Section 8. Amino Acid Metabolism

Section 8. Amino Acid Metabolism

Porphyrins, heme, bile pigmentsPorphyrins, heme, bile pigments

11/22/04

Heme

• In addition to the heme in hemogloblin and myoglobin, molecules with the porphyrin ring structure include cytochromes, and in plants, the chlorophylls.

• Heme is synthesized in most cells. Reticulocytes make ~ 4 x 1012 hemes per second.

• In all cases, the precursors are glycine and succinyl CoA.

1

Heme Structure

• The core protoporphyrin ring structure is constant.

• The sidechains vary in other porphyrins.

• The protein structure determines the properties of the prosthetic group. (p270)

2

Protoporphyrin IX Structure

• Fe(II) displaces the two protons in the center.

Stryer 4th

3

N NH

NNH

OO

O O

B12 Coenzyme

Structure• Not heme, but

similiar.• Compared to a

protoporphyrin ring, the corrin ring has smaller space in center to fit Co(II), smaller than Fe(II).

• Vitamin B12, cobalamin, may have Co(III) with 5’-deoxyadenosine substituted by CN or CH3. Stryer 4th

4

Succinyl CoA Biosynthesis

• One heme precursor, succinyl CoA. can be made from propionyl CoA, or from -ketoglutarate in the Krebs cycle.

• Propionyl CoA can be made from amino acids, or from the oxidation of odd-numbered fatty acids.

• After propionyl CoA is carboxylated, the conversion of methylmalonyl CoA to succinyl CoA requires vitamin B12.

Valine

Methionine

Isoleucinepropionyl CoA

SCoA

methylmalonyl CoA

HCO3- ADP

+ ATP + Pi

SCoA

B12methylmalonyl CoA mutase

succinyl CoASCoA

propionyl CoAcarboxylase

OO

O

CH3H

CH3

CH2O

OO

O

5

Heme Biosynthesis: Part 1

• -ALA synthetase catalyzes the committed step.

• Reactions occur in the mitochondrial matrix.

• 8 succinyl CoA & 8 glycine.

NH NH NH NHNH3

+

A PA A APPP

+

OO

O

succinyl CoASCoA

OO

O

-aminolevulinate (-ALA)

NH

O

OOO

NH3

+

porphobilinogen

O

O

glycine

a tetrahydropyrrole

-ALA synthetase

-ALA dehydrase

porphobilinogendeaminase

PLP

A P

+

+

CoA + CO2 +

NH3

NH3

6

Heme Biosynthesis: Part 2

• Notice that one of the pyrrole rings was “rotated” by the uroporphyrinogen cosynthetase making the cyclic uroporphyrinogen III asymmetric (look at the A-P distribution).

• The multiple arrows are several decarboxylations.

NH NH NH NHNH3

+

A PA A APPP

Uroporphyrinogen IIISynthetase and Cosynthetase

ferrochelatase

2 H+

protoporphyrin IX HEMEuroporphyrinogen III

Fe(II)

N NH

NNH

OO O O

N NH

NNH

OO O O

O

O

O

O

OOOO

O

O

O

O

N N

NN

OO O O

Fe

7

Ferrochelatase Mechanism

• Ferrochelatase (FC) binds Fe(II), displacing 2 H+ on the enzyme.

• Next it “domes” the protoporphyrin IX.• Fe(II) is inserted, exchanging 2 H+.• It appears that distortion of the

protoporphyrin planar structure occurs after the metal binds to FC.

• From Blackwood etal, Biochemistry 37:779 (1998).

8

Stoichiometry

• This ignores the conversion of some of the acetate and propionate groups to methyl and vinyl groups.

8 glycine + 8 succinyl CoA + Fe(II)

1 heme

9

Control Mechanisms

• -ALA synthetase catalyzes the committed step.

• Heme inhibits the activities of -ALA synthetase, -ALA

dehydrase and ferrochelatase.• Heme also binds a nuclear receptor, which

reduces the transcription of the mRNA for -ALA synthetase.

• Heme inhibits transport of -ALA synthetase into the mitochondria.

10

OO

CH2CH2

OCH2

NH3

+

Diseases Related to Heme Synthesis

• Congenital erythropoietic porphyria is due to deficient uroporphyrinogen III cosynthetase activity in red blood cell precursors.

• The symmetric product uroporphyrinogen is not degraded.• Short rbc lifetime, photosensitive skin, red enamel (UV light).• Deficient ferrochelatase causes erythropoietic protoporphyria,

which has similar but milder symptoms.

Asymmetric Symmetric

N NH

NNH

OO O O

O

O

O

O

OO

OO

O

O

O

O

N NH

NNH

OO

O O

O

O

OO

OO

O

O

O

OO

O

11

Acute Intermittent Porphyria

• Due to deficient uroporphyrinogen synthetase activity in the liver, coupled to elevated -ALA synthetase activity.

• The result is high [-ALA] and [porphobilinogen].

• Symptoms are abdominal pain, dark urine, neurological aberrations.

• King George III ? Vincent Van Gogh ?

12

Heme Degradation

• Heme is degraded in the liver.• The final product, bilirubin

diglucuronide, is transported in bile from the liver to the gall bladder and excreted.

• It, and several related structures, are bile pigments.

• Notice energy is used to reduce biliverdin to a waste product.

O2

+ NADPH

H2O

+ NADP+ Fe(III)

heme oxygenase

M VP P MMMV

biliverdinNADPH

NADP+

biliverdinreductase

M VP P MMMV

bilirubin

2 UDP-glucuronate

2 UDP

M VPG PG MMMV

bilirubin diglucuronide

+ CO

NH NH NH NH OO

NH N NH NH OO

NH NH NH NH OO

N N

NN

OO

O O

Fe

13

Hb Heme Life Cycle • Synthesized in red blood cell precursors.• Spends about 120 days in the bloodstream.• Identified as “old” by the spleen, which disrupts the

rbc membrane, freeing the Hb.• Transported to the liver as a haptoglobin-hemoglobin

complex.• Degraded in the liver to amino acids, Fe(III) and

bilirubin diglucuronide.• The iron is transported to other cells, bound to

transferrin, for reuse.• Bilirubin diglucuronide excreted by gall bladder.

14

Bile

• About 500 ml per day of bile is made by the liver, which excretes it and transfers it to the gall bladder.

• The gall bladder concentrates bile and excretes it into the intestinal lumen.

• The three main dissolved constituents are glycocholate (~80%), phospholipids (~15%) and cholesterol (~5%).

• Glycocholate, a fat-solubilizing detergent, is reabsorbed by the intestinal epithelium.

• The other constituents, such bile pigments, are minor.

15

GallstoneFormation

• At typical total concentrations in bile, the solubility of the constituents is limited (Percent is percent of total solid).

• The hatched area includes the combinations of cholesterol, phospholipid and bile salt that are soluble.

• Precipitation can lead to gallstones, which are painful if they block the bile duct.

100

0100

100

0

0

5050

50Percent Bile Salt

PercentCholesterol

Percent Phospholipid

*

* 80% bile salt 15% phospholipid 5% cholesterol

16

Neonatal Jaundice

• The immature liver cannot add glucuronate to bilirubin.• Bilirubin circulates in the blood instead of being transported

to the gall bladder.• Bilirubin can cross the immature blood brain barrier and

causes brain damage.• Two therapies are successful :

– Visible light irradiation. – Sn-substituted protoporphyrin.

• Sn-protoporphyrin inhibits heme oxygenase.

bilirubin

NH NH NH NH OO

OO O

O

17

Next topic: One-carbon metabolism, purine metabolism