Metabolism VI: Lipid Metabolism

Question 1

What is the function of HSTL (hormone-sensitive triacylglycerol lipase)?

Answer 1

It clips off fatty acids from triacylglycerols to begin mobilizing the stored fats.

Question 2

What does acetyl-CoA carboxylase catalyze?

Answer 2

the irreversible conversion of acetyl-CoA to fatty acyl-CoA’s to begin the process of synthesizing FAs and TAGs

Question 3

How do triacylglycerols travel in the bloodstream?

Answer 3

They are packaged into very low density lipoproteins (VLDLs) and/or chylomicrons.

Question 4

What are the 2 main breakdown products of triacylglycerols? What happens to these products during a fasted state?

Answer 4

• 2 products: fatty acids and glycerol
• In a fasted state, fatty acids escape into the bloodstream and are transported by albumin (and can then enter the liver for breakdown into ketone bodies, or to muscle tissue for energy), while glycerol enters the liver to feed directly into gluconeogenesis to produce glucose

Question 5

Fatty acid biosynthesis requires what?

Answer 5

• acetyl groups (transported by citrate from MT cytoplasm)
• acetyl-CoA carboxylase (rate-limiting enzyme)
• biotin (carrier of activated CO2)
• fatty acid synthase complex
• NADPH

Question 6

In fatty acid biosynthesis, all carbons are ultimately derived from ________.

Answer 6

acetyl-CoA (and most acetyl-CoA’s come from carbohydrate metabolism)

Question 7

How does citrate affect fatty acid biosynthesis?

Answer 7

It stimulates FA biosynthesis and allosterically activates acetyl-CoA carboxylase (rate-limiting enzyme).

Question 8

Citrate carries ________ from ________ to _________.

Answer 8

acetyl groups; mitochondria; cytoplasm

Question 9

Acetyl-CoA carboxylase catalyzes the formation of _______ from _______.

Answer 9

malonyl-CoA; acetyl-CoA

Question 10

What are the short-term activators vs. inhibitors of acetyl-CoA carboxylase ?

Answer 10

• activators: citrate (allosteric) and insulin (hormonal)

- inhibitors: palmitoyl-CoA and AMP (allosteric), glucagon and epinephrine (hormonal)

Question 11

What is the relevance of malonyl-CoA?

Answer 11

• product of the rxn catalyzed by acetyl-CoA carboxylase
• only physiological inhibitor of the rate-limiting enzyme in FA oxidation
• key regulator of FA oxidation and ketone body formation

Question 12

What are the long-term regulators of acetyl-CoA carboxylase?

Answer 12

• increased enzyme synthesis: high carb diet, insulin, thyroid hormone, fat-free diet
• decreased enzyme synthesis: high fat diet, fasting, glucagon

Question 13

The FAS complex catalyzes the formation of ________ from ______ + ________.

Answer 13

fatty acids (ex: palmitate); 1 acetyl-CoA; 7 malonyl-CoA

Question 14

What is the first enzyme involved in formation of a fatty acid?

Answer 14

transferase (acetyltransferase and malonyltransferase)

*the first step just involves transferring CoA

Question 15

After transferase activity, what are the main steps of fatty acid synthesis?

Answer 15

1) condensation
2) reduction
3) dehydration
4) reduction
(then repeat x6 to synthesize palmitoyl-ACP)

Question 16

What are some of the fates of fatty acids following fatty acid synthesis?

Answer 16

• can be elongated or desaturated

- they are ultimately esterified to TAGs, phospholipids, cholesterol, etc.

Question 17

Mammals must obtain some polyunsaturated fatty acids (essential fatty acids) from which source?

Answer 17

diet

Question 18

Endogenous synthesis of TAGs is regulated primarily by what?

Answer 18

substrate availability (peroxisome proliferator-activated receptors, or PPARs, promote TAG deposition)

Question 19

How are dietary TAGs and synthesized TAGs transported in the circulation?

Answer 19

they are packaged into lipoproteins and transported that way