Garrett R.H., Grisham C.M. - Biochemistry (1999)(2nd ed.)(en)

FMN, and FAD are shown in Figure 18.21. The isoalloxazine ring is the core structure of the various flavins. Because the ribityl group is not a true pentose sugar (it is a sugar alcohol) and is not joined to riboflavin in a glycosidic bond, the molecule is not truly a “nucleotide,” and the terms flavin mononucleotide and dinucleotide are incorrect. Nonetheless, these designations are so deeply ingrained in common biochemical usage that the erroneous nomenclature persists. The flavins have a characteristic bright yellow color and take their name from the Latin flavus for “yellow.” As shown in Figure 18.22, the oxidized form of the isoalloxazine structure absorbs light around 450 nm (in the visible region) and also at 350 to 380 nm. The color is lost, however, when the ring is reduced or “bleached.” Similarly, the enzymes that bind flavins, known as flavoenzymes, can be yellow, red, or green in their oxidized states. Nevertheless, these enzymes also lose their color on reduction of the bound flavin group.

Flavin coenzymes can exist in any of three different redox states. Fully oxidized flavin is converted to a semiquinone by a one-electron transfer, as shown in Figure 18.22. At physiological pH, the semiquinone is a neutral radical, blue in color, with a max of 570 nm. The semiquinone possesses a pKa of about 8.4. When it loses a proton at higher pH values, it becomes a radical anion, displaying a red color with a max of 490 nm. The semiquinone radical is particularly stable, owing to extensive delocalization of the unpaired electron across the -electron system of the isoalloxazine. A second one-electron transfer converts the semiquinone to the completely reduced dihydroflavin as shown in Figure 18.22.

Access to three different redox states allows flavin coenzymes to participate in one-electron transfer and two-electron transfer reactions. Partly because of this, flavoproteins catalyze many different reactions in biological systems and work together with many different electron acceptors and donors. These include two-electron acceptor/donors, such as NAD and NADP , oneor two-elec-

592 Chapter 18 ● Metabolism—An Overview

tron carriers, such as quinones, and a variety of one-electron acceptor/donors, such as cytochrome proteins. Many of the components of the respiratory electron transport chain are one-electron acceptor/donors. The stability of the flavin semiquinone state allows flavoproteins to function as effective electron carriers in respiration processes (Chapter 21).

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Nucleophilic

attack

Y

FIGURE 18.24 ● Acyl transfer from acyl-CoA to a nucleophile is more favorable than transfer of an acyl group from an oxygen ester.

594 Chapter 18 ● Metabolism—An Overview

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Vitamin B6: Pyridoxine and Pyridoxal Phosphate

The biologically active form of vitamin B6 is pyridoxal-5-phosphate (PLP), a coenzyme that exists under physiological conditions in two tautomeric forms (Figure 18.25). PLP participates in the catalysis of a wide variety of reactions involving amino acids, including transaminations, - and -decarboxylations,- and -eliminations, racemizations, and aldol reactions (Figure 18.26). Note that these reactions include cleavage of any of the bonds to the amino acid alpha carbon, as well as several bonds in the side chain. The remarkably versatile chemistry of PLP is due to its ability to

(a)form stable Schiff base (aldimine) adducts with -amino groups of amino acids, and

(b)act as an effective electron sink to stabilize reaction intermediates.

The Schiff base formed by PLP and its role as an electron sink are illustrated in Figure 18.27. In nearly all PLP-dependent enzymes, PLP in the absence of substrate is bound in a Schiff base linkage with the -NH2 group of an active site lysine. Rearrangement to a Schiff base with the arriving substrate is a transaldiminization reaction. One key to PLP chemistry is the protonation of the Schiff base, which is stabilized by H bonding to the ring oxygen, increasing the acidity of the C proton [as shown in (3) of Figure 18.27]. The carbanion formed by loss of the C proton is stabilized by electron delocalization into the pyridinium ring, with the positively charged ring nitrogen acting as an electron sink. Another important intermediate is formed by protonation of the aldehyde carbon of PLP. As shown, this produces a new substrate–PLP Schiff base, which plays a role in transamination reactions and increases the acidity of the proton at C , a feature important in -elimination reactions.

The versatile chemistry of pyridoxal phosphate offers a rich learning experience for the student of mechanistic chemistry. William Jencks, in his classic text, Catalysis in Chemistry and Enzymology, writes:

It has been said that God created an organism especially adapted to help the biologist find an answer to every question about the physiology of living systems; if this is so it must be concluded that pyridoxal phosphate was created to provide satisfaction and enlightenment to those enzymologists and chemists who enjoy pushing electrons, for no other coenzyme is involved in such a wide variety of reactions, in both enzyme and model systems, which can be reasonably interpreted in terms of the chemical properties of the coenzyme. Most of

COO–