Carbohydrates higher plants

Aldol reactions occur in many biological pathways, but are particularly important in carbohydrate metabolism, where enzymes called aldolases catalyze the addition of a ketone enolate ion to an aldehvde. Aldolases occur in all organisms and are of two types. Type 1 aldolases occur primarily in animals and higher plants type II aldolases occur primarily in fungi and bacteria. Both types catalyze the same kind of reaction, but type 1 aldolases operate place through an enamine, while type II aldolases require a metal ion (usually 7n2+) as Lewis acid and operate through an enolate ion. 

Plants were probably the first to have polyester outerwear, as the aerial parts of higher plants are covered with a cuticle whose structural component is a polyester called cutin. Even plants that live under water in the oceans, such as Zoestra marina, are covered with cutin. This lipid-derived polyester covering is unique to plants, as animals use carbohydrate or protein polymers as their outer covering. Cutin, the insoluble cuticular polymer of plants, is composed of inter-esterified hydroxy and hydroxy epoxy fatty acids derived from the common cellular fatty acids and is attached to the outer epidermal layer of cells by a pectinaceous layer (Fig. 1). The insoluble polymer is embedded in a complex mixture of soluble lipids collectively called waxes [1], Electron microscopic examination of the cuticle usually shows an amorphous appearance but in some plants the cuticle has a lamellar appearance (Fig. 2). 

Precursors of phenylpropanoids are synthesized from two basic pathways the shikimic acid pathway and the malonic pathway (see Fig. 3.1). The shikimic acid pathway produces most plant phenolics, whereas the malonic pathway, which is an important source of phenolics in fungi and bacteria, is less significant in higher plants. The shikimate pathway converts simple carbohydrate precursors into the amino acids phenylalanine and tyrosine. The synthesis of an intermediate in this pathway, shikimic acid, is blocked by the broad-spectrum herbicide glyphosate (i.e., Roundup). Because animals do not possess this synthetic pathway, they have no way to synthesize the three aromatic amino acids (i.e., phenylalanine, tyrosine, and tryptophan), which are therefore essential nutrients in animal diets. 

Phenol-carbohydrate derivatives, in higher plants, 20, 371-408 Photochemistry, of carbohydrates, 18, 9-59 Physical chemistry, of carbohydrates, 15, 11-51 of starch, 11, 335-385 Physical properties, of solutions of polysaccharides, 18, 357-398... 

Higher plants make large amounts of L-ascorbate, which in leaves may account for 10% of the soluble carbohydrate content.28 However, the pathway of synthesis differs from that in Fig. 20-2. Both D-mannose and L-galactose are efficient precursors. The pathway in Eq. 20-4, which starts with GDP-d-mannose and utilizes known enzymatic processes, has been suggested.28 29 The GDP-D-mannose-3, 5-epimerase is a well documented but poorly understood enzyme. Multistep mechanisms related to that of UDP-glucose 4-epimerase (Eqs. 20-1,15-14) can be envisioned. 

Glycosides are formed by higher plants from a variety of externally applied substances, regardless of whether or not the substance is toxic or growth active. The herbicidal chemicals have been discussed in other Sections, and this Section will deal with glycoside formation of some other compounds, and with some carbohydrates and derivatives that have shown growth activity. 

Pridham, J. B., Phenol-Carbohydrate Derivatives in Higher Plants, 20, 371-408... 

The neutral and nonspecific FDPases may function in nonphotosynthetic carbohydrate metabolism in higher plants. In the germinating castor bean acetate is utilized for the synthesis of sucrose 107, 108), and the presence of the AMP-sensitive FDPase in plant embryo tissues has been demonstrated by Bianchetti and Satirana 109). The changes in levels of this enzyme in response to changes in physiological conditions 109) support a gluconeogenic role for this enzyme. The role of the nonspecific acid FDPase in plant tissues remains unknown. 

Primary metabolites are produced in larger quantities than the secondary metabolites and have specific metabolic functions. Primary metabolites obtained from higher plants are used as foods, food additives, and industrial raw materials, such as carbohydrates, vegetable oils, protein, and fatty acids. They are generally high-volume, low-value bulk materials. 

This cycle represents the quantitatively most important C02 fixation pathway in Nature. It is found in most aerobic autotrophic organisms, ranging from diverse photosynthetic and chemolithoautotrophic bacteria to chloroplasts of eukaryotic algae and higher plants [5]. It is centered around carbohydrates, with ribulose 1,5-bisphosphate being the C02 acceptor (Figure 3.1). 

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