13
Polygalacturonases i n H i g h e r Plants
Downloaded by UNIV OF SOUTHERN CALIFORNIA on August 23, 2013 | http://pubs.acs.org Publication Date: June 5, 1986 | doi: 10.1021/bk-1986-0310.ch013
Russell Pressey Agricultural Research Service, Richard B. Russell Agricultural Research Center, U.S. Department of Agriculture, Athens, GA 30613
Polygalacturonase (PG) is widely distributed in higher plants. The richest plant source of PG is ripe tomato f r u i t . Extacts of tomatoes contain two forms of the enzyme (PG I and PG II) which differ markedly in molecular size and stability to heat. A protein (PG converter) has been isolated from tomatoes that reacts with PG II to form PG I. PG II can be recovered by treating PG I with mild alkali (pH 11). Both tomato enzymes are endo-PG's. Some fruits that soften markedly during ripening such as pears and freestone peaches contain not only endo-PG but also exo-PG. Other fruits such as apples and clingstone peaches contain only exo-PG, consistent with slow softening characteristics. Low levels of exo-PG are found in many vegetative and storage tissues. Pectin occurs in most plant materials but is particularly high in young and fruit tissues. It is an important constituent of the cell wall where it may be involved in interlinking of other structural polysaccharides and proteins (I) . Pectin is also the major component of the middle lamella which has led to the classical view that it is an intercellular adhesive. Pectin appears to vary greatly in composition from source to source and even from the same plant at different stages of growth. A serious problem in characterization of pectin is that the native polysaccharide is essentially insoluble and solubilization can be accomplished only by degradation. Other polysaccharides are solubilized along with pectin and separation is difficult. Thus the pectin that is isolated is partly degraded, quite heterogeneous in term8 of molecular size, and may contain contaminating neutral polysaccharides. Nevertheless, it is known that pectin is basically a linear o^-l ,4-galacturonan in which at least some of the carboxyl groups are esterified with methanol. Variable amounts of neutral sugars are usually associated with the galacturonan (2). The galacturonan chain is interrupted by «C-1,2-linked This chapter not subject to U.S. copyright. Published 1986, American Chemical Society In Chemistry and Function of Pectins; Fishman, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
Downloaded by UNIV OF SOUTHERN CALIFORNIA on August 23, 2013 | http://pubs.acs.org Publication Date: June 5, 1986 | doi: 10.1021/bk-1986-0310.ch013
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CHEMISTRY AND FUNCTION OF PECTINS
rhamno8e r e s i d u e s , but the amount of rhamnose i n pectin i s highly v a r i a b l e from source to source. For example, the p e c t i n from suspension c u l t u r e d sycamore c e l l s contains nearly ten times more rhamnose t h a n the p e c t i n from apple f r u i t (2., 3.) . Two other neutral sugars, arabinose and galactose, are also covalently bonded to the g a l a c t u r o n a n . These s u g a r s o c c u r as s i d e c h a i n s o f < - l , 5 - a r a b i n a n and (3-1,4-galactan (1., 3., 4., 5.). I t has been proposed that branching of the galacturonan chain i s more prevalent i n the pectin from c e l l walls than that from the middle lamella (6., 1). Two c l a s s e s of enzymes degrade the
