De novo-biosynthesis

Biosynthesis of Tea Flavonoids. The pathways for the de novo biosynthesis of flavonoids in both soft and woody plants (Pigs. 3 and 4) have been generally elucidated and reviewed in detail (32,51). The regulation and control of these pathways in tea and the nature of the enzymes involved in synthesis in tea have not been studied exhaustively. The key enzymes thought to be involved in the biosynthesis of tea flavonoids are 5-dehydroshikimate reductase (52), phenylalanine ammonia lyase (53), and those associated with the shikimate/arogenate pathway (52). At least 13 enzymes catalyze the formation of plant flavonoids (Table 4). 

FATP5 KO mice have been characterized in two studies focusing on the role of FATP5 in hepatic lipid and bile metabolism. LCFA uptake in primary hepato-cytes isolated from FATP5 KO mice was reduced by 50% and hepatic lipid content in the KO mice was significantly reduced despite an increased fatty acid de novo biosynthesis. Detailed analysis of the hepatic lipidome of FATP5 KO mice revealed significant... 

Marine organisms frequently contain a complex mixture of sterols. The biosynthetic origin of these compounds is complicated by the fact that there are four possible sources. The sources by which marine invertebrates may obtain sterols, first described by Goad [3], are (1) de novo biosynthesis, (2) assimilation from diet, (3) modification of dietary sterols and (4) assimilation of symbiont-produced sterols or sterol precursors. In any examination of the biosynthetic origin(s) of sterols one must consider all of these factors needless to say, a complex picture is emerging. 

The scope of the majority of the papers is limited to the ascertainment of the existence of de novo biosynthesis of a particular compound. In this respect it has recently been proposed that in many cases it would be possible to predict the origin of secondary metabolites in nudibranch molluscs by examining their geographical variations in a given species [10]. Those molluscs that exhibit considerable variation in their chemical constituents undoubtedly obtain these from dietary sources, while those that have the same substances wherever they are collected are most likely capable of de novo biosynthesis. 

Shaw and co-workers during studies into the de novo biosynthesis of purine nucleotides demonstrated that 4(5)-aminoimidazole (25 R = H) on treatment with a saturated aqueous solution of potassium bicarbonate at 70°C for 15 min gave 4-aminoimidazole-5-carboxylic acid (38) in an estimated yield of 40% [71JCS(C)1501]. This and related reactions are discussed in more detail in Section V,B,4. 

The major difference between purine and pyrimidine de novo biosynthesis is that the pyrimidine ring is assembled and then added to PRPP (Fig. 20-1). With purines, the purine ring is built directly on the PRPP. 

Mutations in the gene for adenylosuccinate lyase (ASL), inherited as an autosomal recessive disorder in purine metabolism, are associated with severe mental retardation and autistic behavior, but apparently not self-mutilation [10, 11]. This enzyme catalyzes two distinct reactions in the de novo biosynthesis of purines the cleavages of adenylosuccinate (S-Ado) and succinylaminoimidazole carboxamide ribotide (SAICAR), both of which accumulate in plasma, urine and cerebrospinal fluid of affected individuals [12]. Measurements of these metabolites in urine... 

In resting neutrophils, about 50% of the total cellular FcyRIII pool is expressed on the cell surface. There is considerable variation in this value because many methods used to isolate neutrophils can also inadvertently mobilise these subcellular receptors. The remainder of the total cellular FcyRIII that is not expressed on the plasma membrane is present in the subcellular pool. However, if the FcyRIII normally present on the plasma membrane is cleaved (e.g. via the action of elastase or pronase) and the cells subsequently activated, then FcyRIII reappears on the cell surface via the mobilisation of these pools. Thus, the expression can be restored to up to 70% of the resting level within 15 min via such a translocation. During activation (and presumably priming), FcyRIII (together with other plasma membrane markers) is also translocated to the plasma membrane however, because the receptor is also shed from the cell, the total number of receptors on the cell surface remains largely unchanged. There is also some evidence that continued expression of FcyRIII on the cell surface requires de novo biosynthesis of this receptor (see Fig. 7.8). 

Thus, their continued expression on the plasma membrane requires translocation from preformed pools and/or de novo biosynthesis. 

The fact that receptors need to be replaced via de novo biosynthesis can be demonstrated in experiments where neutrophils are cultured in the presence and absence of cycloheximide. When protein biosynthesis is blocked by this inhibitor, the expression of FcyRIII on the cell surface cannot be maintained as the cells age in culture (Fig. 7.8), indicating that continued expression of this receptor is a balance between the amount shed and the amount replaced via new biosynthesis. Newly synthesised receptors appear to be functional, because they can be detected within the biosynthetic machinery of the cell, and newly made (i.e. newly labelled) receptors are detected in the plasma membrane. 

Lehtonen, J.Y.A., Horiuchi, M., Daviet, L., AMshita, M. and Dzau, V.J., 1999, Activation of the de novo biosynthesis of sphingolipids mediates angiotensin 11 type 2 receptor-induced apoptosis. J. Biol. Chem. 274 16901-16906. 

The significance of these in vitro enzyme inhibition studies is uncertain, in view of the evidence that has been presented concerning the sensitivity of cancer cells to feedback inhibition by these nucleotides. On the other hand, 6-chloropurine inhibits the de novo biosynthesis of nucleic acid guanine but not of nucleic acid adenine in sarcoma 180 ascites cells [319],... 

Kubanek, J. Andersen, R.J. (1999) Evidence for de novo biosynthesis of the polyketide fragment of... 

Overexpression of key genes of the synthetic pathways Heterologous gene expression/use of engineered enzymes Knockouts of genes involved in product degradation Precursor approach instead of de novo biosynthesis... 

Fig. 23.1 Microbial routes from natural raw materials to and between natural flavour compounds (solid arrows). Natural raw materials are depicted within the ellipse. Raw material fractions are derived from their natural sources by conventional means, such as extraction and hydrolysis (dotted arrows). De novo indicates flavour compounds which arise from microbial cultures by de novo biosynthesis (e.g. on glucose or other carbon sources) and not by biotransformation of an externally added precursor. It should be noted that there are many more flavour compounds accessible by biocatalysis using free enzymes which are not described in this chapter, especially flavour esters by esterification of natural alcohols (e.g. aliphatic or terpene alcohols) with natural acids by free lipases. For the sake of completeness, the C6 aldehydes are also shown although only the formation of the corresponding alcohols involves microbial cells as catalysts. The list of flavour compounds shown is not intended to be all-embracing but focuses on the examples discussed in this chapter... 

---