Plastoquinones

Two groups of substituted l,4-ben2oquiaones are associated with photosynthetic and respiratory pathways the plastoquinones, eg, plastoquinone [4299-57-4] (34), and the ubiquinones, eg, ubiquinone [1339-63-5] (35), are involved in these processes. Although they are found in all living tissue and are central to life itself, a vast amount remains to be learned about their biological roles. 

FIGURE 8.18 Dolichol phosphate is an initiation point for the synthesis of carbohydrate polymers in animals. The analogous alcohol in bacterial systems, undecaprenol, also known as bactoprenol, consists of 11 isoprene units. Undecaprenyl phosphate delivers sugars from the cytoplasm for the synthesis of cell wall components such as peptidoglycans, lipopolysaccharides, and glycoproteins. Polyprenyl compounds also serve as the side chains of vitamin K, the ubiquinones, plastoquinones, and tocopherols (such as vitamin E). 

FIGURE 22.15 The structures of plasto-quiuoue and its reduced form, plastohydro-quiuoue (or plastoquiuol). The oxidation of the hydroquiuoue releases 2 as well as 2 c. The form shown (plastoquinone A) has nine isoprene units and is the most abundant plastoquinone in plants and algae. Other plasto-quinones have different numbers of isoprene units and may vary in the substitutions on the quinone ring. 

Photosynthesis is the process that plants use to convert sunlight into chemically useful energy. One part of this process involves using sunlight to convert water and a plastoquinone, Q, into oxygen and a hydroquinone, QH2 (R = (CH2CH=C(Me)CH2) H where n = 6-10). 

All these latter centers were seen to titrate at around 150 mV, that is, some 150 mV lower than the traditional centers, and thus form a separate subclass of this type of redox proteins (see Fig. 7). Since similar downshifts were observed for almost all redox components in the mentioned species (for a compilation, see 133), it is generally assumed that the differences between the two groups represent an adaptation to the difference in value of the quinone pool, which is plastoquinone(PQ)/ubiquinone(UQ) E 100 mV) in the traditional species and menaquinone (MK) Em,i---70 mV) in the other... 

Ubiquinone or Q (coenjyme Q) (Figure 12-5) finks the flavoproteins to cytochrome h, the member of the cytochrome chain of lowest redox potential. Q exists in the oxidized quinone or reduced quinol form under aerobic or anaerobic conditions, respectively. The structure of Q is very similar to that of vitamin K and vitamin E (Chapter 45) and of plastoquinone, found in chloroplasts. Q acts as a mobile component of the respiratory chain that collects reducing equivalents from the more fixed flavoprotein complexes and passes them on to the cytochromes. 

Chlorophyll, plastoquinone, and cytochrome are complicated molecules, but each has an extended pattern of single bonds alternating with double bonds. Molecules that contain such networks are particularly good at absorbing light and at undergoing reversible oxidation-reduction reactions. These properties are at the heart of photosynthesis. 

Many of the Lewis structures in Chapter 9 and elsewhere in this book represent molecules that contain double bonds and triple bonds. From simple molecules such as ethylene and acetylene to complex biochemical compounds such as chlorophyll and plastoquinone, multiple bonds are abundant in chemistry. Double bonds and triple bonds can be described by extending the orbital overlap model of bonding. We begin with ethylene, a simple hydrocarbon with the formula C2 H4. 

Plastoquinone, whose line structure appears in the margin, has ten double bonds. Unlike the bonds in the tail of retinal, however, the bonds in the long tail of plastoquinone are not delocalized because s p -hybridized carbon atoms separate them. The delocalized 7t system of plastoquinone is its planar ring of six carbon atoms, with two of the carbon atoms double-bonded to outer oxygen atoms. 

The shikimate pathway is the major route in the biosynthesis of ubiquinone, menaquinone, phyloquinone, plastoquinone, and various colored naphthoquinones. The early steps of this process are common with the steps involved in the biosynthesis of phenols, flavonoids, and aromatic amino acids. Shikimic acid is formed in several steps from precursors of carbohydrate metabolism. The key intermediate in quinone biosynthesis via the shikimate pathway is the chorismate. In the case of ubiquinones, the chorismate is converted to para-hydoxybenzoate and then, depending on the organism, the process continues with prenylation, decarboxylation, three hydroxy-lations, and three methylation steps. - ... 

The third pathway involved in the quinones biosynthesis is the isoprenoid route. This pathway is primarily important for the formation of prenyl side chains of prenylquinones (ubiquinone, menaquinone, plastoquinones, etc.). The side chains of ubiquinones and prenylated naphthoquinones derive from polyprenyl diphosphates. 

In a partly biological, partly artificial model (page 397) reduced anthraquinone-2-sulphonate plays the role of NAD+ and tetramethyl-p-phenylenediamine that of plastoquinones. 

From the plastoquinone pool, the electrons pass through the cyt b6f complex, which generates much of the electrochemical proton gradient that drives the synthesis of ATP. 

The high mobilities of plastoquinone (Q) and PC, the electron carriers that shuttle electrons between these particles permit the photosynthesis to proceed at a reasonable rate. 

PS II absorbs more light than PS I. As such PS I cannot take electrons as fast as PS II can supply, leaving plastoquinone in its reduced state. This reduced plastoquinone activates a protein kinase that phosphorylates the threonine (Thr) residue of the LHCs that in turn migrate to the unstacked portion of the thylakoid membrane where it binds to PS I. As a result, a large portion of incident light is funneled to PS I. 

Under the conditions of low illumination (normally shady light, which has a high proportion of long-wavelength red light), PS I takes electrons faster than PS II can supply, leaving plastoquinone in its oxidized state. As a result, LHCs are dephosphorylated and migrate to the stacked portion of the thylakoid membrane where they drive to PS II. 

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. 

The reaction-center proteins for Photosystems I and II are labeled I and II, respectively. Key Z, the watersplitting enzyme which contains Mn P680 and Qu the primary donor and acceptor species in the reaction-center protein of Photosystem II Qi and Qt, probably plastoquinone molecules PQ, 6-8 plastoquinone molecules that mediate electron and proton transfer across the membrane from outside to inside Fe-S (an iron-sulfur protein), cytochrome f, and PC (plastocyanin), electron carrier proteins between Photosystems II and I P700 and Au the primary donor and acceptor species of the Photosystem I reaction-center protein At, Fe-S a and FeSB, membrane-bound secondary acceptors which are probably Fe-S centers Fd, soluble ferredoxin Fe-S protein and fp, is the flavoprotein that functions as the enzyme that carries out the reduction of NADP+ to NADPH. 

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