Kaplan Ch. 11 - Lipid And Amino Acid Metabolism

Question 1

Describe the general steps of lipid digestion and absorption.

Answer 1

Lipid digestion begins to a small extent in the mouth with salivary lipases. But this is very minimal and most fats reach the small intestine intact. Once in the small intestine, the fats are emulsified in the duodenum (this process is aided by bile salts). Then pancreatic lipases break down the fats into free fatty acids and cholesterol.

Next, the fatty acids and cholesterol form micelles and travel through the remainder of the intestine in this form. They also contain any fat soluble compounds like vitamins. Fat that does not incorporate into micelles is excreted in stool.

Micelles diffuse through brush border cells of intestinal mucosa where they are converted back into triacylglycerols and cholesteryl esters. These are packages into chylomicrons which leave the intestines via lacteals.

Question 2

Hormone sensitive lipase

1) what is it?
2) what stimulates it?

Answer 2

1) enzyme that hydrolyzes triacylglycerols to free fatty acids and glycerol
2) low insulin, high cortisol, high epinephrine

Question 3

Once HSL breaks TAGs into glycerol and free fatty acids, what is the pathway of the glycerol? Of the free fatty acids?

Answer 3

Glycerol is transported to the liver to make glucose via gluconeogenesis or to make pyruvate via glycolysis

Free fatty acids are bound to albumin and transported to liver to undergo beta oxyidation to yield acetyl co A, which then enters the citric acid cycle or is made into ketone bodies for energy

Question 4

Lipoprotein lipase

1) what is it?

Answer 4

1) enzyme that breaks down TAGs in chylomicrons and VLDL

Question 5

How do free fatty acids travel through the body?

How do TAGs and cholesterol travel?

Answer 5

In the blood, bound to albumin

In lipoproteins

Question 6

List the types of lipoproteins from lowest density to highest density.

Answer 6

Chylomicrons, VLDL, IDL, LDL, HDL

Question 7

What is the function of chylomicrons?

Answer 7

Transport cholesterol and TAGs from intestines to tissues

Question 8

What is the function of VLDL?

Answer 8

Transport TAGs and fatty acids from liver to tissues

Question 9

How is an IDL formed?

Answer 9

Once TAGs are removed from VLDL, it becomes and IDL

Question 10

What is the purpose of LDL?

Answer 10

Delivers cholesterol to cells

Question 11

What is the function of HDL?

Answer 11

Picks up cholesterol accumulating in blood vessels and delivers to liver.

Question 12

What is an apolipoprotein?

Answer 12

They form the protein component of lipoproteins. They are receptor molecules and are involved in signaling.

Question 13

Most cells receive cholesterol from LDL or HDL. But how else can cholesterol be made?

Answer 13

De novo - happens in liver cells, driven by acetyl co A and ATP.

Question 14

What is lectithin cholesterol acyltransferase?

Answer 14

An enzyme found in the blood stream that is activated by HDL and adds a fatty acid to cholesterol so it can be distributed to other lipoproteins

Question 15

If a fatty acid is listed as (18:2 cis, cis-9,12) what does that mean?

Answer 15

18 carbons in the fatty acid, 2 double bonds, both are in the cis configuration and one is between carbons 9 and 10 and another is between carbons 12 and 13

Question 16

What are 2 important unsaturated fatty acids to know? What are their functions?

Answer 16

1) alpha linolenic acid
2) linoleic acid

They are important in maintaining cell membrane fluidity

Question 17

Fatty acids used by body for fuel are supplied by diet mostly. What is the other way they can be supplied?

Answer 17

Excess carbohydrates and proteins ingested can be converted to fatty acids and stored.

Question 18

Where does fatty acid synthesis occur?

Where are it’s products transported?

Answer 18

The liver

Adipose tissue

Question 19

Which hormone stimulates the enzymes of fatty acid synthesis?

Answer 19

Insulin

Question 20

What is the primary end product of fatty acid synthesis?

Answer 20

Palmitic acid

Question 21

What starts the process of fatty acid synthesis?

Answer 21

Acetyl co-A is made from pyruvate. Following a large meal, when the energy needs of the cell have been met, there is a build up of acetyl CoA in the mitochondria because the kreb’s cycle slows down with increasing ATP production and so acetyl co A does not enter the cycle. The acetyl CoA can then be combined with OAA to form citrate and shuttles into the cytoplasm. There, the citrate is cleaved back into acetyl co A and OAA by citrate lyase.

Question 22

Acetyl CoA carboxylase

1) what is important about this enzyme?
2) what 2 molecules does it require to function?
3) what is it activated by?
4) what is it’s product

Answer 22

1) it is the rate limiting step of fatty acid synthesis
2) ATP and biotin
3) insulin and citrate
4) Malonyl CoA

Question 23

Fatty acid synthase

1) what is the ONLY fatty acid that humans can synthesize on our own?
2) what hormone stimulates this enzyme?
3) what 2 molecules does this enzyme require?
4) what is the product?

Answer 23

1) palmitate (16 carbon saturated FA)
2) insulin
3) vitamin B5 and NADPH
4) palmitate

Question 24

The first step in beta oxidation is activation of the fatty acid. Describe this step.

Answer 24

Fatty acids are in the cytoplasm following entry into the cell. Small FAs can diffuse directly across mitochondria membrane, large FAs cannot. Large FAs are attached to CoA and that activates the FA.

Question 25

Once activated, FAs can enter the mitochondria. Describe this process.

Answer 25

FA attaches to CoA can pass through the outer membrane but not the inner membrane. To get into the mitochondrial matrix, the FA is attached to carnitine. Then the FA-carnitine can pass through a carnitine transporter to the matrix. THIS IS THE RATE LIMITING STEP OF BETA OXIDATION.

Question 26

Once in the matrix, beta oxidation can occur. Describe this process.

Answer 26

2 carbons are removed from the FA to make acetyl CoA. In the process, NAD+ and FAD are reduced to NADH and FADH2 which enter the ETC to produce ATP.

Question 27

Where are ketone bodies produced? Where are ketone bodies metabolized? How do ketone bodies get from their production site to their consumption site?

Answer 27

Produced in mitochondrial matrix of liver cells Consumed in the mitochondrial matrix of renal cells, muscle cells and the brain (during prolonged fasts of 1+ week) Ketone bodies are soluble in blood so they travel via the blood stream

Question 28

Describe the process of how ketone bodies are made.

Answer 28

In fasting state, liver converts excess acetyl co A (from beta oxidation of fatty acids) to acetoacetate and 3-hydroxybutarate. These travel via blood to target tissues. At target tissues, 3-HBT is converted back to acetoacetate and acetoacetate is converted to 2 molecules of acetyl co-A which enter the kreb’s cycle

Question 29

Where is bodily protein primarily catabolized?

Answer 29

The liver and muscle

Question 30

What mode of transport is used to move amino acids from the small intestine into the epithelial cells of lining? To move AAs from epithelial lining to blood?

Answer 30

Secondary active transport coupled with sodium transport Simple and facilitated diffusion

Question 31

Amino acids can be either ____genic or ____genic.

Answer 31

Glucogenic (carbon skeletons can be converted into glucose through gluconeogenesis) Ketogenic (can be converted into acetyl co A and ketone bodies)

Question 32

What happens to the amino end of AAs?

Answer 32

They are removed via deamination/transamination. Nitrogen containing compounds are potentially toxic because they can form ammonia, so the amino group is made into urea via the urea cycle in the liver. The urea is excreted in urine.