Biochemistry : Sterols and Steroids
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Example Question #1 : Sterols And Steroids
Which of the following would be characteristic of a steroid hormone?
Binding to cytosolic or nuclear receptors
Effects are short term and quickly reversed
Not requiring any proteins to transport them to target cell in the bloodstream
Binding to receptors on the outside of the cell membrane
Binding to cytosolic or nuclear receptors
Steroid hormones are derivatives of cholesterol and therefore can pass through the plasma membrane of cells since they are lipophilic. Steroid hormones generally don't bind on the surface of plasma membranes, but instead to receptors inside the cell itself. Since they are nonpolar, they are carried by proteins in the blood.
Example Question #21 : Lipid Structures And Functions
A steroid molecule consists of how many fused carbon rings?
2
4
5
6
3
4
Steroids are derivatives of the molecule cyclopentanoperhydrophenanthrene which is composed of 4 fused carbon rings. There are three hexane rings and one pentane ring in the structure of steroids.
Example Question #2 : Sterols And Steroids
Cholesterol has a structure most similar to which of these?
Glycolipid
Steroid
Free fatty acid
Carbohydrate
Phospholipid
Steroid
Although it is one of the three main types of membrane lipids (along with phospholipids and glycolipids) cholesterol's structure is quite different. It is a steroid, and so it is made from four linked hydrocarbon rings, including three six-membered rings and one five-membered ring.
Example Question #3 : Sterols And Steroids
Which of the following is not made from cholesterol?
Cortisol
Insulin
Testosterone
Progesterone
Estrodiol
Insulin
Cholesterol is a very important molecule for a number of reasons. One function of cholesterol is to regulate membrane fluidity in the plasma membrane. Another very important role that cholesterol plays is as the precursor to a number of steroid hormones including cortisol, testosterone, estradiol, and progesterone. Insulin, however, is a peptide hormone made in the pancreas.
Example Question #4 : Sterols And Steroids
All of the carbons in cholesterol are derived from carbons in what molecule?
Mevalonate
Glucose
Acetoacetate
Acetyl-CoA
Glycerol
Acetyl-CoA
Cholesterol is a molecule with 27 carbon atoms. In order to synthesize this steroid, a three step process must be completed. First, isopentenyl pyrophosphate is synthesized. Second, six isopentenyl pyrophosphates form squalene. Third, squalene cyclizes and the product of this is converted to cholesterol. The building blocks that began this whole process were carbons from acetyl-CoA.
Example Question #5 : Sterols And Steroids
The first of three steps of cholesterol synthesis occurs in the cytoplasm. Where do the following two steps occur, respectively?
Endoplasmic reticulum
Endoplasmic reticulum; Golgi apparatus
Cytoplasm
Cytoplasm; endoplasmic reticulum
Endoplasmic reticulum; cytoplasm
Endoplasmic reticulum
The fist step of cholesterol synthesis is the synthesis of isopentenyl pyrophosphate from acetyl-CoA molecules, and this occurs in the cytoplasm. The second step of cholesterol synthesis is six isopentenyl pyrophosphate molecules converging to form squalene. This occurs in the endoplasmic reticulum. The third step of cholesterol synthesis is the cyclization of squalene and the final conversion of the molecule to cholesterol. This occurs in the endoplasmic reticulum.
Example Question #6 : Sterols And Steroids
Which step in cholesterol synthesis is the committed step?
Synthesis of squalene from isopentenyl pyrophosphate units
Isomerization of isopentenyl pyrophosphate to dimethyallyl pyrophosphate
Condensation of two farnesyl pyrophosphates by squalene synthase
Synthesis of mevalonate by HMG-CoA reductase
Cyclization of squalene
Synthesis of mevalonate by HMG-CoA reductase
Once acetyl-CoA molecules have been converted to 3-hydroxy-3-methylglutary-CoA (HMG-CoA), reduction to mevalonate occurs. This synthesis is an irreversible step in the pathway to create cholesterol, and therefore it is the committed step in cholesterol synthesis.
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