Independent Pathways

Both prokaryotes and eukaryotes are capable of introducing a single cis double bond in a newly synthesized fatty acid. Bacteria such as E. coli carry out this process in an Og-independent pathway, whereas eukaryotes have adopted an Og-dependent pathway. There is a fundamental chemical difference between the two. The Og-dependent reaction can occur anywhere in the fatty acid chain. 

Consider the two diagrams in Fig. 6-3. Both are said to feature upward bends. In terms used earlier, Fig. 6-3a represents a transition with increasing [H+] from zeroth to first order in [H+], and Fig. 6-3b from negative first to zeroth. (Of course, the use of the pH scale reverses their appearance.) As noted previously (Rule 8, Section 6.2), this observation signifies a mechanism with two independent pathways. 

Each term in this equation represents an independent pathway. The low-pH arm in the figure is equivalent to reaction (6-57), or one similar to it, in which the proton attacks the substrate directly. The high-pH pathway represents the unimolecular reaction of the substrate or else its reaction with water. As this discussion illustrates, a reaction whose pH profile shows upward bends can be analyzed in terms of separate pathways. A complex profile can be separated into regions at each upward bend each region is a distinct pathway. 

An enantioconvergent transformation leads to a single enantiomeric product from a racemate [51]. Each enantiomer is transformed via independent pathways by the same catalyst or by two different catalysts (Figure 6.6). For example, the hydrolysis of epoxides may proceed with high regio- and stereoselectivity vdth inversion or retention of configuration. Several enantioconvergent transformations of epoxides are reported in the last section of this chapter. 

Proteins are degraded by both ATP-dependent and ATP-independent pathways. Ubiquitin targets many intracellular proteins for degradation. Liver cell surface receptors bind and internalize circulating asialoglycoproteins destined for lysosomal degradation. 

Schols D, Proost P, Struyf S et al (1998) CD26-processed RANTES(3-68), but not intact RANTES, has potent anti-HIV-1 activity. Antiviral Res 39 175-187 Schonbeck U, Mach F, Libby P (1998) Generation of biologically active IL-1 beta by matrix metaUoproteinases a novel caspase-1-independent pathway of IL-1 beta processing. J Immunol 161 3340-3346... 

This dependence was interpreted in terms of an [ ]-independent pathway Fe3++Fe ... 

HAGUE A, MANNING A M, HANLON K A, HUSCHTSCHA L I, HART D, PARASKEVA C (1993) Sodium butyrate induces apoptosis in human colonic tumour cell lines in a p53-independent pathway impUcations for the possible role of dietary fibre in the prevention of large-bowel cancer. /ni J Cancer. 55 498-505. 

Rohmer, M., The discovery of a mevalonate-independent pathway for isoprenoid biosynthesis in bacteria, algae and higher plants, Nat. Prod. Rep., 16, 565, 1999. 

Sprenger, G.A. et al.. Identification of a thiamin-dependent synthase in Escherichia coli required for the formation of the 1-deoxy-D-xylulose 5-phosphate precursor to isoprenoids, thiamin, and pyridoxol, Proc. Natl. Acad Sci. USA 94, 12857, 1997. Lange, B.M. et al., A family of transketolases that directs isoprenoid biosynthesis via a mevalonate-independent pathway, Proc. Natl. Acad Sci. USA 95, 2100, 1998. Lois, L.M. et al., Cloning and characterization of a gene from Escherichia coli encoding a transketolase-like enzyme that catalyzes the synthesis of D-1- deoxyxylulose 5-phosphate, a common precursor for isoprenoid, thiamin, and pyridoxol biosynthesis, Proc. Natl. Acad. Sci. USA 95, 2105, 1998. 

Hirshman CA, Zhu D, Panettieri RA, Fmala CW. Actin depolymerization via the beta-adrenoceptor in airway smooth muscle cells a novel PKA-independent pathway. Am J Physiol Cell Physiol 2001 281(5) C1468-1476. 

Independent Pathways of Retinol and Carotenoid Absorption in Caco-2 Cells ... 

Assembly and lysis In the case of T4, assembly of heads and tails occur on independent pathways. DNA is packaged into the assembled head and the tail and tail fibers are added subsequently. Somehow, the DNA is packed tightly and inserted into the empty phage head. 

Dang, Y., Kedersha, N., Low, W. K., Romo, D., Gorospe, M., Kaufman, R., Anderson, P., and Liu, J. O. (2006). Eukaryotic initiation factor 2alpha-independent pathway of stress granule induction by the natural product pateamine A. J. Biol. Chem. 281, 32870-32878. 

As well as an acid-independent pathway, an acid-dependent pathway also occurs for the dissociation of a number of the alkali and alkaline earth metal cryptates (Table 7.1). Similar behaviour to that just discussed for methanol also occurs for reactions in a range of other non-aqueous solvents (Cox, Truong Schneider, 1984). 

Stewart, E. 2001. Gonadotrophins and the uterus is there a gonad-independent pathway Journal of the Society for Gynaecologic Investigation 8(6), 319-326. 

The biosynthesis of monoterpenes, the major components of peppermint essential oils, can be divided into four stages (Fig. 9.4). Stage 1 includes the formation of isopentenyl diphosphate (IPP) and dimethylallyl alcohol (DMAPP). In plants, two separate pathways are utilized for the synthesis of these universal C5 intermediates, with the cytosolic mevalonate pathway being responsible for the formation of sterols and certain sesquiterpenes, and the plastidial mevalonate-independent pathway being involved in the biosynthesis of isoprene, monoterpenes, certain sesquiterpenes, diterpenes, tetraterpenes, as well as the side chains of chlorophyll and plastoquinone.16 In peppermint oil gland secretory cells, however, the mevalonate pathway is blocked and the biosynthesis of monoterpenoid essential... 

It had been suggested that the second step of the mevalonate-independent pathway involved an intramolecular rearrangement and subsequent reduction of DXP to yield 2-C-methyl-D-erythritol 4-phosphate (Fig. 9.6A).29-3 Seto and co-workers reported the isolation, by using a mutant complementation approach, and characterization of such a reductoisomerase gene from E. coli)2 Based on likely... 

---