Polysaccharides functional groups

Other immobilization methods are based on chemical and physical binding to soHd supports, eg, polysaccharides, polymers, glass, and other chemically and physically stable materials, which are usually modified with functional groups such as amine, carboxy, epoxy, phenyl, or alkane to enable covalent coupling to amino acid side chains on the enzyme surface. These supports may be macroporous, with pore diameters in the range 30—300 nm, to facihtate accommodation of enzyme within a support particle. Ionic and nonionic adsorption to macroporous supports is a gentle, simple, and often efficient method. Use of powdered enzyme, or enzyme precipitated on inert supports, may be adequate for use in nonaqueous media. Entrapment in polysaccharide/polymer gels is used for both cells and isolated enzymes. 

A regular fine structure causing maximal hydrogen bonding between the molecules reduces the possibility of reversible processes and therefore reduces the gel-forming properties of the compound. Other polysaccharides with different types of monomers or with branched chain structures can be treated in a similar manner. However, much more study of their fine structure and the accessibility of their functional groups is necessary. 

The carbonyl-reactive group on these crosslinkers is a hydrazide that can form hydrazone bonds with aldehyde residues. To utilize this functional group with carbohydrate-containing molecules, the sugars first must be mildly oxidized to contain aldehyde groups by treatment with sodium periodate. Oxidation with this compound will cleave adjacent carbon-carbon bonds which possess hydroxyl groups, as are abundant in polysaccharide molecules (Chapter 1, Sections 2 and 4.4). 

PDPH also may be used as a thiolation reagent to add sulfhydryl functional groups to carbohydrate molecules. The reagent can be used in this sense similar to the protocol described for AMBH (Chapter 1, Section 4.1). After modification of an oxidized polysaccharide with the hydrazide end of PDPH, the pyridyl group is removed by treatment with DTT, leaving the exposed sulfhydryl (Figure 5.15). 

The organic fraction composition may influence the exchange capacity. A key contribution to the exchange capacity of humus is given by the carboxyl and phenolic hydroxyl functional groups. Under appropriate pH conditions, uranic acids in polysaccharides or carboxy-terminal structures in peptides can contribute to the... 

At this point it is convenient to reiterate the major features and relationships normally encountered for polysaccharide structures. Each saccharide residue is normally linked through the reducing (hemi-acetyl) functional group (the anomeric position) to the hydroxy position of another residue. Each residue contains a number of hydroxy positions, but only one anomeric position. 

Polysaccharides are ubiquitous in nature. They can be classified into three separate groups, based on their different functions. Structural polysaccharides provide mechanical stability to cells, organs, and organisms. Waterbinding polysaccharides are strongly hydrated and prevent cells and tissues from drying out. Finally, reserve polysaccharides serve as carbohydrate stores that release monosaccharides as required. Due to their polymeric nature, reserve carbohydrates are osmotically less active, and they can therefore be stored in large quantities within the cell. 

A good example of this interaction in catalysis is the hydrolysis of the bacterial cell wall polysaccharide by lysozyme. This enzyme contains two carboxylic gronps at its active site and, in active enzyme one must be in dissociated—COO, the other in the undissociated—COOH form. Therefore, the pK s of the two carboxylic groups ate different. This difference in dissociation constant is a consequence of the neighbouring amino acid residues and of the interactions between the functional groups in the microenvironment. 

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