## 2 Prenylquinones and the Study of These Compounds  
As the name implies, prenylquinones are structurally composed of an aromatic naphthoquinones or benzoquinones attached to an isoprene chain (Figure 1). The classic nomenclature classifies these substances based on their naphthoquinone/benzoquinone group (example: ubiquinone, UQ; menaquinone, MQ) and numerically indicates the number of side-chain isoprene units (UQ-n, UQn). Prenylquinones are apolar compounds that have the ability to protonate and deprotonate and, thus, most prenylquinones are involved in electron transfer reactions in the cell lipid phase. Many prenylquinones are commonly known as vitamins because these compounds are necessary components of animal dietary intake [41; 42; 43; 44]. It seems that the first prenylquinones were MQs, which appeared in the context of a primitive atmosphere that lacked oxygen [45]. Changes in the Earth's gaseous composition led to the appearance of UQs among other prenylquinones [45]. Currently, many different prenylquinones exist in nature that, because of their chemical characteristics, are capable of developing specific activities in each organism. Among others, the prenylquinone group includes UQs, plastoquinones (PQs), plastocromanol, MQs, phylloquinone (PK),tocopherol, rhodoquinones, chlorobiumquinone (ChQ), and thermoplasmaquinone (TpQ) [45]. Each substance presents a different polarity and redox potential, which determines the cellular location of the compound and the ability of the compound to protonate and deprotonate at different oxygen saturations [45; 46]. The isoprenic chain size has also been shown to influence catalytic activity, activation energy, and kinetics among other chemical characteristics [46]. Even in the same organism, the biosynthesis and proportions of different prenylquinones can be modified as a result of environmental changes. For example, _Escherichia coli_ modifies the UQ-MQ pool ratio as a function of environmental oxygen saturation, and _Euglena gracilis_ also modifies rhodoquinone and UQ biosynthesis for similar purposes [47; 48]. Among other functions, prenylquinones are involved in lipoperoxidation defense, mitochondrial respiration and photosynthesis and act as enzymatic cofactors [45]. For example, PQs in plants and cyanobacteria are involved in photosynthetic electronic transport (PSII) and cell respiration and participate in antioxidant defense, similar totocopherol [49]. Ubiquinone is described to mainly participate in mitochondrial electronic transport between complex I or II and complex III in eukaryotes [50]. However, UQ can also be involved in photosynthesis in some organisms, such as _Rhodospirillum rubrum_[45]. Similarly, MQs are mainly involved in photosynthesis and respiration processes in prokaryotes [45; 51]. The existence of trans-plasma membrane electron transport (TPMET) chains between extracellular and intracellular compounds in several organisms is known. Some electron carriers identified in TPMET are NADH, glutathione, and several prenylquinones, such as topchrol, UQ, PK, and MQ [52; 53]. Among other factors, TPMET is essential for maintenance of internal homeostasis and prevention of oxidative stress as well as ferric ion reduction for intracellular incorporation [52]. There are specialized enzymes involved in these types of reactions, such as NAD(P)H reductase, which reduces UQ to its semiquinone form, which in turn reduces topchrol [52]. Subsequently, topchrol reduces extracellular ascorbic acid [52]. Several researchers have focused on the identification of new prenylquinones in different organisms [41; 45; 54]. The goal of these studies has been to better understand the phylogeny, bioenergetics and electron transfer metabolism of the studied organisms. In pathogenic organisms, the functions and biosynthetic pathways of prenylquinones are interesting drug targets. Aurachin is an MQ biosynthesis inhibitor that has been previously investigated for antimycobacterial activity [55].