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Module 4 Biochemistry Review Part I

Office of Academic Enhancement Sara Koenig

Structure of Cholesterol

· Lipid structure that is different from phospholipids · STEROID! · 4 rings, 27 carbons · Steroid end = hydrocarbon tail! · Non-steroid end = OH group on C3 · Cholesterol is an amphipathic molecule!

Cholesterol = amphipathic

· The hydrocarbon tail likes the inside portion of the membrane bilayer · The OH group on the non=steroidal end likes the outside portion of the membrane and interacts w/ carbonyl groups on phospholipids.

The Job of Cholesterol

· Controls membrane fluidity = KEY REGULATOR! · Why? b/c it disrupts regular interactions between fatty acid chains and forms specific complexes with phospholipids (by the OH group on C3). · Made into:

­ Steroid hormones

· · · · Progesterone Testosterone Estradiol cortisol

­ Bile salts ­ Cholesterol esters for lipoproeins

· ABSENT IN PROKARYOTES, but present in all animals.

Synthesis of Cholesterol

· Step 1 = REGULATED step = synthesis of Mevalonate

­ Done by HMG CoA reductase in the cytosol!! ­ ****if HMG-CoA is in the mitochondrion it is broken down into Acetyl CoA and Acetoacetate (ketone bodies), NOT converted in the mevalonate.

Step 1a Step 1b


Synthesis of Cholesterol

· Step 2 = Synthesis of Isopentenyl pyrophosphate

­ Needs THREE reactions that require ATP in cytosol!!! ­ *******The THIRD Pi allows for the decarboxylation of 5pyrophospho mevalonate in one step ­ END PRODUCT = 3-isopentenyl pyrophosphate = 5C building block of cholesterol vitamin K2, ConeyzmeQ10, farnesyl or geranylgeranyl attachment.

Synthesis of Squalene cholesterol

· Making squalene

­ 1. Make geranyl pyrophosphate = 2 IPP stuck together

· ***one of the IPP is AKA Dimethylallylpyrophosphate, an isomer of IPP!

­ 2. Make Farnesyl phyrophosphate = Geranyl PP + IPP ­ 3. Make Squalene = 2 Farnesyl PP stuck together!




Oxidosqualene synthase

Synthesis of Cholesterol from Squalene

· Squalene cyclizes to make lanosterol, and then cholesterol.

­ 1. Oxygen + NADPH are used to make squalene epoxide. ­ Squalene epoxide converts to lanosterol w/ oxidosqualene cyclase. ­ Lanosterol is converted into cholesterol w/ 19 more steps.

Oxidosqualene synthase

Regulation of Cholesterol Synthesis

· HMG CoA reductase is in the ER membrane; the active site on the C-terminus faces the cytoplasm. 1. Feedback regulation:

­ You should only eat 300 mg/day of cholesterol ­ When cholesterol levels are low, the SREBP is released from the ER or nuclear membrane by proteolytic cleavage.

· HIGH CHOLESTEROL = SREBP is anchored to the ER or nuclear membrane tightly or is quickly degraded if free. · LOW CHOLESTEROL = SREBP migrates to the nucleus and binds the SRE of the HMG-CoA reductase gene and other genes in the cholesterol biosynthetic pathway to enhance transcription.

­ Translation Rate of HMG-CoA reductase mRNA is inhibited by metabolites of mevalonate and dietary cholesterol. ­ Degradation of HMG-CoA reductase is regulated by the Nterminus in the ER high cholesterol levels change its structure making the enzyme more likely to be destructed. ­ Low ATP or High AMP causes phosphorylation of enzyme = SWITCHED OFF

Regulation of Cholesterol Synthesis

Feedback Inhibition:

1. Decreased cholesterol causes release of SREBP which upregulates the txn of HMG-CoA Reductase.



HMG Co-A reductase

3' DNA

2. Increased metabolites of cholesterol and mevalonate inhibit the translation of HMG Co-A reductase.



3. Increased cholesterol levels cause a special oligomerization of the Nterminus of the enzyme enhancing its degradation. 4. High AMP levels or Low ATP levels enhances phosphorylation of HMG-CoA reductase turning it off!!

Bile Salts

· Bile salts are polar derivatives of cholesterol which have both polar and non polar regions · Detergents · Water soluble · Made in liver, stored in gallbladder, secreted in small intestine. · Major constituent of bile; Increases the surface area of lipid for easier digestion. · Low bile salt production = fat excretion! (up to 30g/day) · Bile acids = major fate of cholesterol (800mg/day), hormones only take up 30mg/day.

Bile Salts

· Bile salts are created from catalytic reactions carried out by microsomal p450 enzymes:


7 - hydroxylase

7 - cholesterol Trihydroxycoprostanoate

Cholyl CoA

Glycine Taurine



************7 - hydroxylase is the rate limiting step!!!

Heme Synthesis

· What you need to know:

­ The pathway and roles of enzymes ­ Pathway for the degradation of bilirubin diglucuronide ­ Regulation of heme synthesis in both erythroid and non-erythroid cells.

Heme Synthesis

· Basics: page 87

­ Pyrrole = 5 membered ring, 4C and 1N ­ Porophyrinogen = ring formed form four pyrrole rings joined by methylene bridges ­ Methenyl = =CH­ Vinyl = -CH=CH2 ­ Page 89: The prefix (-uro, copro-, proto-) refers to the number and nature of the side chains attached to the porphyrin or porphyrinogen. ­ Roman Numerals refer to the arrangement fo the side chains.

Heme Synthesis

ferrochelatase Protoporphyrinogen oxidase


Coproporphyrinogen oxidase


Porphobilinogen deaminase

Uroporphyrinogen III Cosynthase

Uroporphyrinogen Decarboxylase

Heme Synthesis Enzymes

· ALA synthase ­ FIRST and KEY REGULATED step.

· · Makes ­ Aminolevulinic acid, sticks succinyl CoA & Glycine together. In the mitochondria!!!

· ALA dehydratase ­ sticks two d-ALA together, takes away a water = porphobilinogen, a PYRROLE RING! · Porphobilinogen deaminase = sticks 4 porphobilinogens together, taking away 4 NH4s.

­ First hydroxymethylbilane is made but it cyclizes to make uroporphyrinogen I spontaneously IF the person is NOT healthy. YOU SHOULD NOT HAVE UROPORHYRINOGEN I!!

· Urophorphyrinogin III Cosynthase ­ takes the hydroxymethylbilane and converts it to Urophorphyrinogen III and not I b/c it flips one of the pyrrole rings so that the product of the cyclization is Uro III.

Heme Synthesis Enzymes

· Uroporphyrinogen Decarboxylase

· Makes Coproporphyrinogen III by removing 4 CO2s.

· Coproporphyrinogen oxidase (BACK into the MITOCHONDRION!)

­ Makes protoporphyrinogen IX by removing 2CO2s, turning two of the propionyl groups into vinyl groups.

· Protoporphyrinogen Oxidase (MITOCHONDRIA) ­ causes oxidation of the methenyl bridges, turning this compound from a porphyrinogen into a porphyrin. · Ferrochelatase (MITOCHONDRIA) ­ sticks an Fe2+ into the center of protoporphyrin IX into HEME!

Regulaton of Heme Synthesis

· ALA synthase

­ Has TWO isoenzymes ­ ALA synthase 2 = in erythroid cells = for hemoglobin ­ ALA synthase 1 = most other cells = for heme-containing proteins like cytochromes ­ ALA synthase of both types has a fast turnover rate.

Regulaton of Heme Synthesis

· ALA synthase 2 ­ in Erythroid Cells

­ 3. )Heme and globin synthesis are balanced

· Protein kinase phorphorylates eIF2 (translation factor!) to inactivate its action. This occurs when [heme] is low. · When [heme] is elevated, it binds the protein kinase, thus increasing the action of eIF2, causing increased globin synthesis. HEME Protein



Low Heme

Protein Kinase


5' mRNA




Regulaton of Heme Synthesis

· ALA synthase 2 ­ in Erythroid Cells

­ 1.) Iron induces heme synthesis! - IBP (iron binding protein) binds the 5' UTR of the mRNA for ALA synthase 2 when iron is present.



Fe2+ IBP


ALA Synthase 3'

­ 2.) Excess heme inhibits uptake of iron by interfering with Transferrin interaction with the transferrin receptor.

· Low iron in the cell causes mechanism one to become inactive.

Regulaton of Heme Synthesis

· ALA synthase 1 ­ in Non-erythroid Cells

­ 1.) Heme inhibits the transport of ALA synthase I into the mitochondria!! - Major control mechanism ­ 2.) Heme inhibits synthesis of ALA synthase I ­ 3.) Heme inhibits the activity of ALA synthase I. This is an allosteric effect!

OVERALL, in Non-Erythroid Cells HEME blocks: 1) Transport 2). Synthesis 3). Activity of ALA synthase

Heme Degradation ­ in the RE

· Heme oxygenase ­ inducible by heme, tin, cobalt, or stress in the RE system to break down Heme into biliverdin by releasing Fe and CO.

­ Three isozymes of heme oxygenase:

· RE system · Brain - uninducible · Testes ­ uninducible

· Biliverdin reductase in the RE ­ makes Bilirubin!

­ Bilirubin carried to liver for further metabolism

Heme Degradation ­ in the RE

· UDP-glucuronyl transferase (UDPGT) in the liver:

­ Bilirubin is conjugated with two sugars making it water soluble. ­ Secreted into the intestine as part fo the bile. ­ Gives bile the yellow color. ­ Urine gets some yellow color from reabsorped biproducts of bilirubin metabolism from bacteria in the gut, and the feces as well!

HEME Biliverdin Bilirubin Bilirubin Diglucuronide

DISORDERS of HEME Metabolism

· Porphyrias = lesions in heme biosynthetic pathway

­ Lesions in ALA synthase = no heme ­ Lesions past ALA synthase step = less heme made ­ Less heme made? This upregulates ALA synthase and you get a cluster mess!

DISORDERS of HEME Metabolism

1. X-linked Anemia ­ x-linked deficiency of ALA synthase = not enough heme = not enough Hemoblogin = anemia! 2. ALA dehydratase = aminolevulinic acid dehydratase porphyria.

- Rare w/ abdominal and neurological symptoms. Buildup of d-ALA occurs; susceptible to lead poisoning!

3. Porphobilinogen deaminase = Acute Intermittent Porphyria

- lessened activity of porpho deaminase causes increased porphobilinogen AND d-ALA. - Abdominal and neurological symptoms; psychosis

DISORDERS of HEME Metabolism

4. Uroporphobilinogen III Cosynthase ­ Congenital erythropoetic porphyria. - Rare, causes accumulation of uroporphyrinogen I, uroporphyrin I, coproporphyrinogen I, and coproporphyrin I. - PHOTOSENSITIVE, discolored urine, PIGMENTS IN TEETH

5. Uroporphyrinogen decarboxylase = Porphyria Cutanea Tarda.

Most common! Accumulation of Uroporphyrin III and the symptoms are mainly in the skin. Induced by hexachorlobenzene, treatment = chloroquin and phlebotomy.

6. Coproporphyrinogen oxidase = Hereditary Coproporphyria

- relatively rare, accumulation of coporporphyrin III. Symptoms are largely neurological and abdominal and sometimes involve the skin.

Heme Synthesis

ferrochelatase Protoporphyrinogen oxidase


Coproporphyrinogen oxidase


Porphobilinogen deaminase

Uroporphyrinogen III Cosynthase

Uroporphyrinogen Decarboxylase

DISORDERS of HEME Metabolism

7. Protoporphyrinogen oxidase ­ Variegate Porphyria - Skin lesions, abdominal, neurological due to many porphyrins


8. Ferrochelatase = Erythropoietic Protoporphyria.

Photosensitive, and porphyrins accumulate in the skin!


· Lead Poisoning simulates a porphyria b/c it blocks many

enzymes of heme synthesis: · Mainly d-ALA dehydratase and ferrochelatase · Blocks (The beginning AND the end of the pathway!) · SYMPTOMS:

·Abdominal ­ building up of toxic intermediates and by-products in the liver causing pain · Neurological ­ toxic intermediates build up in the brain causing psychotic behavior or other neurological symptoms · Skin ­ due to accumulation of intermediates and byproducts in the skin. Reaction w/ UV light makes free radicals and causes cell death! · Teeth ­ accumulation of the intermediates in the teeth causes discoloration.

Drug Interactions

· Some porphyrias are very aggravated by drugs that induce

the action of cytochrome p450 in the liver (phenobarbital).

· ALA dehydratase porphyria · Acute intermittent porphyria · Variegate porphyria · Hereditary coproporphyria

· Phenobarbital (and others) make the liver make cytochrome p450, which contains heme. This activates the heme synthetic pathways and causes increased buildup of the toxic intermediates.


· Increased synthesis of sex hormones requires increased

production of enzymes making the hormones. · Some of the hormone synthesis enzymes are types of cytochrome p450. · Puberty induces heme synthesis andaccumulation of toxic intermediates for those who have a lesion in the synthetic pathway. · Look at the Diagram on page 98!

Types of Jaundice


Class I Hemolytic Jaundice Class II Cirrhosis

Bilirubin Glucuronoside


Bilirubin Albumin


Fecal Bilirubin



Erythrocyte destruction Can't react bilirubin w/ glucuronoside " " / Immature liver

Complete loss of UPD glucuronosyltransferase Only 10% activity of UDP glucuronosyltransferase


Neonatal Jaundice

Low Low Low Very Low Low Low High

High High High Very High High High Normal

Low Low Low Low Low Low Low

Crigler-Najjar Syndrome I

Criger-Najjar Syndrome II Gilbert Syndrome Class III Obstructive Jaundice

50% activity of UDPglucuronosyltransferase

Tumor or gallstones blocks bile from intestines


Module 4 Biochemistry Review

32 pages

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