Organic Chemistry

910 CHAPTER 21 More About Amines • Heterocyclic Compounds

more tightly than to Consequently, breathing carbon monoxide can be fatal

because it prevents the transport of oxygen in the bloodstream.

The extensive conjugated system of porphyrin gives blood its characteristic red

color. Its high molar absorptivity (about 160,000) allows concentrations as low as

to be detected by UV spectroscopy (Section 8.10).

The biosynthesis of porphyrin involves the formation of porphobilinogen from two

molecules of -aminolevulinic acid. The precise mechanism for this biosynthesis is as

yet unknown. A possible mechanism starts with the formation of an imine between the

enzyme that catalyzes the reaction and one of the molecules of -aminolevulinic acid.

An aldol-type condensation occurs between the imine and a free molecule of -

aminolevulinic acid. Nucleophilic attack by the amino group on the imine closes the

ring. The enzyme is then eliminated, and removal of a proton creates the aromatic ring.

Four porphobilinogen molecules react to form porphyrin.

repeat three more

times using an

intramolecular reaction for

the third repetition

subsequent oxidation

increases the unsaturation

+ + NH 3

H 3 NCH 2 NH

+

H 3 N CH 2 NH

+

H 3 NCH 2 NH+ CH 2 NH

H

+

NH H 3 NCH 2 NH CH 2 NH

porphyrin

N

H

N

H

HN

d

d

d

1 * 10 -^8 M

O 2 Fe^2 +.

E

EE

E E

−OOCCH 2

−OOCCH 2 CH 2 CH 2 COO−

H 2 NCH 2

CH 2

O C

CH 2

NH 2

E NH 2

NH 2

H 2 NCH 2 NH 2

C C

C

enzyme

a mechanism for the biosynthesis of porphyrin

-aminolevulinic acid

−OOCCH 2

CH 2

C

CH 2

CH 2

NH 2 H 2 O

−OOCCH 2

HC−

C

CH 2

NH 2

base

+

+ H 2 O

NH

N N

+

+

O

CH 2 COO−

CH 2

C

CH 2

NH 2

H 2 NCH 2 NH 2

C C

NH N CCH 2

H OH

−OOCCH 2 CH 2 CH 2 COO− −OOCCH 2 CH 2 CH 2 COO−

H+

−H+

NH 2

porphobilinogen

H 2 NCH (^2) HN+ H H 2 NCH 2 NH
−OOCCH 2 CH
2 CH 2 COO−
−OOCCH 2 CH
2 CH 2 COO−
E