1.1. Structure of Tetrapyrroles
Tetrapyrroles are a group of organic molecules that includes chlorophyll (Figure 1), hemes (Figure 2), bilins (Figure 3), and corrins, such as vitamin B12 (37). These molecules are also often referred to as por-phyrins, although strictly, these are only those compounds with the same oxidation state as heme. Chlorophyll, for example, has one more saturated bond and is therefore a chlorin (30).
A pyrrole is a 5-membered ring containing one nitrogen, which is colorless, but when four pyrroles are linked by unsaturat-ed methine groups, the properties of the tetrapyrrole macrocycle are changed dramatically, and two extremely important characteristics emerge. Tetrapyrroles contain a ring rich in conjugated double bonds that absorb light strongly, and they have four nitrogens oriented towards a cavity that may accommodate metal ions and allow coordination of the metal ion above or below the plane of the macrocycle.
Heme, Chlorophyll, and Bilins: Methods and Protocols Edited by A.G. Smith and M. Witty ©2002 Humana Press, Totowa, NJ
These metals have stabilized oxidation states and solubility. Aside from these two important properties, tetrapyrroles also have a subtly substituted ring structure which alters the light absorbance properties of the conjugated double bond system, the geometry of metal ion binding (and therefore the type of metal bound), and mediates interactions of the tetrapyrrole with proteins.
Most metals and metalloids in the periodic table have been incorporated into complexes with tetrapyrroles (27), and many metals are observed in mineral por-phyrins (10). However, because of the differences in abundance and differential stability of the complexes, nickel and vanadium are the most common ions in natural abiotic porphyrins, whereas the following seven have been seen in living systems: Mg, Fe, Mn, Co, Zn, Ni, and V (6).
Porphyrins are spontaneous products of organic chemical reactions which can be synthesized in Urey-Miller type experiments that mimic prebiotic atmospheric conditions: UV irradiation of 5-aminole-
vulinic acid (ALA) has produced pyrroles (33), while electrical discharge in the presence of pyrrole and formaldehyde has produced porphyrins (14), and they have been detected in sterile meteorites (14,15).
Porphyrins are chemically stable (30) and can persist in the environment for many millions of years. Porphyrins are found in large fossils such as mollusk shells (17) and also in molecular fossil forms in geological strata. The best examples of these are coal and oil deposits, where they are found as mostly nickel(II) and vanadyl complexes (9). Mineral porphyrins have been detected in sedimentary deposits with high organic content laid down as early as precambrian times (8). They may precipitate to form distinct bedding planes and, although most deposits contain only a few parts per million, some contain significant amounts of free or complexed porphyrins, for example the Gibellina sedimentary deposits, which contain 24 mg/g copper and nickel porphyrins (29).
Although they are found in abiotic systems, most tetrapyrroles are biological, and indeed they are the most conspicuous living molecule on earth. Chlorophylls can be seen from satellites in space, where vegetation types can be identified and used to predict underlying geology (31). Even when viewed from outside, the Earth looks enticing because of tetrapyrroles. If there are Men from Mars, they would pick on Earth for special interest, and they would be right to do so (25).
1.3. Importance of Tetrapyrroles in Nature
Although there are a large number of chemical types and ionic conjugates of tetrapyrroles, only a few species and their derivatives are very abundant in nature: chlorophylls, hemes, and linear tetrapyrroles, the bilins. Tetrapyrroles are important in living cells because of their physical properties. The tetrapyrrole macrocycle can be highly conjugated and absorb visible light strongly, therefore many tetrapyrroles are photochemically active, the most important interaction with light being the capture of energy by chlorophyll in photosynthesis.
Chlorophylls are an essential part of the photosynthetic apparatus, and the heme of cytochromes is an essential part of electron transfer chains in both respiration and photosynthesis. These two tetrapyrrole types are essential for the most significant reduction and oxidation processes in nature. Tetrapyrroles are also essential in many other biochemical processes. They form the prosthetic groups of metalloen-zymes such as sulfite reductase, nitrite reductase, peroxidase, and catalase, which carry out a wide range of oxidation and reduction reactions. Vitamin B12 is a cobalt tetrapyrrole complex that acts as a cofactor in methyltransferases, and factor F430 is a nickel tetrapyrrole that is involved in methane formation in certain bacteria. Bilins are linear tetrapyrroles with no tightly bound metal and are important as the accessory pigments in algae and as phy-tochromobilin, the red-light receptor of higher plants (Chapters 12-14) (21).
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