Metabolic pathways named examples

Divalent sulfur compounds are achiral, but trivalent sulfur compounds called sulfonium stilts (R3S+) can be chiral. Like phosphines, sulfonium salts undergo relatively slow inversion, so chiral sulfonium salts are configurationally stable and can be isolated. The best known example is the coenzyme 5-adenosylmethionine, the so-called biological methyl donor, which is involved in many metabolic pathways as a source of CH3 groups. (The S" in the name S-adenosylmethionine stands for sulfur and means that the adeno-syl group is attached to the sulfur atom of methionine.) The molecule has S stereochemistry at sulfur ana is configurationally stable for several days at room temperature. Jts R enantiomer is also known but has no biological activity. 

The current state of Ser/Thr phosphorylation of a protein is determined by the relative activity of Ser/Thr-specific protein kinase and protein phosphatase. It is therefore imderstandable that the cell has had to develop special mechanisms to balance the two activities with one another, and, when needed, to allow kinase or phosphatase activity to dominate. One of the best investigated examples of coordinated activity of protein kinases and protein phosphatases is the regulation of glycogen metabolism in skeletal muscle. Glycogen metabolism is an example of how two different signals, namely a cAMP signal and a Ca signal meet in one metabolic pathway and control the activity of one and the same enzyme. 

Individual enzymes are assigned a four-digit number, a systematic name, and a trivial name more commonly used by biochemists. For example, EC (Enzyme Commission) 2.7.1.2 denotes a transferase (major class 2) and indicates that a phosphate group is transferred (subclass 7) and that an alcohol group accepts the phosphate (sub-subclass 1). The final digit denotes that the enzyme is ATP D-glucose-6-phosphotransferase (glucokinase). For the most part, we will use the trivial names accepted by the IUB in our subsequent discussion of enzymes that participate in metabolic pathways. 

A second challenge is the toxitity potential of some prodrugs, namely, a toxic metabolite formed from the promoiety or a reactive metabolic intermediate generated during the activation of some bioprecursors. The former case is illustrated by the liberation of formaldehyde, as seen with Mannich bases or some double esters [1,4]. The latter case involves a very few known examples of failed bioprecursors whose activation was via a reactive and toxic intermediate. Thus, arylacetylenes were examined as potential bioprecursors of nonsteroidal anti-inflammatory agents [1]. Although the nature of the final (and stable) metabolite (an arylacetic add) was known, researchers at the time were not aware that the metabolic pathway involved an intermediate and highly reactive ketene. 

Another example where metabolic pathway engineering has made a dramatic impact is in the biodegradable polymer field. The polymer of this family most widely studied is poly-P-hydroxybutyrate (PHB) (46). Another member of the PHA family commercialized by Imperial Chemical Industries (ICI), which later became Zeneca under the trade name Biopol, is a copolymer consisting of p-hydroxybutyric acid and P-hydroxyvaleric acid. This biodegradable polymer was first used in plastic shampoo bottles by the Wella Corporation [198]. In the early part of 1996, the Biopol product line, was purchased from Zeneca by the Monsanto Company. 

In Table 14.3, six types of chemical reactions are listed. In metabolic pathways, these reactions would be catalyzed by enzymes. Enzymes are given functional names. For example, isomerization is catalyzed by an isomerase. What would be the enzyme name, ending in -ase for the following oxidation, ligation, group transfer, hydrolytic. 

An example of a process based on the causative relationship is one referenced earlier in this chapter, named Biogeochemical Reductive Dechlorination (BiRD). This method can be viewed as an alternative method to MRD for destructing CAHs (Brown et al., 2009 Kennedy et al., 2006). The main concept in BiRD is based on this fact that carbon is used by bacteria for growth and for producing energy via different reductive metabolic pathways that can affect the abiotic processes. Reduced minerals, in this case, mostly iron sulfides (FeS and FeS2) are produced through bacterial respiration these minerals aid the abiotic reductive dechlorination process (Kennedy et al., 2006 Lee and Batchelor, 2002). BiRd occurs via three steps ... 

---