Acyl CoA is then reformed in the matrix by the action of the enzyme carnitine acyltransferase
   II
4. Acyl CoA then enters the B-Oxidation pathway

Question 15

What are the reactants and products of B-oxidation? Where do they products go?

Answer 15

Reactants:
Acyl CoA

Products:
1xFADH2
2xNADH
Acetyl CoA
Acyl-CoA (minus 2 carbons)

*All products except for Acyl-CoA enter the citric acid/TCA cycle. Acetyl CoA cycles back to the B-oxidation pathway again until it is degraded to a 2 carbon molecule (Acetyl CoA)

Question 16

Is B-oxidation energetically favored?

Answer 16

No. Because ATP is required to bind the acyl groups to CoA in the cytosol in order for the molecule begin the process of entering the mitochondria, the initial rctn is not favored

Question 17

What factors during exercise speeds up fatty acid oxidation in muscle?

Answer 17

Exercise stimulates lipolysis in adipose and skeletal muscle by:

1) Increasing blood flow to active muscle thus increasing FFA flow

2) Enhancing translocation of fatty acid binding protein at the plasma membrane (FABP-PM) for FFA uptake into muscle

3) [Epinephrine] binds to the B-adrenergic receptor located in the plasma membrane of adipocytes which increases the activity of AMP-activated protein kinase which phosphorylates three lipases that breakdown TAG in the cytosol

Question 18

How does exercise intensity change whether carb or fat oxidation occurs?

Answer 18

As the exercise intensity increases carbohydrate metabolism increases. It increases exponentially after 60% VO2 max

Lipid metabolism increases as exercise intensity increases until it drops off at 60% VO2 max

*At lower intensities lipid oxidation > carb oxidation

Question 19

What is the Max Fat Oxidation/Fat max?

Answer 19

This is the point around 55% VO2max that fat oxidation is at its maximum and then its contribution drops off shortly thereafter

Question 20

Is it possible to convert glucose to fatty acids and vice versa?

Answer 20

It is possible to convert glucose to fatty acids through the intermediate of glycolysis - glycerol-3-phosphate- although this is quite minimal

It is not possible to covert fatty acids into glucose from acetyl CoA because it loses its only carbons to CO2 in the TCA cycle

Question 21

What is an apolipoprotein?

Answer 21

An apolipoprotein is a protein embedded in the membrane of the lipoprotein which act as recognition/signal sites that allow for interaction btw other lipoproteins, enzymes and receptors

Question 22

What is a lipoprotein?

Answer 22

Lipoproteins allow the transport of lipids other than fatty acids (which employ albumin) in plasma for delivery to tissues.

Question 23

How are lipoproteins classed and what are the four classes?

Answer 23

Lipoproteins are classed by density, that is, the proportion of lipid to apoprotein mass. The more apoprotein, the higher the density.

The major lipoprotein classes, in order
of increasing density, are:
Chylomicrons = 0.92-0.94 g/ml
Very low-density lipoproteins (VLDL) = 0.96-1.00 g/ml
Low-density lipoproteins (LDL) = 1.02-1.06 g/ml
High-density lipoproteins (HDL) 1.06-1.21 g/ml

Question 24

What are the basic roles of the classes of lipoprotein?

Answer 24

Chylomicrons - carry dietary fat to extrahepatic tissues (outside the liver) and then remnants are taken up by liver

VLDL - deliver hepatic triacylglycerols to extrahepatic tissues i.e remnants of chylomicrons

LDL - main carriers of cholesterol that is delivered to cells

HDL - deliver cholesterol esters back to the liver collected from circulating dead cells and restructured membranes

Question 25

How does acute and chronic exercise affect plasma lipids?

Answer 25

Acute:

Acute exercise can suppress post-prandial (after meal) plasma lipid profiles

Chronic:

Chronic exercise can decrease the lipid profile of VLDL and LDL, but can increase the HDL profile (which is considered ‘good cholesterol’ because it brings cholesterol to liver to be broken down instead of to other tissues)

Question 26

What can chronic positive energy balance cause in adipocytes?

Answer 26

• Adipocyte tissue can become inflamed
• Blood flow is disturbed/blockages in blood vessels
• Adipocytes can rupture
• Increased disease risk

Question 27

What are some factors that contribute to obesity?

Answer 27

• Food intake
• E expenditure
• Low PA

Question 28

What is adiponectin? What factor can increase it?

Answer 28

Adiponectin is a hormone released from the adipocytes that lowers inflammation, can increase lipid oxidation and insulin sensitivity

Exercise can increase adiponectin levels

Question 29

What does a healthy lipidemic profile look like?

Answer 29

Low LDL, high HDL