Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 432

Biochemistry

Spring 2009

Lecture Notes: 4 February

© R. Paselk 2006
 
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Cholesterol Biosynthesis, cont.

Last time we left off with the labeling pattern in Cholesterol:

Structural drawing of Cholesterol and acetyl CoA

Now we want to look at how the synthesis of cholesterol from acetyl CoA results in this pattern.

The biosynthesis of cholesterol can be looked at in terms of a particular strategy involving the construction of building blocks with the subsequent joining of these building blocks to give a 30 carbon linear molecule (squalene) which is then induced to fold up and fuse into the familiar fused ring of steroid: Lanosterol. The lanosterol will then undergo rearrangement involving a complex series of methyl and hydrogen migrations to give the final product.

The strategy for the assembly of squalene can be outlined as below as four sequences of reactions:

  1. Synthesis of 3-Isopentyl PPi (IPP, 5 C) from acetyl CoA.
  2. IPP (5 C) + IPP (5 C) right arrow Geranyl PPi (GPP, 10 C).
  3. GPP (10 C) + IPP (5 C) right arrow Farnasyl PPi (FPP, 15 C).
  4. FPP (15 C) + FPP (15 C) right arrow Squalene (30 C).

1. The biosynthesis of 3-Isopentyl PPi takes place in a series of six reactions, beginning with:

a. the condensation of two acetyl CoA's to give acetoacetyl CoA (4 C) in a reaction familar from ketone body synthesis, but using a cytosolic isozyme:

structural diagram of the condensation two acetyl CoA's to give acetoacetyl CoA

b. In the next step a third acetyl CoA is condensed to give HMG (3-hydroxy-3-methylglutaryl CoA, 6 C) in another familar reaction. (Note however that this reaction takes place in the cytosol rather than the mitochondrion, and it uses a different isozyme.):

Structural diagram of the condensation of acetoacetyl CoA and acetyl CoA to give HMG (3-hydroxy-3-methylglutaryl CoA

c. HMG-CoA is now reduced by HMG-CoA reductase using two NADPH's with the loss of CoASH to give mevalonate:

Structural diagram of the reaction sequence converting HMG-CoA to Mevelonate

d. Mevalonate is now phosphorylated twice, first by by mevalonate-5-phosphotransferase at the cost of one ATP, and then,

e. by phosphomevalonate kinase at the cost of a second ATP to give 5-pyrophophomevalonate:

Structural diagram of the reactions phosphorylating mevelonate to 5-Pyrophophomevelonate

f. Finally, the 5-pyrophophomevalonate is decarboxylated with the hydrolysis of an additional ATP by pyrophophomevalonate decarboxylase to give 3-Isopentyl PPi (3-Isopentene is the so-called "isoprene unit", 5 C):

Structural diagram of the decarboxylation of 5-pyrophophomevelonate to give delta 3- isopentyl pyrophosphate

We have now completed the reactions in the first step below:

  1. Synthesis of 3-Isopentyl PPi (IPP, 5 C) from acetyl CoA.
  2. IPP (5 C) + IPP (5 C) Geranyl PPi (GPP, 10 C).
  3. GPP (10 C) + IPP (5 C) Farnasyl PPi (FPP, 15 C).
  4. FPP (15 C) + FPP (15 C) Squalene (30 C).

2. In the next reaction sequence two isopentyl units (5 C) are joined to give geranyl PPi in two reactions:

a. First a 3-Isopentyl PPi is isomerized by isopentyl PPi isomerase to give Dimethylallyl pyrophosphate (DPP):

Structural diagram of the isomerization of IPP to DPP

b. IPP and DPP then condense to give geranyl pyrophosphate (GPP,10 C):

Structural diagram of the condensation of IPP and DPP to give GPP

3. We now add a third IPP to the GPP to give farnesyl pyrophosphate (FPP, 15 C).

This is an unusual reaction with the loss of the pyrophosphate resulting in a carbocation intermediate-one of the few seen in enzyme mechnisms. Redrawing GPP closer to the final conformation and adding IPP gives:

Structural diagram of the condensation of IPP and GPP to give FPP

4. Squalene synthase now catalyzes the head-to-head condensation of two farnesyl pyrophosphates:

This is a complex reaction involving a cyclopropene intermediate (presqualene pyrophosphate) which is then reduced and dephosphorylated to give squalene (30 C):

Structural diagram of the condesation of two FPP to give squalene

Finally squalene is converted to a sterol, Lanosterol (30 C) by two enzymes:

1. Squalene epoxidase (a mixed function oxidase) inserts an oxygen across the 2,3 double bond to give 2,3-Oxidosqualene:

Structural diagram of the Squalene epoxidase reaction

2. Squalene oxidocyclase then catalyzes the cyclization of this structure by protonating the epoxide oxygen which opens the epoxide leaving an electron deficient carbon. Migration of the resulting electron deficiency sequentially cyclizes the rings, leaving a carbocation on the Protosterol cation:

Structural diagram of the cyclization reaction catalyzed by Squalene oxidocyclase

The carbocation then drives a series of hydride and methyl migrations, finally resulting in Lanosterol:

Structural diagram of the hydride and methyl migrations catalysed by Squalene oxidocyclase

Lanosterol is now converted to cholesterol in a 19 step process in which three methyl groups are removed by sequential oxidations catalyzed by mixed-function oxidases.


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Last modified 2 February 2009.