Biosynthetic pathways, eukaryotic pathway

Addition of Isoprenyl Groups A number of eukaryotic proteins are modified by the addition of groups derived from isoprene (isoprenyl groups). A thioether bond is formed between the isoprenyl group and a Cys residue of the protein (see Fig. 11-14). The isoprenyl groups are derived from pyrophosphorylated intermediates of the cholesterol biosynthetic pathway (see Fig. 21-33), such as famesyl pyrophosphate (Fig. 27-30). Proteins modified in this way include the Ras proteins, products of the ras oncogenes and proto-oncogenes, and G proteins (both discussed in Chapter 12), and lamins, pro-... 

S-Adenosylmethionine decarboxylase is the first enzyme in the biosynthetic pathway to spermidine (Chapter 24). Whether isolated from bacteria, yeast, animals, or other eukaryotes, this enzyme always contains a bound pyruvoyl group.273 274b Both the... 

Glycolysis is a set of reactions that take place in the cytoplasm of prokaryotes and eukaryotes. The roles of glycolysis are to produce energy (both directly and by supplying substrate for the citric acid cycle and oxidative phosphorylation) and to produce intermediates for biosynthetic pathways. 

Depict biosynthetic pathways for eukaryotic and prokaryotic tRNA and highlight the differences between the two systems. 

The other lipid commonly present in eukaryotic membranes is cholesterol, a polycyclic structure produced from isoprene by a complex biosynthetic pathway. It is interesting to ask whether it is conceivable that prebiotically plausible reactions might also produce complex amphiphiles. The earliest investigations aiming to answer this question were carried out by Hargreaves et al. [36], Oro and coworkers [37,38], and, more recently, Ourisson et al. [39] and Conde-Frieboes and Blochliger [40]. In all such reactions,... 

In conclusion, sterol biosynthesis in Bacteria is probably limited to a small number of taxa that either have an incomplete sterol biosynthetic pathway or lack the capacity to alkylate the side chain. Therefore, steranes ((66b)-(66e) and (69b) and (69c)) in bitumens and oils can be reliably attributed to the activity of eukaryotic organisms (contra Cavalier-Smith, 2002). Additionally, some steranes with a diagnostic alkylation pattern ((66d)-(66e), and (70)) have taxonomic value below domain level. 

The biosynthetic pathway for this pterin cofactor, which was described in Chapter 4, appears to be universally conserved in biology, underlining its importance. Interestingly however, baker s yeast, a much used model eukaryote, is the only organism known which does not contain Mo enzymes (it is also one of the few organisms which does not contain ferritin). The MoCo cofactor can exist in the fully oxidised (Mo ) and fully reduced (Mo ") forms, with some enzymes generating the (Mo ) form as a catalytic intermediate. 

Finally, we briefly consider the way in which Sec is generated and co-translationally incorporated in selenoproteins. There are 25 selenoprotein genes in humans, and Sec has been found in the active site of those to which a function has been attributed. Sec does not occur as the free amino acid, and the biosynthetic pathway of Sec from serine on tRNA in eukaryotes requires four enzymes, as illustrated in Figure 18.8. The specific tRNA is aminoacylated with serine by the conventional Seryl-tRNA synthetase (SerRS) and the... 

Figure 10 The sialoside biosynthetic pathway in eukaryotic cells A, UDP-GlcNAc-2-epimerase B, ManNAc-6-kinase C, Neu5Ac-9-P04 synthase D, NeuSAc-b-POj phosphatase E, CMP-Neu5Ac-synthetase F, sialyltransferase. Enzymes A and B function together as a bifunctional enzyme.

---