Porphyrin Ring Synthesis and Breakdown

Daniel Wellman

What are they?

• Porphyrin rings are biological molecules used in a variety of essential chemical processes

•The two most well-known porphyrins are heme and chlorophyll

Because of their large conjugated double bond system, porphyrins typically absorb visible light

Chlorophyll’s green color, heme’s red, and the blue blood of some sea creatures are all a result of this absorbance

Additionally, the 4 nitrogen atoms at the center of the ring are excellent at conjugating metals because of their lone pairs

As a result, porphyrins are a common way to attach metals to proteins

Heme

• Porphyrin synthesis is still poorly understood• Intermediates are known, but reaction

mechanisms are generally still speculative• Plants and animals follow different synthetic

pathways in the early stages of ring formation• Once delta-aminolevulinate is synthesized, the

synthetic pathways are identical• Delta-aminolevulinate is used only for porphyrin

synthesis, so its synthesis is tightly regulated—at least 6 enzymes or products are controlled

Beginning steps in plants

O

NH3

O

O

O

O

NH3

t RNA-Glu

O

O

ATP->AMP

NADPH

O

O

NH3

OH

O

O

O

NH3

Glutamate glutamyl-tRNA

glutamate 1-semialdehydedelta-aminolevulinate

O

NH3

GluANRt

O

O

NADPHO

O

NH3

OH

glutamyl-tRNA glutamate 1-semialdehyde

This reaction uses Glu tRNA as a leaving group, which is very unusual

Guu tRNA acts like a phosphate group, facilitating transformation of a carboxylic acid to an aldehyde

Because of the size of the tRNA molecule, the enzyme must be highly flexible

The only well-understood mechanism is the last step

Structure of glutamate-1-semialdehyde 2,1-

aminomutase To limit access to the highly reactive PLP active site, the enzyme uses a “gating loop,” an alpha-helix that changes into a beta-sheet to expose the active site of the enzyme

This prevents a variety of side reactions from happening

What about people?

• For animals and some eukaryotes, synthesis of delta-aminolevulinate follows a different path

O

O

O

S-CoA

Succinyl-CoA

+O

NH3

O

Glycine

O

O

O

NH3

O

O

O

O

O

NH3

delta-aminolevulinate

What’s Next?The next step is the synthesis of Porphobilinogen

from 2 molecules of delta-aminolevulinate

O

O

O

NH3

delta-aminolevulinate

N

NH3

O

O

O

O

H

O

O

O

NH3

Proposed mechanism for Porphobilinogen synthesis

• Another cool thing about this enzyme is that it contains zinc, but the zinc does not react in any way

• Experiments determined that zinc is used to control reactivity and make the catalytic site more selective

N

NH3

O

O

O

O

H

4 NH HN

NHHN

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

HO

This reaction mechanism is fairly complicated, but isotope experiments have confirmed which atoms come from which precursor molecules

The results of these studies are shown in blue and red

Ring closure

NH HN

NHHN

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

H

H NH HN

NHHN

O

O

O

O

O

O

O

O

O

O

O

O

O

O

Mechanism for ring closure

Steps after ring formation

Essentially all that is needed after these steps is insertion of the metal at the center of the ring

Additional covalent modification of the outside of the ring is often performed

Examples of this modification include siroheme and chlorophyll

Heme Iron Insertion

• Metal insertion in porphyrins is performed by a class of complexes called chelatases

Ferrochelatase

Ferrochelatase activity

To force iron (II) into the heme ring, ferrochelatase holds the ring in a bent conformation

This bending causes the ring to pop out of the enzyme once iron insertion is complete

What do you do with it once you’re done with it?

• Recycling the iron in heme is critical for survival

• To make sure iron is not unnecessarily wasted, a complex system for recycling the iron in heme is employed

• The ring itself is solubilized and eliminated—only the metal is reused

Heme Breakdown

Followed By…

• Reaction with 2 molecules of sugar to yield:

Bilirubin diglucuronide

So what happens if you have a problem making or breaking down porphyrins?

A variety of medical conditions can occurBabies often suffer from jaundice, which is caused by an

accumulation of heme breakdown products in fatJaundice is typically harmless, and typically is cured with

exposure to lightThe condition fades once proper liver function develops in

the infantThere are some serious conditions that result from

accumulation of porphyrinsThe most severe of these is porphyria, which is caused by

a problem in one of the first 8 steps of synthesis

Porphyria• Porphyria is assumed to be the root of our myths about vampires

and werewolves• In all cases, the urine changes colors, typically to red, but

occasionally to pink or purple, and the teeth become reddish-brown, and the lack of heme means that sufferers are anemic

• In erythroid porphyrias, the skin becomes extremely sensitive to light, and will blister or scar if exposed, and hair will grow in unusual places

• Ulcers may also cause their hands to become deformed and paw-like and mutilate the nose, ears, eyelids, and fingers

• In hepatic porphyrias, damage to the central nervous system results, often leading to seizures or insanity

• Because of the wide variety of symptoms, porphyria has been associated with insane rulers like King George III