The Link to Oxygen

The citric acid cycle plays a central role in anabolic pathways as well as in catabolism. 

Pathways that give rise to sugars, fatty acids, and amino acids aU originate with components of the citric acid cycle. 

Recall the classic equation for the aerobic oxidation of glucose  

We have seen the metabolism of glucose through glycolysis. Now we see where the GOg comes from—namely, the three decarboxylation reactions associated with the citric acid cycle. In the next chapter, we will see where the water and oxygen come from. 

The citric acid cycle is considered part of aerobic metabolism because of the link to the electron transport chain and oxidative phosphorylation. NADH and FADHg generated by the citric acid cycle ultimately pass their 

---

Structures in which the linking glycosidic oxygen is replaced by -CH2- may be named by use of the replacement prefix carba- (c/. 2-Carb-34.2) for emphasis of homomorphic relationships. The oxygen replaced is given the locant of the carbon atom to which it is attached in the residue with the lower roman numeral (cited as superscript) (cf 2-Carb-37.2), with suffix a1. 

Thiemens MH, Heidenreich JE (1983) The mass independent fractionation of oxygen a novel isotope effect and its possible cosmochemical implications. Science 219 1073-1075 Thiemens MH, Jackson TL, Brenninkmeijer CAM (1995) Observation of a mass-independent oxygen isotopic composition in terrestrial stratospheric COj, the link to ozone chemisdy, and the possible occurrence in the Martian atmosphere. Geophys Res Lett 22 255-257 Timmes FX, Woosley SE, Weaver TA(1995) Galactic chemical evolution hydrogen through zinc. Astrophys J Suppl 98 617-658... 

The attachment of carbohydrate to nitrogen is invariably by A -acetyl-D-glucosamine and the attachment to oxygen is invariably by A/ -acetyl-D-galactosamine. A -linked carbohydrates are invariably composed of a core pentasaccharide [142] of the following structure ... 

These oxides contain d and d transition metal ions with a high formal valence state which are linked to oxygens by strong covalent bonds. Their framework is depicted as an array of distorted tetrahedra, pentahedra, or octahedra as in the Wadsley phases" belonging to the (V-0), (Mo-0), (V-Mo-0), (W-0) systems (44,46). 

As discussed in the introduction, any enzymatic activity that is direcdy (or indirectly) linked to oxygen consumption could be measured by using the Hb02 method. However, it is not our purpose to shift all analytical procedures using manometric or polarographic techniques to spectro-photometric assays merely because it is more sensitive. Therefore, in the following sections we will discuss those applications where HbC>2 method could offer obvious advantages over other procedures. 

The earliest studies relative to the mechanism was the stereochemical work of Conforth, Popjak, and their collaborators [49]. They concluded that there was an inversion of configuration at C-1 of the allylic substrate, consistent with a concerted process, the new carbon-to-carbon bond being formed as the carbon-to-oxygen bond is cleaved. They also felt that elimination of a proton from C-2 of the isopentenyl moiety would not be concerted with this, since suprafacial (same side) reactions are generally considered unfavorable. To circumvent this, a 2-stage mechanism involving an electron donor X , with X being covalently linked to the initial condensation product, was proposed. The X residue is then lost simultaneously with elimination of the proton in an anti-mode (Fig. 12). 

---