Plants, higher Carbohydrate metabolism

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. 

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. 

In higher plants several glucan metabolizing enzymes occur as plastid- and cytosol-specific isozymes (1,2). The dual intracellular location of these enzyme activities suggests that both the plastidic and the cytosolic compartment contain a pool of polysaccharides. However, until now a cytosolic starch-like polysaccharide which functions as the physiological carbohydrate substrate of the cytosol-specific isozymes has not been identified and, therefore, the metabolic function of these enzyme forms remains enigmatic. 

The metabolic pathway responsible for biosynthesis of aromatic amino acids and for vitamin-like derivatives such as folic acid and ubiquinones is a major enzyme network in nature. In higher plants this pathway plays an even larger role since it is the source of precursors for numerous phenylpropanoid compounds, lignins, auxins, tannins, cyano-genic glycosides and an enormous variety of other secondary metabolites. Such secondary metabolites may originate from the amino acid end products or from intermediates in the pathway (Fig. 1). The aromatic pathway interfaces with carbohydrate metabolism at the reaction catalyzed by 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthase, the condensation of erythrose-4-phosphate and PEP to form... 

Carbohydrates are aldehydes or ketones of higher polyhydric alcohols or components that yield these derivatives on hydrolysis. They occur naturally in plants (where they are produced photosynthetically), animals and microorganisms and fulfil various structural and metabolic roles. Monosaccharides are the simplest carbohydrates and they often occur naturally as one of their chemical derivatives, usually as components of disaccharides or polysaccharides. 

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. 

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