Metabolism 6

Question 1

Draw out an overview of lipid metabolism starting with glucose to acetyl-CoA.

Which paths will insulin promote?

Answer 1

p 4
FED:

In the fed state, insulin drives synthesis of cholesterol and fatty acyl-CoA’s.

Question 2

In the fed state which enzymes will be utilized? Which inhibited?

Answer 2

Fed state: use HMG-CoA reductase to synthesize cholesterol from Acetyl-CoA

use acetyl-CoA carboxylase to syn. fatty acyl CoA’s

inhibit CPT-1/CAT-1 from backwards path of fatty acyl-CoA back to acetyl-CoA.

In the fed state, insulin drives synthesis of cholesterol and fatty acyl-CoA’s.

Question 3

In the fasting state, what hormone will act? Which enzymes are utilized to promote which pathways?

Answer 3

FASTING:
In the fasting state, epinephrine drives the formation of acetyl-CoA and ketone bodies.

-succinyl-CoA transferase (absent in liver) to synthesize ketone bodies from acetyl-CoA

use camitine palmitoyltransferase I (CPT1/CAT1) to change fatty acyl-CoA’s to acetyl-CoA.

inhibit acetyl-CoA carboxylase.

Question 4

Describe inter-organ transport of fatty acids in the fed state. How are FA transported in bloodstream?

(small intestine, liver, adipose tissue, muscle)

Answer 4

FA transported as part of TAGs on lipoproteins

p 6, Slide 7/8

Question 5

Describe inter-organ transport of fatty acids in the fasted state. How are FA transported in bloodstream?

(small intestine, liver, adipose tissue, muscle)

Answer 5

FA transported bound to albumin.

p 6, slide 7/8

Question 6

What are the important fatty acids?

Answer 6

Slide 9, p 7

Question 7

What is the equation for fatty acid biosynthesis?

Answer 7

Acetyl-CoA + 7 malonyl-CoA + 14 NADPH + 14H+ —–>
Palmitate + 7 CO2 + 14 NADP+ + 8 CoA + 6 H2O

Note: Malonyl-CoA is a coenzyme A derivative of malonic acid.

Question 8

Where must acetyl groups be in order for fatty acid biosynthesis to take place? What enzyme is necessary for this process?

Answer 8

Acetyl groups must be transported from the
mitochondria into the cytoplasm. Citrate is the
carrier; it also activates acetyl-CoA carboxylase.

Question 9

Describe the role of acetyl-CoA carboxylase in fatty acid biosynthesis.

Answer 9

Acetyl groups must be transported from the
mitochondria into the cytoplasm. Citrate is the
carrier; it also activates acetyl-CoA carboxylase.

Acetyl-CoA Carboxylase, the rate-limiting enzyme,
catalyzes the formation of malonyl-CoA.

Question 10

Describe the fatty acid synthase complex. What does it do?

Answer 10

Fatty Acid Synthase Complex, a multifunctional
enzyme complex, utilizes 1 acetyl-CoA and
7 malonyl-CoA to synthesize even-numbered,
saturated fatty acids (i.e., palmitate).

Question 11

What is the role of NADPH in fatty acid biosynthesis?

Answer 11

NADPH is required; it is generated by the pentose
phosphate pathway & the cytoplasmic reaction
malate —-> pyruvate.

Question 12

What does biotin do?

Answer 12

Biotin is the carrier of “activated” CO2.

Question 13

Where are all carbons derived from?

Answer 13

All carbons are derived (ultimately) from acetyl-CoA.

Question 14

Where are most acetyl-CoA’s derived from?

Answer 14

Most acetyl-CoA’s derive from carbohydrate metabolism.

Question 15

Describe 3 roles of citrate..

Answer 15

• Stimulates fatty acid synthesis
• transports acetyl groups from mitochondria
• allosterically activates acetyl-CoA carboxylase

Question 16

Describe Acetyl CoA carboxylase.

It’s role, its cofactor…

Answer 16

• Is the rate-limiting enzyme in fatty acid biosynthesis.
• Catalyzes the formation of Malonyl-CoA from Acetyl CoA
• Biotin, a cofactor, serves as a carrier of “activated” CO2.

The rate-limiting step in fatty acid biosynthesis is catalyzed by the enzyme
acetyl-CoA carboxylase. The cofactor biotin serves as a carrier of “activtated”
CO2.

Question 17

Draw out the citrate shuttle (regeneration of pyruvate).

Answer 17

p 9

Slide 15

Question 18

Describe the regulation of acetyl-CoA carboxylase.

(What reaction does it catalyze?)

What hormones and factors will promote the reaction, which inhibit?

Answer 18

acetyl CoA + HCO3- + ATP. —> malonyl CoA + H2O + ADP + Pi

Acetyl CoA carboxylase moves this reaction forward.

• This is an irreversible reaction.
• AcetylCoA carboxylation is a rate-limiting step of FA biosynthesis
• AcetylCoA carboxylase is under allosteric regulation.

citrate and insulin and biotin promote

palmitoyl CoA, glucagon and epinephrine inhibit

Slide 17
p 10

Question 19

Describe the role of malonyl CoA.

Answer 19

Note: The product of the reaction, malonyl CoA, plays a critical role in the regulation (inhibition) of fatty acid degradation (Beta-oxidation) and ketone body formation.

• Is the product of the above reaction, catalyzed by Acetyl CoA Carboxylase.
• Is the only physiological inhibitor of rate-limiting enzyme in fatty acid oxidation.
• Is the key regulator of fatty acid oxidation and ketone body formation.

Question 20

In the short term, describe the following regulators of Acetyl-CoA carboxylase (ACC):

Allosteric activator:

Allosteric inhibitor:

Hormonal activator:

Hormonal inhibitor:

Answer 20

Allosteric activator: citrate

Allosteric inhibitor: palmitoyl-CoA (long chain C16-C18) fatty acyl CoA) AMP

Hormonal activator: insulin (dephosphorylation of ACC)

Hormonal inhibitor: (covalent modification- PKA-mediated phosphorylation of ACC)
glucagon, epinephrine

Question 21

In the long term, describe what factors will increase or decrease enzyme synthesis.

Answer 21

Increased Enzyme Synthesis:

• high carbohydrate
• insulin
• thyroid hormone

Decreased enzyme synthesis:

• high fat diet (via low insulin and high glucagon)
• fasting glucagon

Question 22

The fatty acid synthase complex catalyzes the formation of fatty acids (e.g.
palmitate, 16C). The seven enzyme activities of the fatty acid synthase
complex are found in one protein.

Describe what happens in the first cycle, and each remaining cycle.

Answer 22

In the first cycle, acetyl-CoA combines with a 2-carbon acetyl group (derived from
malonyl-CoA) to ultimately yield the 4-carbon butyryl group.

In each of the remaining six
cycles, a 2-carbon acetyl group is transferred from malonyl-CoA to a growing fatty acid chain.

After 7 cycles of reactions catalyzed by the fatty acid synthase complex (the first reaction + 6 more cycles), a 16-carbon saturated fatty acid (palmitate) is formed.

Question 23

Show the stoichiometry of the reaction of the formation of palmitate.

Answer 23

Acetyl-CoA (2 C) + 7 malonyl-CoA (21 C) + 14 NADPH + 14 H+ —>

Palmitate (16 C) + 7 CO2
+ 14 NADP+ + 8 CoA + 6 H2O

Question 24

What are the major reactions that fatty acid synthase complex synthesizes? (6)

Answer 24

Slide 27

p 14

Question 25

What is the short term allosteric activator of fatty acid synthase (FAS) complex?

Answer 25

fructose-1,6-bisphosphate

Question 26

In the long term, what will increase FAS activity?

Answer 26

• high carbohydrate diet via insulin

- fat free diet

Question 27

In the long term, what will decrease FAS enzyme synthesis?

Answer 27

glucagon, high fat diet via low insulin and high glucagon

Question 28

Mammals must obtain some polyunsaturated fatty acids (essential fatty acids) from dietary sources.

Some polyunsaturated fatty acids must be obtained from dietary sources; they
are called “essential fatty acids.”

Show the positions on the fatty acid chain where desaturation can occur in the human.

Answer 28

essential linoleate ?
alpha linolenate ?

p 15, slide 29

Question 29

Finish:

Following fatty acid synthesis fatty acids may be…. (2 things).

Ultimately they are esterified to ….(3 things).

Answer 29

elongated, desaturated

ultimately esterified to triacylglycerols, phospholipids, or cholesterol

Slide 30, p 16

Palmitate and ATP to Palmitoyl-CoA and AMP… which can undergo esterification to acylglycerols-TAGS, cholesterol esters

or chain elongation desaturation to acyl-CoA

Question 30

How is the endogenous synthesis of TAGS primarily regulated?

What promotes TAG deposition?

Answer 30

Endogenous synthesis of TAGS

• Regulated primarily by substrate availability
• PPARs (peroxisome proliferator-activated receptors) promote TAG deposition.

Question 31

Describe structure of the following acyl-glycerols:

triacylglycerol
diacylglycerol
1-monoacylglycerol
2-monoacylglycerol
glycerol

Answer 31

p 17

slide 33

Question 32

How are fatty acids stored? How are they transported in circulation? In Liver? In intestines?

Answer 32

Both dietary triacylglycerols and triacylglycerols synthesized in the body are packaged into
lipoproteins for transport in the circulation.

In liver synthesized TAG are packaged into VLDL.

In the intestines, dietary triacylglycerols are packaged into chylomicrons

Question 33

In liver synthesized TAG are packaged into VLDL.

Show the mechanism and pathway for this.

Answer 33

Slide 34

p 17

Question 34

How is most glycerol-3- P generated?

Answer 34

Most glycerol 3-phosphate is generated via glyceroneogenesis rather than via glycolysis. In this
process, gluconeogenic precursors (lactate, pyruvate, alanine) are converted first to
dihydroxyacteone phosphate (DHAP). Some DHAP is then reduced to glycerol 3-P for the synthesis of TAGs..

Question 35

In the intestines, dietary triacylglycerols are packaged into chylomicrons.

Show the mechanism and pathway for this.

Answer 35

Slide 35

p 18