Biochem Flashcards
What are the functions of cholesterol?
Structure of membranes, bile acids, steroid hormones
What organ is responsible for controlling body levels of cholesterol? Is this where de novo synthesis occurs?
Liver; yes
Describe the structure of cholesterol
4 rings (steroid nucleus) with an 8-carbon alkyl side chain hanging off of ring D and a solitary polar group on the opposite side hanging on to ring A
How is acetyl-CoA turned into cholesterol?
Acetyl-CoA becomes acetate which is converted into a 5 carbon molecule called isoprene
Six isoprene molecules come together to form a 30-carbon long chain called squalene.
The four rings cyclize into the 27-carbon cholesterol structure
Describe the first step of cholesterol synthesis including enzyme names
What is the rate limiting step of cholesterol synthesis?
2 Acetyl-CoAs come together via Thiolase to form acetoacetyl-CoA (one CoASH is released)
One more acetyl-CoA is added via HMG-CoA synthetase to form HMG-CoA (another CoASH released)
HMG-CoA becomes Mevalonate via NADPH and HMG-CoA reductase (RATE LIMITING STEP)
Alternatively, HMG-CoA can be turned back into acetylacetate via HMG-CoA lyase
What is the second step of cholesterol synthesis (involving Mevalonate)?
Mevalonate is converted into the 5-carbon Isoprene pyrophosphate
This requires four additional enzymes and 3 molecules of ATP
One carbon is released as CO2
How do statins work to lower cholesterol? Name a few
Statins are reversible, competitive inhibitors of HMG-CoA reductase.
Used in patients with hypercholesterolemia
Lipitor, Pravachol, and Zocor
Statins also have anti-inflammatory properties; increase NO production and decrease circulating levels of C-reactive protein, a serum marker of inflammation. Also limits T-cell activation
How does isoprene become squalene? How does squalene become cholesterol? What intermediates are passed?
6 isoprenes condense to form a straight 30 carbon squalene molecule by passing through Geranyl Pyrophosphate (10 carbon) and Farnesyl pyrophosphate (15 carbon)
Monooxygenases add O2 to the straight Squalene molecule in order to get the 4 rings to close which forms cholesterol.
SREBP, SCAP, Insig
Sterol regulatory element binding protein
SREBP cleavage-activating protein
The first one binds the SRE on the HMG-CoA reductase gene to increase transcription. SREBP is an integral ER membrane protein and is bound to SCAP. When cholesterol levels are low, the protein complex moves to the Golgi where SREBP is cleaved and can travel to the nucleus to increases transcription
When sterols are abundant, SCAP binds another ER membrane protein called Insig. This causes retention of the SCAP-SREBP complex and HMG-CoA reductase levels fall.
Also, Insig can bind HMG-CoA reductase and cause ubiquitination.
What effects do the well-fed and fasting state have on HMG-CoA reductase?
In the well-fed state, high ATP causes phosphoprotein phosphatase to be active which keeps HMG-CoA reductase in its active state (producing cholesterol to replace membranes, etc.)
High AMP levels (fasting state) causes activation of AMP activated protein kinase (AMPK). This kinase phosphorylates and inactivates HMG-CoA reductase. Makes sense because cholesterol synthesis requires ATP and Acetyl CoA
Insulin activates
Glucagon inactivates
Why is it easy for activated isoprene to be converted into other molecules? Besides cholesterol (and bile acids, steroids, etc.) what can isoprene become?
The pyrophosphate molecule provides a lot of energy when hydrolyzed.
The isoprene can also become any of the fat soluble vitamins (A,D,E,K)
Or it can become a quinone electron carrier for the ETC, or Dolichol which aids in the formation of N-linked oligosaccharides.
Describe the flow of cholesterol to cortisol, estradiol, and aldosterone during pregnancy
Cholesterol is converted to progesterone and estradiol in the corpus luteum. Progesterone is the precursor for other steroid hormones including estradiol.
In the adrenal cortex, progesterone is converted to cortisol (a glucocorticoid) or corticosterone and aldosterone (both mineralcorticoids).
In the testes, progesterone becomes testosterone which is responsible for the development of secondary male sex characteristics
In the ovaries, progesterone becomes estradiol (estrogen) which is responsible for the development of secondary female characteristics and regulation of ovarian cycle.
What does cortisol do?
Released in response to stress and low glucose levels
Increases carb, fat, and protein metabolism. Suppresses the immune system.
Increases during week 30-32 of pregnancy causing the production of fetal lung surfactant and promoting lung maturation
Most PEDs are derivatives of what? What are the possible side effects?
Testosterone derivatives
Liver and kidney cancer, jaundice, fluid retention, high BP, increase in LDL and decrease in HDL, aggression, depression, acne, and trembling.
Males: premature balding, breasts, testicular atrophy, infertility
Females: premature baldness, deepening of the voice, increase in body hair, enlarged clitoris, decrease in menstrual cycle.
What effect does cortisol have on the liver and adipose tissue?
Glucocorticoids increase the activity of PEP carboxykinase (PEPCK) in the liver which causes production of DHAP and subsequently glycerol 3 phosphate.
The glycerol 3 phosphate is turned into TGs which are then exported into the blood. Lipoprotein lipases in the plasma separate the FAs from the glycerol backbone.
In the adipose, glucocorticoids inhibit PEPCK which means glycerol 3 phosphate is not available to make TGs.
The end result is that fFAs increase in the blood and the glycerol is converted to glucose by gluconeogenesis. The excess FAs can lead to decreased carbohydrate metabolism and increased insulin resistance.
What does Thiazolidinedione do? What effect does it have on Type 2 diabetics?
It is a glucocorticoid derivative which activates a nuclear receptor called peroxisomes proliferator-activated receptor y (PPARy). This receptor induces the activity of PEPCK.
Therapeutically, this increases the rate of glyceroneogenesis in adipose tissue which allows TGs to be synthesized and fFAs levels in the blood are reduced. Therefore, glucose metabolism can continue and some insulin sensitivity is conferred in those with type 2 diabetes (as well as an improved lipid profile)
What happened when pregnant rats were given a cholesterol biosynthesis inhibitor? What happened when they were given exogenous cholesterol after that?
Led to embryonic malformations such as holoprosencephaly (HPE) and genitourinary abnormalities
Proper phenotype rescued
Conclusion: cholesterol is super important for development of several organs
What is holoprosencephaly? Rate?
The mirror image hemispheres of the brain do not form. Abnormal specification of the ventral-dorsal axis of the neural tube
1/16,000 live births; 1/250 during embryogenesis
Usually autosomal dominant
Clinical variability broad: fetal lethality at one end; central incisor tooth at the lighter of the spectrum
Often manifested as center line cleft (unusual type of cleft)
What gene mutations are causative for HPE?
Sonic hedgehog (Shh) gene mutations
Signaling protein which establishes ventral identity in developing neural tube
What molecule does Shh require to function properly?
Cholesterol (Shh is cleaved an cholesterol attaches to glycine residue to activate it)
Smith-Lemli-Opitz syndrome
Multiple congenital malformations with severe mental retardation
- CNS malformation and HPE
- cleft palate, micrognathia
- cardiac anomalies
- polydactyly (or oligodactyly)
- genital anomalies
AR, 1/1700-13000 live births
Defect in 7-sterolreductase (7 dehydrocholesterol reductase) leads to cholesterol deficiency and build up of 7-dehydrocholesterol
What are the secondary effects of cholesterol deficiency in SLOS?
Adrenal insuffiency (low aldosterone)
Testosterone insufficiency (ambiguous genitalia)
Cortisol insuffiency (growth deficiency and infant death)
Desmosterolosis
Looks similar to SLOS but without the HPE (may be macro or microcephaly)
Facial dysmorphology
Skin and limb dysmorphology
Growth retardation
Mental/communication retardation
Defective enzyme leads to desmosterol build up and cholesterol insufficiency. Affects Shh gene just like SLOS.
Increased desmosterol metabolism.
Adrenal insuffiency (low Aldo/renin ratio), but cortisol and testosterone are pretty normal
Lathosterolosis
Another build up product of blocked cholesterol synthesis
Defect in 3B-OHsteroid-delta5-desaturase—> Lathosterol builds up
In SLOS, is it the lack of cholesterol or build up of 7DHC that’s the problem?
Cholesterol deficiency affects Shh which has downstream effects on growth enzymes. 7DHC doesn’t affect Shh
What is the common structure of all nuclear receptors (steroid receptors)?
Three domain structure
- ligand binding domain— binds lipophilic ligand
- DNA-binding domain— binds to specific DNA consensus sequences
- Variable region— interacts with other proteins to activate or inhibit transcriptional machinery
What are the two types of Nuclear receptors? Their differences? Which ligands do they bind?
Type I: Estrogens, androgens, glucocorticoids, mineralocorticoids
- bind inverted repeat DNA consensus sequences
- in the absence of ligand, found in cytoplasm bound to inhibitors on the LBD
- when a ligand binds, it displaces the inhibitor and exposes the DBD; the complex moves to nucleus and forms homodimer that binds inverted consensus; also recruits transcriptional activators and chromatin remodeling complexes to unwind DNA and increase transcription
Type II: Vitamin D, Retinoic Acid, Thyroxine, Orphans
- bind direct repeat DNA consensus sequence
- found always as a heterodimer in the nucleus bound to a structurally related protein, RXR
- in absence of ligand, complex binds corepressors that suppress gene expression
- upon ligand binding, corepressors replaced with coactivators to upregulated transcription and gene expression
What are the three receptors commonly altered in Breast cancer? What are the four types of breast cancer?
Estrogen Receptor (ER) Human Epidermal Growth Factor Receptor (Her2/Neu) Progesterone Receptor (PR)
Endocrine Receptor Positive (ER or PR)
HER2/Neu Positive
Triple Negative
Triple Positive
What is Tamoxifen? Against which type of BC is it effective? What are the potential dangers? What is a SERM?
Tamoxifen is an estrogen analog that can bind estrogen receptors and block transcription in ER(+) BC; also strengthens bones and lowers LDL cholesterol
Does not work against ER(-) BC
However, doesn’t work against ER(+) uterine endometrial cancer and can actually increase the risk of developing that cancer
Tamoxifen is considered a SERM: Selective Estrogen Receptor Modulator
