## 4 Specific Prenylquinones
### Vitamin K  
#### 4.2.1 Biosynthesis and Distribution of Vitamin K  
Natural occurring methyl-1,4-naphthoquinones include PK (or vitamin K1) and MQs (including MQ-4, vitamin K2). Some of these substances are considered vitamins that are mostly exogenously obtained and are involved in several essential physiological processes in animals [45]. Naturally, these compounds are derived from the diet or the intestinal microbiota [172], are used as cofactors for \(\gamma\)-glutamyl carboxylase among other enzymes [45; 173] and can be \"redox-recycled\" by the epoxide reductase vitamin K [173]. Although the chemical structures of PK and MQs are similar, the functions, origins and distribution of these compounds in nature are different (Figure 1). Reduced MQs rapidly become oxidized and inefficient in rich oxygen-saturated environments, and these compounds appear to have originated in the reducing atmosphere before the appearance of oxygenic photosynthesis [174]. The isoprenoid side chains of MQs can differ in the number of isoprene units in the lateral chain and in the degree of saturation [147]. Modified MQs such as the proteobacterial TpQ biosynthesized by _Thermoplasma_[175] have also been identified and are discussed in detail in Section 4.3.1.  
Several MQs can be found in archaea [175; 176; 177] and bacteria [147]. The different MQs are mainly involved in respiratory and photosynthetic processes in prokaryotes but are also found in diatoms and red algae [45; 51; 178]. Inhibitors of MQ biosynthesis are also of clinical interest with regard to the pathogenic actinomycetes and _Corynebacterium_ species [55; 179]. For example, MQ and its sulfated derivate produced by _Mycobacterium tuberculosis_ are believed to act in both respiration and in attenuation of the host immune response [180]. Phyloquinone appears to have originated later than MQs in Earth's history, but the structure of PK differs from that of MQs only by the phytol chain. The biosynthesis of PK in photosynthetic organisms capable of performing oxygenated photosynthesis can be detected. PK essentially participates in oxygenated photosynthesis and is mainly located in chloroplasts linked to photosystem I [181; 182]. It is believed that PK could also be involved in (ROS) production under stress and in the resistance to pathogenic attack [183]. Some authors have suggested that there are PK-mediated electron transport chains in plant plasma membranes [183].  
There is only one fully characterized pathway for MQ biosynthesis, and another remains poorly understood. The first pathway forms the head group of prenylquinone from chorismate via the activity of several enzymes: MenF (isochorismate synthase), MenD (2-succinyl-5-enolpyruvateyl-6-hydroxy-3-cyclohexene-1-carboxylate synthase), MenH (2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylate synthase), MenC (succinyl benzoate synthase), MenE (succinyl benzoic CoA synthase), MenB (1,4-dihydroxy-2-naphthoyl-CoA synthase) and DHNA-CoA thioesterase [184]. Finally, the head group is prenylated by a DHNA prenyltransferase, and the naphthoate is methylated. The second pathway, known as the fatalosine pathway, diverges from the classic pathway until the formation of DHNA [185]. The pathway differs from the classic pathway fundamentally by the condensation of a chorismate molecule, an inosine and a two-carbon molecule that remains unidentified for the fatalosine pathway [186]. Phyloquinone is biosynthesized by a pathway that is analogous to the previously described classic MQ biosynthetic pathway. The major difference between the pathway for vitamin K1 biosynthesis and the fully characterized pathway for vitamin K2 biosynthesis is the isoprenic chain condensed to the aromatic head group [45; 187].