|Lecture Notes: 4 February||
Last time we left off with the labeling pattern in Cholesterol:
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:
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:
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.):
c. HMG-CoA is now reduced by HMG-CoA reductase using two NADPH's with the loss of CoASH to give mevalonate:
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:
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):
We have now completed the reactions in the first step below:
a. First a 3-Isopentyl PPi is isomerized by isopentyl PPi isomerase to give Dimethylallyl pyrophosphate (DPP):
b. IPP and DPP then condense to give geranyl pyrophosphate (GPP,10 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:
This is a complex reaction involving a cyclopropene intermediate (presqualene pyrophosphate) which is then reduced and dephosphorylated to give squalene (30 C):
1. Squalene epoxidase (a mixed function oxidase) inserts an oxygen across the 2,3 double bond to give 2,3-Oxidosqualene:
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:
The carbocation then drives a series of hydride and methyl migrations, finally resulting in Lanosterol:
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.
Last modified 2 February 2009.