Steroid plant, biosynthesis

McCue, K. F., Shepherd, L. V. T., Allen, P. V, Maccree, M. M., Rockhold, D. R., Corsini, D. L., Davies, H. V, Belknap, W. R. (2005). Metabolic compensation of steroidal glycoalkaloid biosynthesis in transgenic potato tubers using reverse genetics to confirm the in vivo enzyme function of a steroidal alkaloid galactosyltransferase. Plant science, 168, 267-273. 

Bishop, G.J. (2007) Refining the plant steroid hormone biosynthesis pathway. Trends Plant Sci. 12, 377-80. 

Kohara, A., Nakajima, C., Hashimoto, K., Ikenaga, T, Tanaka, H., Shoyama, Y, Yoshida, S. and Muranaka, T. (2005) A novel glucosyltransferase involved in steroid saponin biosynthesis in Solanum aculeatissimum. Plant Mol. Biol., 57, 225-39. 

Steroids are a class of organic compounds that contain a characteristic arrangement of four cycloalkane rings joined to each other. All steroids are synthesized inside cells either from the sterols lanosterol (in animals and microorganisms) or from cycloartenol (in plants). Both lanosterol and cycloartenol are derived from the cycli-zation of the triterpene squalene (six isoprene units, 30C) [3-5]. In other words, squalene is the biochemical precursor to the whole family of steroids. Among all of the steroid biosynthesis, steroid hormone biosynthesis is the most concerned. 

Saponins are glycosylated secondary metabolites that are widely distributed in the Plant Kingdom.3,4 They are a diverse and chemically complex family of compounds that can be divided into three major groups depending on the structure of the aglycone, which may be a steroid, a steroidal alkaloid, or a triterpenoid. These molecules have been proposed to contribute to plant defense.3 6 Saponins are also exploited as drugs and medicines and for a variety of other purposes.4 Despite the considerable commercial interest in this important group of natural products, little is known about their biosynthesis. This is due in part to the complexity of the molecules, and also to the lack of pathway intermediates for biochemical studies. 

Donath J, Boland W (1994) Biosynthesis of acyclic homoterpenes in higher plants parallels steroid hormone metabolism. J Plant Physiol 143 473 78... 

The biosynthesis of synthetic natural rubber has been completely determined and appears in Fig. 15.1. Many plants and animals use this same biosynthetic pathway to make hundreds of terpenes and steroids from their common isoprenoid building blocks. 

Cholesterol is a soft waxy substance that is a steroidal alcohol or sterol. It is the most abundant steroid in the human body and is a component of every cell. Cholesterol is essential to life and most animals and many plants contain this compound. Cholesterol biosynthesis occurs primarily in the liver, but it may be produced in other organs. A number of other substances are synthesized from cholesterol including vitamin D, steroid hormones (including the sex hormones), and bile salts. Cholesterol resides mainly in cell membranes. 

In the biochemistry of D-fucose and its derivatives, there are larger lacunae in our knowledge. The biosynthesis of D-fucose has not been examined, so that it is not yet known how this sugar is produced in the organisms that contain it (mainly plants and some micro-organisms). In plants, it occurs especially in the form of steroid glycosides, whereas, in micro-organisms, it has been particularly located in antibiotic substances. It has not been identified in animals. 

In addition to these major processes, many other chemical events also occur. Mitochondria concentrate Ca2+ ions and control the entrance and exit of Na+, K+, dicarboxylates, amino acids, ADP, P and ATP, and many other substances.16 Thus, they exert regulatory functions both on catabolic and biosynthetic sequences. The glycine decarboxylase system (Fig. 15-20) is found in the mitochondrial matrix and is especially active in plant mitochondria (Fig. 23-37). Several cytochrome P450-dependent hydroxylation reactions, important to the biosynthesis and catabolism of steroid hormones and... 

The terpenes, carotenoids, steroids, and many other compounds arise in a direct way from the prenyl group of isopentenyl diphosphate (Fig. 22-1).16a Biosynthesis of this five-carbon branched unit from mevalonate has been discussed previously (Chapter 17, Fig. 17-19) and is briefly recapitulated in Fig. 22-1. Distinct isoenzymes of 3-hydroxy-3-methylglutaryl-CoA synthase (HMG-CoA synthase) in the liver produce HMG-CoA destined for formation of ketone bodies (Eq. 17-5) or mevalonate.7 8 A similar cytosolic enzyme is active in plants which, collectively, make more than 30,000 different isoprenoid compounds.910 However, many of these are formed by an alternative pathway that does not utilize mevalonate but starts with a thiamin diphosphate-dependent condensation of glyceraldehyde 3-phosphate with pyruvate (Figs. 22-1,22-2). 

Figure 22-9 Structures and routes of biosynthesis for a few plant steroids.

The concentration of dolichyl phosphate in eukaryotic tissues is very low and is probably rate-limiting for the glycosylation processes. Variation of the concentration in the endoplasmic-reticulum membranes is a possible way of controlling the rate of glycosylation. It is important to point out that the early steps in the dolichol biosynthesis are common to such other prenyl derivatives in plants as steroids, essential oils, hormones, phytol, and carotenes (see Scheme 1), and parameters affecting those reactions that may control the dolichol to dolichyl phosphate step could be another mechanism for regulation of the level of dolichyl phosphate. 

Cholesterol and the C29 sterols are used by plants as the starting materials for the biosynthesis of other steroids with the same number of carbons, such... 

Sterols and Cholesterol. Natural sterols are crystalline C76 C1(1 steroid alcohols containing an aliphatic side chain at C17. Sterols were first isolated as lionsaponifiable fractions of lipids from various plant and animal sources and have been identified in almost all types of living organisms. By far, the most common sterol in vertebrates is cholesterol (8). Cholesterol serves two principal functions in mammals. First, cholesterol plays a role in the structure and function of biological membranes.. Secondly, cholesterol serves as a central intermediate in the biosynthesis of many biologically active steroids, including bile acids, corticosteroids, and sex hormones. 

Plant Sterols. Sterols have been identified in almost all types of living organisms and can be isolated, in varying quantities, from many different plants. Similar to cholesterol, plant sterols have a structural and functional role in biological systems and serve as intermediates in the biosynthesis of an assortment of biologically active steroids. 

The biosynthesis of alkaloids has been extensively studied, and although for a time it was thought that alkaloids arose primarily from amino acid precursors, strong evidence now is available that ethanoate also is involved. The mode of alkaloid biosynthesis is not yet as well understood as that of the terpenes and steroids. One experimental problem is the difficulty of feeding suitably labeled precursors to plants. 

The formation of plant cellulases has been found to be closely regulated by different growth hormones, particularly auxin (6,11), steroids (12), or ethylene gas (13). The hormones act in different tissues under different circumstances, and they seldom lead to such high cellulase activity that there is a net decline in total cellulose. Indeed, cellulose biosynthesis usually continues even while partial hydrolysis occurs, and net cellulose deposition often keeps pace with growth under all of these conditions (14). 

The earliest steps (MVA to GGPP) for polyisoprenoid biosynthesis are identical for all plants and animals (12,13). They involve the well-known diterpene pathway, MVA — MVAP — MVAPP — IPP + DMAPP — GPP — FPP — GGPP. The enzymes catalyzing these steps have been studied extensively, especially from animals (liver) and yeast, and to a more limited extent from higher plants. In some cases the enzymes have been purified to homogeneity most have been only partially purified. In both plants and animals a major branch at FPP leads to the production of squalene and the steroids. In plants, three major branches occur at GGPP, of which one leads to the carotenoids via phyto-ene, a second to the phytyl group of chlorophyll, and a third to the GAs. 

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