Upload
dream10f
View
573
Download
1
Tags:
Embed Size (px)
DESCRIPTION
Citation preview
BiochemistrySixth Edition
Chapter 26The Biosynthesis of Membrane
Lipids and Steroids
Copyright © 2007 by W. H. Freeman and Company
Berg • Tymoczko • Stryer
Cholesterol is made from Acetyl CoA
27 C atoms of cholesterol are derived from AcetylCoA in a 3 stage synthetic process.
Stage 1: Synthesis of isopentenyl pyroP, an activated isoprene unit that is the key building block of cholesterol
Stage 2: Condensation of 6 molecules of isopentenyl pyroP to form squalene
Stage 3: Squalene cyclizes and the tetracyclic product is subsequently converted into cholesterol
The synthesis of mevalonate, which is activated as isopentenyl pyrophosphate, initiates the synthesis of cholesterol
Stage 1: Making isopentenyl pyrophosphate from acetylCoA
The synthesis of mevalonate is the committed step in cholesterol formation
• Enzyme: HMG-CoA reductase• Mevalonate is converted into 3-isopentenyl pyroP in 3
consecutive reactions requiring ATP.
Squalene (C30) is synthesized from 6 molecules of isopentenyl pyroP (C5)
Stage 2: Squalene is synthesized from isopentenyl pyrophosphate
Reaction series: C5 C10 C15 C30
1. Isopentenyl pyrophosphate isomerization2. Dimethylallyl pyroP and isopentenyl pyroP condenses to form geranyl
pyroP.3. The same kind of attack takes place again
– Geranyl pyroP is converted into an allylic carbonium ion and attacked by isopentenyl pyroP, resulting in C15 farsenyl pyroP
– Geranyl transferase catalyzes each of these reactions. The last step in the synthesis of squalene is a reductive tail-to-
tail condensation of two molecules of farnesyl pyroP catalyzed by the ER enzyme squalene synthase
Squalene cyclizes to form cholesterol
Stage 3: The final stage of cholesterol biosynthesis starts with the cyclization of squalene
• Squalene is activated by conversion into squalene epoxide.• Squalene epoxide is then cyclized to lanosterol by
oxidosqualene cyclase.• Lanosterol is converted to cholesterol in a multistep process by
– removal of the 3 methyl groups– the reduction of 1 double bond by NADPH– the migration of the other double bond.
Cholesterol Synthesis Broken down into Five Steps
1. Synthesis of mevalonic acid from Acetyl-CoA • Conversion of HMG-CoA into mevalonate by HMG-CoA
reductase is the rate limiting step!
2. Formation of isoprenoid units from mevalonic acid.3. Six activated isoprene units undergo condensation
to form Squalene.4. Squalene is converted into Lanosterol (in animals).5. Cholesterol is formed from lanosterol after several
further steps that includes the loss of three methyl groups.
Regulation of cholesterol biosynthesis
In mammals, it is regulated by intracellular cholesterol concentration and by the hormones glucagon and insulin.• The rate limiting step is the conversion of HMG-CoA into mevalonate;
the enzyme is HMG-CoA reductase.– It is allosterically inhibited by cholesterol and mevalonate.
• Insulin favors cholesterol synthesis.• Glucagon inhibits cholesterol synthesis.
High intracellular cholesterol – activates ACAT (acyl CoA-cholesterol acyl transferase), increasing
esterification of cholesterol for storage.– causes reduced production of the LDL receptor, slowing the uptake of
cholesterol from the blood.
HMG-CoA reductase
Regulation of cholesterol biosynthesis
Translational control of cholesterol
Feedback regulation is primarily mediated by the amount of HMG-CoA reductase activity. This enzyme is controlled in multiple ways.
1. The rate of synthesis of reductase mRNA is controlled by the sterol regulatory element binding protein (SREBP). • SREBP is a transcription factor and binds to a specific site on DNA called
SRE (sterol regulatory element). • SREBP is attached to ER or nuclear membrane when cholesterol level is
normal.• When cholesterol is low, SRBEP is released by proteolytic cleavage and
moves to DNA and binds to SRE to start making HMG-CoA reductase.• When cholesterol is high, the proteolytic release of the SREBP is blocked,
and the SREBP in the nucleus is rapidly degraded.
Regulation continue
2. Nonsterol metabolites derived from mevalonate inhibit translation of reductase mRNA
3. Degradation of reductase carefully controlled.
4. Phosphorylation decreases the activity of the reductase. This enzyme, like Acetyl CoA carboxylase, is turned off by an AMP-activated protein kinase. Therefore, cholesterol synthesis stops when the ATP level is low.
Lipoproteins
Cholesterol is carried in the blood plasma by plasma lipoproteins.
They are molecular aggregates of specific carrier proteins called “apolipoproteins”
Different combinations of lipids and proteins produce particles of different densities, ranging from VLDL to HDL which may be separated by ultracentrifugation and visualized by EM.
At least 9 different apoproteins are found.
The protein component of lipoprotein has a specific function determined by its point of synthesis, lipid composition, and apolipoprotein content.
Composition of serum lipoproteins
CM85% TAG2% protein8% phospholipid5% Cholesterol-esters
VLDL60% TAG10% protein15% phospholipid15% Cholesterol
LDL8% TAG22% protein20% phospholipid50% Cholesterol
HDL3% TAG50% protein30% phospholipid17% Cholesterol
The blood levels of certain lipoproteins can serve diagnostic purposes
Bad cholesterol (LDL) Good cholesterol (HDL)
– HDL functions as a shuttle that moves cholesterol throughout the body.
– HDL binds and esterifies cholesterol released from the peripheral tissues and then transfers cholesteryl esters to the liver or to tissues that use cholesterol to synthesize steroid hormones.
– HDL protects us from heart attacks. Why??
For a healthy person LDL/HDL ratio is 3.5
Serum lipoproteins
They are complexes of lipids and specific proteins called “apoproteins”
Classified according to increasing density– CM (chylomicron)– VLDL (very low density)– LDL (low density), IDL (intermediary density)– HDL (high density)
They function to keep lipids soluble as they transport them in the serum.
LDL plays a central role in cholesterol metabolism
Cells outside the liver and intestine obtain cholesterol from the plasma
Specifically, their primary source of cholesterol is the LDL
The process of LDL uptake is called receptor mediated endocytosis.
Uptake of cholesterol by receptor mediated endocytosis
LDL receptor
Domain 1 – Amino-terminal region• Cys rich sequence of about 40 residues that is
repeated , with some variation, 7times• Site of LDL binding• Ca also binds here.
Domain 4• Very rich in Ser and Thr• Contains O-linked sugars that may function as
struts to keep the receptor extended from the membrane so that the LDL-binding domain is accessible to LCL
Domain 2• Homologous to EGF• Repeated 3 times , and in between the second
and third repeat is the third domain
Domain 5• 22 hydrophobic residues• Spans the plasma membrane
Domain 3• Similar to the blades of the transducin β
subunit• Exposure to the low-pH environment of the
lysosomes causes the propeller-like structures to interact with the LDL-binding domain. This interaction displaces the LDL, which is then digested by the lysosome.
Domain 6• Has 50 residues• Emerges on the cytoplasmic side of the
membrane• Controls the interaction of the receptor with
coated pits and participates in endocytosis
115 kd protein 6 domains
LDL receptor
The absence of the LDH receptor leads to hypercholesteremia
In familial hypercholesterolemia:– The total concentration of cholesterol and LDL in the blood plasma is
markedly elevated in this genetic disorder.– The result of a mutation at a single autosomal locus.
The desirable cholesterol level is <200 mg/dL. But the levels for those with the genetic disorder are:– 680 mg/dL in homozygotes– 300 mg/dL in heterozygotes
Cholesterol is deposited in various tissues because of the increased concentration of LDL cholesterol in plasma.– Xanthomas, nodules of cholesterol, are prominent in the skin and tendons.– LDL can be oxidized to form oxLDL that can be taken up by immune system
cells, called macrophages. The engorged macrophages form foam cells that are trapped in the walls of the blood vessels and contribute to the formation of atherosclerotic plaques.
atherosclerosis
Foam cells are trapped in the blood vessel wall making it thick, contributing to the formation of atherosclerosis plaques.
Most homozygotes die of CAD in childhood.
Why HDL is good cholesterol?– Possibly, the HDL-associated protein destroys the oxLDL.
Molecular defect: NO LDL RECEPTOR...
Atherosclerotic plaque
The clinical management of cholesterol levels can be understood at a biochemical level
Homozygous familial hypercholesterolemia can be treated only by a liver transplant!
With heterozygous familial hypercholesterolemia, the goal is to increase the gene expression so that more LDL receptors are made.– If the cells are deprived of cholesterol, mRNA production for the
LDL receptor would increase.– How do we deprive cells of cholesterol?
1. By inhibiting intestinal reabsorption of bile salts. Bile salts are cholesterol derivatives that increase the absorption of dietary cholesterol and dietary fats (achieved by positively charged polymers).
2. Blocking de novo synthesis of cholesterol with statins, like lovastatin. These drugs are competitive inhibitors of HMG-CoA reductase.
Lovastatin
Important derivatives of cholesterol
Bile salts• Highly effective detergents• Made in the liver• Stored in the gallbladder
Building block for 5 major classes of steroid hormones• Progesterone• Androgen• Estrogen• Glucocorticoids• Mineralocorticoids
Vit D
Nomenclature of steroid hormones
The rings in steroids are denoted by the letters A, B, C and D
Cholesterol has 2 angular methyl groups– The C-19 methyl group is attached to C-10.– The C-18 methyl group is attached to C-13.
C18 and C19 methyl groups lie above the plane containing the 4 rings– A substituent above the plane is termed b oriented.– A substituent below the plane is termed a oriented.
How do we make steroid hormones? Cholesterol has 27 carbon whereas steroid hormones contain 21 or
fewer carbon. Need to remove 6C unit from cholesterol to form pregnenolone
– Cholesterol side chain is hydroxylated at C-20 and C-22 – The bond between these carbon atoms is subsequently cleaved by
desmolase
– Rate limiting step
Pregnenolone is next oxidized and then isomerized to progesterone.
Progesterone is further modified by a series of hydroxylation reactions to other steroid hormones. These enzymes are mixed-function oxidases requiring NADPH and oxygen.
desmolasecholesterol pregnenolone
Steroid hormones All steroid hormones are derived from cholesterol.
– Mineralocorticoids• Control mineral absorption • Example: aldosterone
– Glucocorticoids• Regulate gluconeogenesis and decrease inflammatory response• Example: cortizon
– Androgens• Sex hormones
– These hormones are effective at very low concentrations and are, therefore, synthesized in relatively small quantities.
Adrenal cortex has three histological zones that are exclusive steroid producers. – Together, the zones can produce all classes of steroid hormones. – Each zone has cells that make different steroid hormones.
Steroid hormone synthesis
Cholesterol (C27)
pregnenolone
desmolase
3-b-OHsteroiddehydrogenase
DeficiencyNo gluco, mineralo or sex hEarly deathprogesteron
17-a-OHprogesteron
testosteron
estradiol
11-deoxycortisol
cortisol
21-a-OHlase
1-deoxycorticosterone
Aldosteron
11-b-OHlase
17-a-OHlaseCommon form of CAHAldosteron and cortisol sex hormones
Cortisol, aldosteron sex hormones
Congenital AdrenalHyperplasia (CAH)
IMPORTANT A defect in the activity or amount of an enzyme in
steroid hormone synthesis pathway can lead to BOTH– a deficiency in the synthesis of hormones beyond the
affected step AND– an excess in the hormones or metabolites before that
step. Therefore severe metabolic imbalances may
occur.
Secretion of adrenal steroid hormones
Adrenal cortical hormone secretion is controlled by the hypothalamus, to which the pituitary gland is attached.
When the body is stressed, released factors travel to the anterior lobe that produce and secrete ACTH (adrenocorticotropic hormone).
ACTH is often called “stress hormone”• Stimulates adrenal cortex to make mineralocorticoids and
glucocorticoids (collectively called corticosteroids)
Corticosteroids bind to their specific receptors and do their action
Actions of corticosteroids Aldosteron
• Stimulates reabsorption of Na and excretion of K Cortisol
• Increased gluconeogenesis• Anti-inflammatory action
Estrogens• Controls menstrual cycle• Promotes female secondary sex characteristics
Progesteron• Maturation of fertilized ovum
Testosteron• Stimulates spermatogenesis• Promotes male secondary sex characteristics
Vit D is derived from cholesterol by the ring-splitting activity of light
Cholesterol is also Vit D precursor. Vit D plays an important role in Ca and phosphorous
metabolism 7-dehydrocholesterol (provitD3) is photolyzed by the UV
light of sunlight to preVit D3. This spontaneously isomerized to D3, D3 is then converted to Calcitriol, the active hormone in the liver and kidney.
Vit D deficiency in children causes rickets Recommended daily intake of Vit D is 400 iu. In adults Vit D deficiency causes osteomalacia (softening
of bones)
BiochemistrySeventh Edition
CHAPTER 26The Biosynthesis of Membrane
Lipids and Steroids
Copyright © 2012 by W. H. Freeman and Company
Berg • Tymoczko • Stryer
Q.
Mice were divided into four groups, two of which were fed a normal diet and two of which were fed a cholesterol-rich diet. HMG-CoA reductase mRNA and protein from liver were then isolatedAnd quantified. Graph A shows the results of the mRNA isolation.a. What is the effect of cholesterol feeding on the amount of HMG-CoA reductase mRNA?b. What is the purpose of also isolating the mRNA for the protein actin?
HMG-CoA reductase protein was isolated by precipitation with a monoclonal antibody to HMG-CoA reductase. The amount of HMG-CoA protein in each group is shown in graph B.
c. What is the effect of the cholesterol diet on the amount of HMG-CoA reductase protein?d. Why is this result surprising in light of the results in graph A?e. Suggest possible explanations for the results in graph B.