Question 1

The PRIMARY site, but not the only site, of cholesterol synthesis is:

Accepted Answer

The liver is the primary site of cholesterol synthesis, producing approximately 70% of the body's cholesterol.

Question 2

The committed step in cholesterol biosynthesis is catalyzed by:

Accepted Answer

The committed step in cholesterol biosynthesis is the conversion of HMG-CoA to mevalonate, which is catalyzed by HMG-CoA reductase. This step is irreversible and is the target of many drugs used to lower cholesterol levels.

Question 3

Regulation of fatty acid synthesis in humans is affected by all of the following EXCEPT:

Accepted Answer

Insulin receptor binding actually activates acetyl-CoA carboxylase by promoting its dephosphorylation, not maintaining it in the inactive form. This enzyme is crucial for the first step in fatty acid synthesis, converting acetyl-CoA to malonyl-CoA.

Question 4

____ are all derived from 20-carbon fatty acids that are acylated to membrane phospholipids and released by the enzyme ____.

Accepted Answer

Eicosanoids are derived from 20-carbon fatty acids like arachidonic acid, which are released from membrane phospholipids by the enzyme phospholipase A2.

Question 5

Which of the following is correctly paired with its effect upon fatty acid synthesis?

Accepted Answer

Citrate is a molecule that activates acetyl-CoA carboxylase, the rate-limiting enzyme in fatty acid synthesis, by signaling the abundance of acetyl-CoA, which is a substrate for fatty acid synthesis.

Question 6

The carbons of sphinganine are derived from:

Accepted Answer

The carbons of sphinganine are derived from palmitate and serine. Specifically, the amino group of serine combines with the fatty acid palmitate to form 3-ketosphinganine, which is then reduced to sphinganine.

Question 7

All are regulatory mechanisms of HMG-CoA reductase EXCEPT:

Accepted Answer

High cholesterol levels actually lead to a decreased half-life for HMG-CoA reductase, not an increase. This is part of the body's feedback mechanism to reduce cholesterol synthesis when its levels are high.

Question 8

An important mechanism of arachidonate release and eicosanoid synthesis involves ____ and ____.

Accepted Answer

Arachidonate, a polyunsaturated fatty acid, is released from membrane phospholipids following an inflammatory stimulus, such as tissue injury, and is then converted into various eicosanoids, such as prostaglandins and leukotrienes. Insulin and glucagon release are not involved in this mechanism, nor is low blood sugar.

Question 9

All are true for aspirin EXCEPT:

Accepted Answer

Aspirin does not stimulate ring opening in prostaglandin synthesis; instead, it inhibits the synthesis of prostaglandins by acetylating a serine residue in the active site of cyclooxygenase (COX), thereby blocking the enzyme's activity.

Question 10

A lipid that does NOT have a sphingosine backbone is:

Accepted Answer

Phosphatidylinositol does not have a sphingosine backbone. It is a phospholipid that is composed of glycerol, two fatty acids, a phosphate group, and an inositol molecule.

Question 11

Which of the following statements concerning eicosanoids is correct?

Accepted Answer

None of the statements are correct. Thromboxanes do contain a 1,3-dioxane ring but not all of them do. Prostaglandins do contain a cyclic ether but not all of them do. Leukotrienes have three conjugated double bonds but not all of them do. Lastly, leukotrienes and thromboxanes are both derived from arachidonic acid.

Question 12

Platelet-activating factor displays dramatic ability to ____ blood vessels and thus ____ blood pressure, and to aggregate platelets.

Accepted Answer

The answer of Platelet-activating factor displays dramatic ability to ____...

Question 13

Insulin promotes the ____ of acetyl-CoA carboxylase and ____ of triacylglycerol lipase by ____ the phosphodiesterases that converts cAMP to AMP.

Accepted Answer

Insulin promotes the activation of acetyl-CoA carboxylase and the inactivation of triacylglycerol lipase by stimulating phosphodiesterases, which convert cAMP to AMP, thereby reducing cAMP levels and decreasing protein kinase A activity.

Question 14

Both glycerol and dihydroxyacetone phosphate can serve as precursors for phosphatidic acid. Which enzyme is NOT USED to catalyze reactions in the synthesis of phosphatidic acid from these two precursors?

Accepted Answer

Triose phosphate isomerase catalyzes the interconversion of dihydroxyacetone phosphate and glyceraldehyde 3-phosphate in glycolysis, however, it is not involved in the synthesis of phosphatidic acid from glycerol and/or dihydroxyacetone phosphate.

Question 15

CDP-diacylglycerols are precursors for the synthesis of all EXCEPT:

Accepted Answer

Phosphatidylethanolamine is synthesized from CDP-ethanolamine, not CDP-diacylglycerol.

Question 16

All are characteristics of COX-2 inhibitors EXCEPT:

Accepted Answer

COX-2 inhibitors are not NSAIDs, so choice B is incorrect. All other choices are characteristics of COX-2 inhibitors.

Question 17

Components for plasmalogen biosynthesis include all EXCEPT:

Accepted Answer

Plasmalogens are a type of phospholipid found in many tissues throughout the body. Their biosynthesis involves several components, but glycerol is not directly used in their synthesis. Instead, the process includes CDP-ethanolamine (for the head group), fatty acyl-CoA (for the fatty acid component), NADPH and NADH (as reducing agents), and long-chain alcohols (which differentiate plasmalogens from other phospholipids by being incorporated into the vinyl ether bond at the sn-1 position of the glycerol backbone).

Question 18

The correct sequence for synthesis of mevalonate from acetyl-CoA is:
A.HMG-CoA synthase
B.formation of 3-hydroxy-3-methylglutaryl-CoA
C.β-ketothiolase catalyzed condensation
D.HMG-CoA reductase activity
E.Formation of acetoacetyl-CoA

Accepted Answer

The correct sequence for the synthesis of mevalonate from acetyl-CoA involves the formation of acetoacetyl-CoA (E), followed by the β-ketothiolase catalyzed condensation (C), leading to the formation of 3-hydroxy-3-methylglutaryl-CoA (B), which is then catalyzed by HMG-CoA synthase (A), and finally, the HMG-CoA reductase activity (D) converts it into mevalonate.

Question 19

Sphinganine is acylated in the ____-position.

Accepted Answer

Sphinganine is acylated at the 2-NH₂ position, which is the amino group in the sphingoid base structure.

Question 20

In eukaryotes, phosphatidic acid is converted directly either to ____ or to ____ from which all other ____ are derived.

Accepted Answer

In eukaryotes, phosphatidic acid is converted directly to diacylglycerols (DAG) or to CDP-diacylglycerols (CDP-DAG) from which all other glycerophospholipids are derived. This process occurs via the Kennedy pathway.

Quiz 24: Lipid Biosynthesis