Phylloquinone reactions

An example of a biological Friedel-Crafts reaction occurs during the biosynthesis of phylloquinone, or vitamin Kl( the human blood-clotting factor. Phylloquinone is formed by reaction of 1,4-dihydroxynaphthoic acid with phytyl diphosphate. Phytyl diphosphate first dissociates to a resonance-stabilized allylic carbocation, which then substitutes onto the aromatic ring in the typical way. Several further transformations lead to phylloquinone (Figure 16.10). 

Figure 16.10 Biosynthesis of phylloquinone (vitamin K-j) from 1,4-dihydroxynaphthoic acid. The key step that joins the 20-carbon phytyl side chain to the aromatic ring is a Friedel-Crafts-like electrophilic substitution reaction.

As the above mentioned studies with high supplementation dosages exemplarily show, there is no known toxicity for phylloquinone (vitamin Kl), although allergic reactions are possible. This is NOT true for menadione (vitamin K3) that can interfere with glutathione, a natural antioxidant, resulting in oxidative stress and cell membrane damage. Injections of menadione in infants led to jaundice and hemolytic anemia and therefore should not be used for the treatment of vitamin K deficiency. 

Reaction centers of bacteria contain polypeptides, bacteriochlorophylls, bacteriopheo-phytins, two quinines, and nonheme iron atom. In some bacterial species, both the quinones are ubiquinones, whereas in some others one of the quinones is menaquinone [37,39]. Depending on the bacterial species chloroplasts contain plastoquinone and phyl-loquinone. Structures of ubiquinone, menaquinone, and phylloquinone are provided in Figures 7.12 through 7.14, respectively. 

This enzyme [EC 1.6.99.2] (also known as NAD(P)H quinone reductase, DT diaphorase, quinone reductase, azoreductase, phylloquinone reductase, and menadione reductase) catalyzes the reaction of NAD(P)H with an acceptor to produce NAD(P) and the reduced acceptor. This FAD-dependent enzyme is inhibited by dicoumarol. 

Figure 2 Molecular structures and IUPAC numbering scheme of organic cofactors occurring in photosynthetic reaction centres (bRC, PS I, PS II). (Bac-teriolpheophytin is the free base of (bacterio)chlorophyll plastoquinone (PQ) is found in PS If phylloquinone or vitamin K, ( VK,) in PS I many bacteria contain ubiquinone (UQ). Shown is also the amino acid tyrosine (Tyr, Y) that is redox active in PS II.

Radical pairs of phylloquinone (vitamin Ki) were produced by reaction with A1C13 or by photochemical reaction at 77 K.40 The dipolar splitting (D) of 19 0.5 mT for the photochemically produced radical pair corresponds to a point dipole distance of 5.3 A, which is attributed to a solvent-separated radical pair. For radical pairs generated with A1C13, D --= 11.2 + 0.5 mT (r = 6.3 A), which is consistent with the distance between two phylloquinone radicals coordinated to an Al3+ ion.40... 

The electron donor to Chl+ in PSI of chloroplasts is the copper protein plastocyanin (Fig. 2-16). However, in some algae either plastocyanin or a cytochrome c can serve, depending upon the availability of copper or iron.345 Both QA and QB of PSI are phylloquinone in cyanobacteria but are plastoquinone-9 in chloroplasts. Mutant cyanobacteria, in which the pathway of phylloquinone synthesis is blocked, incorporate plasto-quinone-9 into the A-site.345a Plastoquinone has the structure shown in Fig. 15-24 with nine isoprenoid units in the side chain. Spinach chloroplasts also contain at least six other plastoquinones. Plastoquino-nes C, which are hydroxylated in side-chain positions, are widely distributed. In plastoquinones B these hydroxyl groups are acylated. Many other modifications exist including variations in the number of iso-prene units in the side chains.358 359 There are about five molecules of plastoquinone for each reaction center, and plastoquinones may serve as a kind of electron buffer between the two photosynthetic systems. 

The Z scheme. [(Mn)4 = a complex of four Mn atoms bound to the reaction center of photosystem II Yz = tyrosine side chain Phe a = pheophytin a QA and Qb = two molecules of plastoquinone Cyt b/f= cytochrome hf,f complex PC = plastocyanin Chi a = chlorophyll a Q = phylloquinone (vitamin K,) Fe-Sx, Fe-SA, and Fe-SB = iron-sulfur centers in the reaction center of photosystem I FD = ferredoxin FP = flavoprotein (ferredoxin-NADP oxidoreductase).] The sequence of electron transfer through Fe-SA and Fe-SB is not yet clear. 

In a previous study we have found that, at low temperature, PS-I electron transfer is largely blocked away from A, and that the state (P-700+, A, ) decays with a half-time of 130us. Analysis of the absorption spectrum of that state showed that A, is presumably a quinone radical anion (Brettel et al, 1986). Chemical analysis, following separation by HPLC, has shown that phylloquinone (a naphthoquinone also named vitamin Kj) is the only quinone present in PS-I. We have found 2 moles of phylloquinone per PS-I. Extraction with dry hexane does not change the electron transfer reactions this treatment only extracts only one phylloquinone per PS-I (Biggins and Mathis, 1987). 

In these studies it appeared that low-temperature photoreduction of the bound iron-sulfur centers is not greatly decreased (about 2x) by the removal of phylloquinone (Setif et al, 1987). Low temperature photochemistry was measured by three methods the total amount of iron-sulfur centers A and B reduced by continuous illumination at 77K, the extent of reduction of these centers by saturating laser flashes at 77K, and the flash-induced formation of triplet P-700. These methods show that all the reaction centers are still able to oxidize P-700 and to reduce the iron-sulfur centers. This observation raises serious questions concerning the role of phylloquinone or the significance of low-temperature photochemistry. For the moment we consider that room temperature data are more reliable in indicating that A] is a phylloquinone. 

As is apparent in Fig. 3, considerable similarity exists in the arrangement of the electron transfer cofactors in PS I and PS n. The main differences between the two systems are as follows 1) PS I has three Pe4S4 iron-sulfur clusters. Ex, Ea, and Eb, located on the stromal side of the complex 2) In PS I the primary acceptor is a chlorophyll, not pheophytin and 3) the distance between the primary acceptor (Aqa3 ) and phylloquinone (Aia,b) in PS I is significantly shorter than the corresponding distance between PheoA,B and Qa.b in PS II and Type II reaction centers. These structural differences correlate with functional differences between the two types of reaction centers. In PS II, the mobile electron carrier on the stromal side of the complex is Qb, which is a lipid-soluble, two-electron acceptor. In contrast, the mobile electron carrier in PS I is ferredoxin, which is a water-soluble, one-electron acceptor. The three iron-sulfur clusters in PS I provide a chaimel by which electrons are funneled out of the reaction center to ferredoxin. On the donor side of the complex, plastocyanin, the reductant that replenishes electrons removed from P700, is also a water-soluble protein and is a one-electron donor. Thus, each photon absorbed by the PS I complex leads to the transfer of one electron from plastocyanin to ferredoxin. In Fig. 2, it is apparent that the midpoint potentials of the acceptors in PS I are about 500 to 700 mV more negative than those in PS II, and the... 

At the time PS 11 absorbs a photon, PS-1 complex also absorbs a photon, resulting in a charge separation to form the oxidized primary electron donor P700 and the reduced primary electron acceptor Aq. The oxidized primary donor P700 is then reduced by an electron from reduced plastocyanin. The electron on the primary acceptor Aq is rapidly transferred step-wise through a series of acceptors consisting of a phylloquinone (0 Q), three iron-sulfur clusters in FeS-X, FeS-A and FeS-B, and finally ferredoxin (Fd) which reduces NADP" to NADPH in a reaction catalyzed by NADP -ferredoxin reductase (FNR). The PS-1 complex is also called the plastocyanin-ferredoxin oxido-reductase. ... 

We have already seen that, like the heterodimer in the bacterial reaction center, heterodimer in PS I binds all the electron-transport pigment molecules and cofactors, with the possible exception of A] (phylloquinone). One difference from the bacterial reaction center, however, is that the PS-I reaction center contains a number of smaller polypeptides with molecular masses in the 3.5-15.5 kDa range. A second distinguishing characteristic of the PS-I reaction center is that it also contains approximately 90 antenna chlorophyll molecules and 10 to 15 P-carotene molecules per P700 , where both PsaA and PsaB contain an unusually large number of histidines, which it is thought may be needed for binding the core-antenna chlorophylls. 

Fig. 12. Transient difference spectra induced by 1-ps flash in PS-I particles with a Chl/P700=30. Spectra for particles with the native phylloquinone extracted (-Q) are shown in panel (A) and those for particles reconstituted with menaquinone-4 (+Q) are shown in panel (A )- Spectra for particles with open (O) and closed ( ) reaction centers are drawn as solid and dotted lines, respectively. For the quinone-depleted and quinone-reconstituted particles, difference spectra were recorded in panels (B) and (B ) at three delay times, as indicated. Excitation intensity corresponds to 1.5 photons absorbed per RC. Panels (C) and (C ) show plots of A[AA]s at 695 and 685 nm vs. various delay times after the excitation flash, showing the rise and decay kinetics of P700 and Ao, respectively. Figure source Kumazaki, Iwaki, Ikegami, Kandori, Yoshihara and Itoh (1994) Rates of primary electron transfer reactions in the photosystem I reaction center reconstituted with different quinones as the secondary acceptor. J Phys Chem 98 11221,11222.

Figure 1. Position of phylloquinone (vitamin KO in the PS-I reaction center.

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