Cholesterol Synthesis

[liveonearth]

Where in the body is cholesterol made?
in the liver, in the cytosol of the cells
also made in intestines, adrenals, gonads, skin, neural tissue and aorta???

What molecule provides all the carbon atoms of cholesterol?
acetate

What cofactor provides the reducing equivalents for the synthesis?
NADPH


acetyl Coa-->-->beta hydroxy beta methyglutaryl CoA (HMG CoA)-->mevalonate

mevalonate-->isopentenyl pyrophosphate (IPP) and 3,3 dimethylallyl pyrophosphate (DPP)
mevalonate is activated (phosphorylated) then carboxylated (needs biotin)
IPP and DPP are isoprenoid isomers

IPP and DPP condense-->squalene-->lanosterol-->cholesterol

Which molecule is this one?

squalene
a triterpene
specific gravity of 0.855
this is the molecule that gets "cyclized" to form rings-->steroid synthesis
has been used as an adjuvant in vaccines (incr immune response) (CMV vaccine)
some veterans say it causes Gulf War Syndrome (study published 2002, use discontinued Oct 04)
double bonds usually saturated-->squalane for stability in personal care products

What is the source for commercial squalene?
shark liver oil

What are other sources?
amaranth seed, rice bran, wheat germ, olives

Which step is the rate limiting one in cholesterol synthesis?
conversion of HMG CoA to mevolonate

What's the rate limiting enzyme?
HGM CoA reductase


What inhibits this enzyme?
presence of cholesterol from any source (endogenous or dietary)

Is free cholesterol in a cell esterified?
no, but it is in the plasma and in the endosomes

How does HDL get cholesterol out of cells???
binds to its receptor on cell (apo-A-1)-->protein kinase C stims transloc of chol to memb

NOTES FROM BIOCHEMI REVIEW

How do you get acetyl CoA out of the mitochondria??
acetyl CoA goes into TCA, joins OAA to make citrate (synthase)
citrate is transported out of mito via special transporter
citrate lyase splits 2C off, back to acetyl CoA and OAA
the rest of these rxns occur in cytosol

How many acetyl CoA's are joined to make HMG CoA?
Acetyl CoA x2 = acetoacetyl CoA
add one more acetyl CoA to make HMG-CoA (6C)
hydroxylmethylglutaryl CoA (gluta = 6C) is an ester

HMG CoA is reduced twice to form what?
mevalonate
uses 2 NADPH
uses HMG-CoA reductase
loose CoA and ketone in the two reductions

What inhibits HMG-CoA reductase?
easy one

statins don't stop the enzyme's manufacture, rather are competitive inhibitors
they are structural analogs
from wiki: Inhibition of this enzyme in the liver results in decreased cholesterol synthesis as well as increased synthesis of LDL receptors, resulting in an increased clearance of low-density lipoprotein (LDL) from the bloodstream. The first results can be seen after one week of use and the effect is maximal after four to six weeks.

How many ATP does it take to make Isopentenyl pyrophosphate from mevalonate?
2, and you loose CO2 and Pi
C5 = isoprene unit

Two isopentenyl pyrophosphates condense to make what?
geranyl phyrophosphate (C10)
then add one more isoprene
Fearneyl phyrophosphate = 15C
put two of those together to get SQUALENE (30C)
use a cyclase to make LANOSTEROL
that converts to Cholesterol

What is cholesterol used for?
steroids
bile salts

How many isoprene units are contained in cholesterol?
six

How many acetyl CoA's are used in making one cholesterol molecule?
18
loose CO2 that's why there's more
also use 18 ATP

What stimulates cholesterol manufacture in body?
regulated by HMG-CoA enzyme
insulin dephosphorylates enzyme thus activating it
(glucagon phosphorylates via a kinase, and deactivates HMG CoA)

Where is HMG CoA located in cell?
in smooth ER of cytosol

bile and chol inhibit DNA manufacture of mRNA for what??? HMG CoA??

research says pts w/ high chol have low vit C ???

What compound is the basis for CoQ10 synthesis?
mevalonate

Comments

[wildrrrumpus.livejournal.com]

hi!

from your post in transhealth, i clicked onto your journal, and i realize this is super geeky, but i'm thrilled by all the biology & chemistry posts and decided to friend you to easily keep up with them. i was planning to go to N.D. school for a few years, but have wound up now as an osteopathic medical student who might eventually go back to N.D. school after paying off my D.O. school debt... so hello!

[liveonearth.livejournal.com]

Hello! I am glad you added me. I just peeked at your journal. Are you working in Palestine? I hope you don't mind if I add you too. It will mean that my personal level posts, which are not so geeky, will also roll across your screen. I have a new housemate moving in and he is Palestinian! Wonderful man. His family relocated to Jordan during the wars. Anyway, I look forward to reading you too. Right now I am studying for basic science boards so most of my posts are supergeek. Once the exam is behind me I'll probably get back to more politics, culture and philosophical questions, which also fascinate me. Good luck paying off those student loans! I have a dark feeling about mine.

[wildrrrumpus.livejournal.com]

Yep, I'm doing a month long exchange in Palestine + doing a little traveling / getting involved in solidarity work for the following month before school starts up again in the fall.

feel free to friend me if you like - i'm happy to read abt politics/culture/philosophy, though my journal's likely headed the other direction - from more of that to more of basic science when i get to school and start prepping for my basic science boards next spring/summer!