Ch 11 - Lipid and Amino Acid Metabolism

Question 1

Where does mechanical digestion of lipids occur?

Answer 1

primarily in the mouth and stomach

Question 2

Where does chemical digestion of lipids occur and what is it facilitated by?

Answer 2

the small intestines

- facilitated by bile, pancreatic lipase, colipase, and cholesterol esterase

Question 3

Why do digested lipids form micelles?

Answer 3

for absorption or to be absorbed directly

Question 4

Where are short chain fatty acids absorbed?

Answer 4

across the intestine into the blood

Question 5

Where are long chain fatty acids absorbed?

Answer 5

as micelles and assembled into chylomicrons for release into the lymphatic system

Question 6

How are lipids mobilized from adipocytes? from lipoproteins?

Answer 6

• hormone sensitive lipase

- lipoprotein lipase

Question 7

What are chylomicrons and how are they transported?

Answer 7

the transport mechanism for dietary triacylglycerol molecules and are transported via the lymphatic system

Question 8

What do VLDL do?

Answer 8

transports newly synthesized triacylglycerol molecules from the liver to peripheral tissues in the bloodstream

Question 9

What is IDL?

Answer 9

• a VLDL remnant in transition between triacylglycerol and cholesterol transport
• picks up cholesteryl esters from HDL

Question 10

What does LDL transport?

Answer 10

primarily cholesterol for use by tissues

Question 11

What is HDL involved in?

Answer 11

involved in the reverse transport of cholesterol

Question 12

What do apoproteins control?

Answer 12

interactions between lipoproteins

Question 13

How is cholesterol obtained?

Answer 13

through dietary sources or through de novo synthesis in the liver

Question 14

What is the key enzyme in cholesterol biosynthesis?

Answer 14

HMG-CoA reductase

- inhibition of this enzyme lowers production of cholesterol

Question 15

What does LCAT catalyze?

Answer 15

the formation of cholesteryl esters for transport with HDL

Question 16

What does CETP catalyze?

Answer 16

the transition of IDL to LDL by transferring cholesteryl esters from HDL

Question 17

What are fatty acids?

Answer 17

carboxylic acids, typically with a single long chain, although they can be branched

Question 18

What is the difference between saturated and unsaturated fatty acids?

Answer 18

• saturated have no double bonds between carbons

- unsaturated have one or more double bonds

Question 19

Where are fatty acids synthesized?

Answer 19

in the cytoplasm from acetyl-CoA transported out of the mitochondria

Question 20

What are the 5 steps of fatty acid synthesis?

Answer 20

• activation (attach to acyl carrier protein), bond formation, reduction (of carbonyl group), dehydration, and a second reduction (of double bond)

Question 21

What is the only fatty acid that humans can synthesize and how is it synthesized?

Answer 21

palmitic acid, formed after 8 repeats of synthesis

• fully saturated, so with no double bonds
• ha 16 carbons and is synthesized from 8 molecules of acetyl-CoA (16:0)

Question 22

Where does fatty acid oxidation occur?

Answer 22

in the mitochondria following transport by the carnitine shuttle

Question 23

What are the steps of beta oxidation?

Answer 23

occurs in mitochondria

• oxidation of the FA to form a double bond
• hydration of the double bond to form a hydroxyl group
• oxidation of the hydroxyl group to form a carbonyl (beta-ketoacid)
• cleavage of the beta-ketoacid into a shorter acyl-CoA and acetyl-CoA

Question 24

How does beta oxidation of unsaturated fatty acids differ from that of saturated fatty acids?

Answer 24

an additional isomerase and an additional reductase for unsaturated that provides the stereochemistry needed for further oxidation

Question 25

When do ketone bodies (ketogenesis) form?

Answer 25

during a prolonged starvation state due to excess acetyl-CoA in the liver

Question 26

What does ketolysis do?

Answer 26

regenerates acetyl-CoA for use as an energy source in peripheral tissues

Question 27

Where does the brain derive its energy from during prolonged starvation?

Answer 27

can derive up to 2/3rds of its energy from ketone bodies

Question 28

Where does protein digestion occur?

Answer 28

primarily in small intestines

Question 29

When does catabolism of cellular proteins occur?

Answer 29

only under conditions of starvation

Question 30

What are carbon skeletons of amino acids used for in protein catabolism?

Answer 30

• energy, either through gluconeogenesis or ketone body formation
• amino groups are fed into the urea cycle for excretion
• the fate of a side chain depends on its chemistry

Question 31

When lipids leave the stomach, what stages of digestion have been accomplished? What enzymes are added to accomplish the next phase?

Answer 31

• physical digestion is accomplished in the mouth and the stomach, reducing the particle size
• beginning in the small intestine, pancreatic lipase, colipase, cholesterol esterase, and bile assist in the chemical digestion of lipids
• in the more distal portion of the small intestine, absorption occurs

Question 32

Do all lipids enter the circulation through the lymphatic system?

Answer 32

No, small free fatty acids enter the circulation directly

Question 33

Describe the structure of a micelle.

Answer 33

• micelles are collections of lipids with their hydrophobic ends oriented toward the center and their charged ends oriented towards the aqueous environment
• micelles collect lipids within their hydrophobic centers

Question 34

What is the expected impact on a patient’s weight when started on insulin injections for management of blood glucose levels?

Answer 34

an increase in insulin levels will increase lipid storage and decrease lipid mobilization from adipocytes, leading to weight gain in diabetic patients who being insulin injections

Question 35

What is the ratio of free fatty acids to glycerol produced through lipid mobilization?

Answer 35

3:1, a triacylglycerol molecule is composed of glycerol and 3 FA

Question 36

What is the primary method of transporting free fatty acids into the blood?

Answer 36

free fatty acids remain in the blood, bonded to albumin and other carrier proteins
- a much smaller amount will remain unbonded

Question 37

Order the lipoproteins from greater percentage of protein to lease percentage of protein. Which are primarily involved in triacylglycerol transport?

Answer 37

HDL > LDL > IDL > VLDL > chylomicrons

• VLDL and chylomicrons are the primary triacylglycerol transporters
• HDL and LDL are mostly involved in cholesterol transport

Question 38

Lipoproteins are synthesized primarily by which 2 organs?

Answer 38

intestine and liver

Question 39

When physicians order a lipid panel to evaluate a patient, which value do they prefer to see over minimum threshold rather than below a maximum?

Answer 39

HDL is often considered the “good” cholesterol because it picks up excess cholesterol from blood vessels for excretion, so its values are checked for being over a minimum value

Question 40

Under what conditions is HMG-CoA reductase most active? In what cellular region does it exist?

Answer 40

• HMG-CoA reductase is most active in the absence of cholesterol and when stimulated by insulin
• cholesterol reduces the activity of HMG-CoA reductase, which is localized in the smooth endoplasmic reticulum

Question 41

What proteins are specific to the formation and transmission of cholesteryl esters and what are their functions?

Answer 41

LCAT catalyzes the esterification of cholesterol to form cholesteryl esters
- CETP promotes the transfer of cholesteryl esters from HDL to IDL, forming LDL

Question 42

How does fatty acid synthesis compare to beta oxidation?

Answer 42

• they are reverse processes
• both involve transport across the mitochondrial membrane, followed by a series of redox reactions, but always in the opposite direction of one another

Question 43

Where are fatty acids synthesized and how are they modified?

Answer 43

• synthesized in the cytoplasm

- modified by enzymes in the smooth endoplasmic reticulum

Question 44

Why are fatty acids used to create ketone bodies instead of creating glucose?

Answer 44

• fatty acid degradation results in large amounts of acetyl-CoA, which cannot enter the gluconeogenic pathway to produce glucose
• only odd-numbered fatty acids can act as a source of carbon for gluconeogenesis; even then, only the final malonyl-CoA molecule can be used
• energy is packaged into ketone bodies for consumption by the brain and muslces

Question 45

What conditions and tissues favor ketogenesis? ketolysis?

Answer 45

• ketogenesis is favored by a prolonged fast and occurs in the liver; stimulated by increasing concentrations of acetyl-CoA
• ketolysis favored during prolonged fast, but is stimulated by low energy state in muscle and brain tissues and does not occur in the liver

Question 46

Are bodily proteins commonly broken down to provide acetyl-CoA for lipid synthesis?

Answer 46

No, proteins are more valuable to the cell than lipids, thus they will not commonly be broken down for lipid synthesis

Question 47

Where does the bulk of protein digestion occur?

Answer 47

small intestine

Question 48

During protein processing, what is the eventual fate the carbon skeleton, amino group, and side chains?

Answer 48

• carbon skeleton is transported to the liver for processing into glucose or ketone bodies
• amino group will feed into the urea cycle for excretion
• side chains are processed depending on their composition (basic side chains will be processed like amino groups, while other functional groups will be treated like the carbon skeleton)

Question 49

What happens during fatty acid mobilization?

Answer 49

• there is a breakdown of triacylglycerols in adipocytes by hormone-sensitive lipase (HSL)
• this breakdown results in the release of 3 FA and a glycerol molecule
• the glycerol may be used by the liver for gluconeogenesis, but adipocytes do not have the ability to carry out gluconeogenesis

Question 50

Where does the majority of triacylglycerols stored in adipocytes originate from?

Answer 50

synthesis in liver

• the liver is the major metabolic organ in the body and is responsible for much of the synthesis and interconversion of fuel sources
• most triacylglycerols that are synthesized in the liver are transported as VLDL to adipose tissue for storage