Biosynthesis of Fatty Acids

Question 1

What is the starting material for fatty-acid synthesis?

Answer 1

Acetyl co-a (in mitochondria), which itself is mostly from pyruvate

Question 2

True or false: acetyl co-a is only found in mitochondria.

Answer 2

False. Citrate from the TCA cycle can pass through the mitochondrial membranes into the cytosol. It is then broken down to oxaloacetate and acetyl co-a by ATP cirate lyase; the oxaloacetate passes back into the mitochondria once converted back to pyruvate (via malate).

Question 3

In the cytosol, acetyl co-a gets converted to ___________ by acetyl co-a carboxylase.

Answer 3

malonyl co-a

Question 4

Fatty acid synthase converts malonyl co-a to __________.

Answer 4

palmitate

Question 5

Desaturation of palmitate is accomplished by ____________.

Answer 5

the ER

Question 6

Both the mitochondria and ER can __________ palmitate.

Answer 6

elongate

Question 7

In naming unsaturated fatty acids, the double bonds are named for ____________.

Answer 7

the carbon they start on; for instance, 20:2(5,8) means a 20-carbon chain with two double bonds that go from 5 to 6 and from 8 to 9

Question 8

Why can’t people make linoleic acid and alpha-linoleic acid?

Answer 8

Because human cells lack enzymes to put double bonds from carbon 9 to the end of the chain.

Question 9

What are three important uses for fatty acids?

Answer 9

• Compose the cell membrane
• Act as triglyceride storage
• Precursors for eicosanoids

Question 10

______________ is the precursor to arachidonic acid.

Answer 10

Linoleic acid

Question 11

Formation of malonyl co-a is stimulated by ____________ and inhibited by __________.

Answer 11

citrate (a sign of nutrient excess); long-chain fatty acyl co-a (the immediate downstream product)

Question 12

____________ has many (seven) different activities.

Answer 12

Fatty acid synthase

Question 13

The first two carbons in fatty acid synthesis are from ___________, while all subsequent carbons are from ____________.

Answer 13

acetyl co-a; malonyl co-a

Question 14

NADPH is used in ______________.

Answer 14

the reducing steps of fatty acid synthesis

Question 15

In addition to being produced in the hexose monophosphate shunt, NADPH can also be made by ____________.

Answer 15

NADH + NADP+ -> NAD+ NADPH

Question 16

What things stimulate and inhibit fatty acid synthesis?

Answer 16

Stimulate:

• high carbohydrate levels leading to lots of acetyl co-a
• high insulin levels

Inhibit:

• high fat/low carb diets leading to increased cytosolic acyl co-a
• high glucagon levels

Question 17

Insulin promotes the conversion of acetyl co-a to malonyl co-a by _______________.

Answer 17

dephosphorylating acetyl co-a carboxylase (via protein phosphatase)

Question 18

Vmax directly correlates with ______________.

Answer 18

enzyme concentration

Question 19

Almost all of the endogenous fatty acids are in the ______ configuration.

Answer 19

cis

Question 20

High levels of ATP lead to increased fatty-acid synthesis by what mechanism?

Answer 20

ATP inhibits isocitrate dehydrogenase, leading to accumulation of citrate.

Question 21

Fatty acid synthase ultimately produces __________.

Answer 21

palmitic acid (a 16-carbon molecule)

Question 22

What enzyme is activated when polymerized and deactivated when depolymerized?

Answer 22

Acetyl co-a carboxylase

Question 23

What enzyme synthesizes triglyceride?

Answer 23

Acyltransferase

Question 24

Why does alcohol consumption promote fatty acid synthesis?

Answer 24

Alcohol is metabolized to acetyl co-a

Question 25

The rate-limiting enzyme in fatty acid synthesis is _____________.

Answer 25

acetyl co-a carboxylase

Question 26

The family of enzymes that elongate fatty acids is called ______________.

Answer 26

fatty acid elongase

Question 27

What do mixed function oxidases do?

Answer 27

Introduce double bonds

Question 28

Fatty acids must be ____________ before being activated to triglycerides.

Answer 28

activated by binding co-a