Block 3 Cholesterol Biosynthesis Flashcards
Describe the structure of cholesterol.
27 C, 4 rings, 8C tail. Hydrophobic except OH at C3, used for esterification with FFA to form CE
What are cholesterol esters?
Very hydrophobic, transported form of Ch (in LPs), stored intracellularly as lipid droplets, processed for excretion as bile acids
What two enzymes are involved in CE formation? What enzyme hydrolyzes CE?
LCAT in plasma (LP formation) & ACAT in some cells (lipid droplets); CE hydrolase
What are the stages of cholesterol biosynthesis? Where are the enzymes located?
Acetate -> isoprenoid intermediates -> squalene -> cyclization product -> cholesterol
Enzymes in cytosol & ER (rxns in cytosol)
What are the required reactants for 1 mole chol synth?
Source of 18 AcCoA (B-ox of FA in perox/mitos, dehydrog of pyr in mito, ox of ketogenic aa’s in cytosol), 16 NADPH, 36 ATP
What is the first stage of chol synth?
Acetate as AcCoA -> isoprenoid intermediate by 4 steps. AcCoA -> HMG-CoA -> mevalonate -> P-mevalonate -> 5-pyro-P-mevalonate -> isopentenyl pryo-P
What enzymes are used in the first stage of chol synth?
HMG-CoA reductase + 2 NADPH; mevalonate P-transferase + ATP; P-mevalonate kinase + ATP; pyro-P-mevalonate DC + ATP
What is the fate of HMG-CoA in mito vs. cytosol?
In mito -> ketone bodies
In cytosol -> chol biosynth
What is the most highly regulated step of stage 1 of chol synth?
HMG-CoA reductase is rate-determining and most elaborately regulated
What is stage 2 of chol synth?
Dimethylallyl-PP (C5) isopentenyl-PP (C5) –> geranyl-PP (C10) -> farnesyl-PP (C15) -> squalene (C30, condensation of 6 isoprene units)
What are stages 3 and 4 of chol synth? Where are the enzymes located?
Squalene cyclization -> lanosterol -> 19 steps to -> cholesterol (requires ox and loss 3 methyl). *All rxns require NADPH and O2. Enzymes are in ER membrane.
What is the role of isopentenyl-PP aside from chol synth?
Makes isopentenyl tRNA
What is the role of farnesyl-PP aside from chol synth?
Farnesylated proteins (15C isoprene addition), dolichol, haem A, ubiquinones
What is the role of intermediates between lanosterol and 7-dehydrocholesterol & 7-DHC?
Int: meiosis activating sterols
7-DHC: vitamin D formation in skin
What is the role of cholesterol?
Bile acids, steroids, hedgehog proteins
How is HMG-CoA reductase regulated short-term? Long-term?
Short: statins (competitive inhibition), allosteric effects (AMP, to conserve energy), covalent mod with reversible P (at Ser 871, P form less active)
Long: modulating synth or degradation rates
What are the allosteric activators and inhibitors of HMG-CoA reductase?
Act: insulin
Inh: glucagon (fasted) or epi (stress) -> cAMP -> PKA
What is SREBP?
Sterol regulatory element binding protein, binds region in HMG-CoA red gene promotor SRE
How does SREBP exert its effects?
ER -> binds SCAP chaperone in ER -> Golgi -> 2 proteolysis (S1P, S2P proteases) -> nucleus
How do cholesterol levels affect SREBP/SCAP?
Low chol: binds COP2 and goes to Golgi
High: interacts with Insig proteins, holds complex in ER by preventing intxn with COP2
How is HMG-CoA Red regulated at post-transcriptional level?
Sterol-depleted cells: slow degradation (half life >12 hr)
Sterol-loaded: rapid degradation (h/l <1 hr), facilitated by Insig binding
*Lanosterol more potent than cholesterol
Describe the circadian rhythm of chol synth.
Peaks 6 hrs after dark, high for 12 hrs after last meal, minimum 6 hr after light exposure. *ACTH, estrogen, other hormones involvement hypothesized
What is Smith-Lemil-Opitz Syndrome (SLOS)?
Most common chol synth disorder, autosomal recessive; deficiency 7-DHC reductase; microcephaly, short nose, small/posterior chin, syndactyly, feeding difficulty, FTT, MR, behavior probs; severity assc. with plasma chol levels
How is SLOS treated?
Chol may help, lovastatin (HMG-CoA inh) in KO mice (suggests acc toxic sterols > reduced chol)
Why doesn’t chol supp to pregnant mothers prevent morphogenic and developmental abnormalities in SLOS?
Transport LDL via megalin limited to 1st trimester, chol not BBB permeable = requires local biosynthesis
How does cholesterol modify hedgehog proteins?
Hh undergoes autocatalytic cleavage at Gly-Cys-Phe motif, C-terminal removed, chol added to C-terminal domain, then palmitoylated at N-terminal Cys. Chol tethers protein to PM (restricts tissue localization to trigger signaling pathway for txn activation for patterning & morphogenesis)
