## 4 Specific Prenylquinones  
Here, we will specifically discuss prenylquinones that are found in several human pathogenic protozoa as well as the therapeutic implications of these compounds (Table 3). To better understand these compounds, first, the well-known biosynthetic pathway, functions and distribution of prenylquinones in nature will be explained. The table shown below contains examples of prenylquinones in pathogenic protozoa that will be cited, discussed and referenced in this review. We show a scheme with most of the biosynthetic pathways for prenylquinone biosynthesis in nature except for the UQ biosynthetic pathway. The mechanism by which tocopherol and methyl-1,4-naphthoquinones are biosynthesized in pathogenic protozoa is poorly understood. Therefore, for these prenylquinones, all the well-characterized pathways in nature are shown in Figure 5. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Organism** & **Parasitic Stage** & **UQ-8** & **UQ-9** & **UQ-10** & **ChQ** & **TpQ** & **MQ-4** & **PK** & \(\alpha\)**-TC, \(\gamma\)**-TC** \\ \hline _L._ & amastigote & & & & & & & \\ \hline _amazonensis_ & promastigote & & & & & & & \\ \hline _L. donovani_ & promastigote & & & & & & & \\ \hline _L. major_ & promastigote & & & & & & & \\ \hline _L. mexicana_ & amastigote & & & & & & & \\ \hline _T. cruzi_ & epimastigote & & & & & & & \\ \hline _T. brucei_ & procyclic & & & & & & \\ \hline _P. falciparum_ & intraerythrocytic & & & & & & \\ \hline _G. lambi_ & trophozoite & & & & & & \\ \hline _E. histolytica_ & trophozoite & & & & & & \\ \hline _Trichomonas_ & & & & & & & \\ _fetus_ & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Examples of the prenylquinone content in pathogenic protozoa. Table indicates in grey squares examples of previously characterized prenylquinones from pathogenic protozoa. White squares indicate still not characterized prenylquinones. All the prenylquinones are discussed in this review [(64; 72; 135; 136; 137; 138; 139; 140; 141)]. Figure 5: Phenylquinone biosynthesis pathways [45]. Some prenylquinones shown have not been found in pathogenic protozoa, but all are cited in this review. The specific pathway of ubiquinone biosynthesis (orange arrows) and the formation of aromatic precursors from amino acid metabolism/shikimate pathways (gray arrows) are simplified because these processes are better described in other figures in this review. The incorporation of isoprene chains and use of S-adenosyl-L-methionine (SAM) by enzymes are indicated. The SAM cofactor is necessary for methylation reactions to produce S-adenosyl homocysteine or for other biochemical processes. Continuous arrows indicate a single enzymatic step, and discontinuous arrows indicate several enzymatic steps, processes that remain poorly understood or processes that are better shown in other figures in this review. The different pathways are distinguishable by the colors of the arrows: Light purple. MQ biosynthesis by the futalosine alternative pathway; Dark blue. Homologous enzymatic steps that are common for PK or MQ biosynthesis by the classic pathway; Dark purple. Specific enzymatic steps for MQ biosynthesis by the classic pathway; Light blue. Enzymatic steps that are specific for PK biosynthesis; Black. Biosynthesis of homogentisate from 4-hydroxyphenylpyruvate; Dark green. Tocopherol biosynthesis [142]; Light green. Shared enzymatic steps for PQ and plastochromanol biosynthesis; Yellow. Plastochromanol biosynthesis [45]; Brown. 4-Hydroxybenzoate biosynthesis from chorismate; Orange. Ubiquinone biosynthesis; Red. Possible rhodoquinone biosynthesis pathway in _Rhodospirillum rubrum_[143]. The figure also indicates processes that remain poorly understood.?A. The final steps of the futalosine alternative pathway [144];  
?B. Biosynthesis of MQ derivatives such as ChQ or sulfated MQs (among other examples) [45];?C. \(\alpha\)-Tocopherol biosynthesis from \(\beta\)-tocopherol;?D. 2-Methyl-6-solanesyl-1,4-benzoquinol biosynthesis from 4-hydroxy-3-polyphenylbenzoate in cyanobacteria [145];?E.