Phylloquinone reductase

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. 

One very active NADHj-cytochrome c reductase from mitochondria recently was separated into two components, a NADHr-ubiquinone reductase and an ubihydroquinone-cytochrome c reductase (see below under Quinone Catalysis ). In addition, non-heme-bound iron was found (about 15 moles/mole of flavin). Martius prepared a highly purified phylloquinone reductase, which contains flavin-adenine dinuoleotide (FAD) and which reduces ubiquinone besides phylloquinone (= vitamin K). The hydroquinone is assumed to be reoxidized to the quinone by cytochrome b. 

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. ... 

Up to this point, we have considered the election carriers bound to the photosystem-I thylakoid membrane. They include the primary electron donor P700 and the series ofelectron acceptors Aq (Chi a). A, (phylloquinone), FeS-X, and FeS-A/B. We now turn to two mobile electron carriers around photosystem I called plastocyanin (PC), a copper protein and ferredoxin (Fd) a [2Fe-2S] irons-sulfur protein, plus the enzyme that catalyzes the reduction of NADP by ferredoxin and called the ferredoxin-NADP -reductase and abbreviated as FNR. 

Fig. 45.5. A. Structures of vitamin K derivatives. Phylloquinone is found in green leaves, and intestinal bacteria synthesize menaquinone. Humans will convert menadione to a vitamin K active form. B. Vitamin K-dependent formation of y-carboxyglutamate residues. Thrombin, Factor VII, Factor IX, and Factor X are bound to their phospholipid activation sites on cell membranes by Ca ". The vitamin K-dependent carboxylase, which adds the extra carboxyl group, uses a reduced form of vitamin K (KH2) as the electron donor and converts vitamin K to an epxjx-ide. Vitamin K epoxide is reduced, in two steps, back to its active form by the enzymes vitamin K epoxide reductase and vitamin K reductase.

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