Food Biochemistry and Food Processing (2 edition)

BLBS102-c34 BLBS102-Simpson March 21, 2012 14:7 Trim: 276mm X 219mm Printer Name: Yet to Come

660 Part 5: Fruits, Vegetables, and Cereals

Table 34.3.Classification of Rye Proteins

Physiological classification Osborne classification Technologically important proteins

Nonstorage proteins Albumins Enzymes

Globulins -amylases

Endoxylanases

Arabinofuranosidases

Xylosidases

Ferulic acid esterases

Proteases

Serine proteases

Metalloproteases

Aspartic proteases

Cysteine proteases

Enzyme inhibitors

-amylase inhibitors

Endoxylanase inhibitors

Protease inhibitors

Storage proteins Secalins -secalins

40k -secalins

75k -secalins

-secalins

HMW secalins

Glutelins -secalins

40k -secalins

75k -secalins

HMW secalins

             {

{ γ

γ

γ

γ

γ

γ

ω

HMW, high molecular weight.

Morey and Evans 1983). The higher proportions of lysine make

rye proteins nutritionally superior to wheat proteins (Chen and

Bushuk 1970, Dexter and Dronzek 1975, Jahn-Deesbach and

Schipper 1980, Morey and Evans 1983).

Table 34.3 gives an overview of the classification of the rye

proteins, which are of technological importance and are further

discussed in detail below.

Storage Proteins

Secalins The rye alcohol-soluble prolamins, also called “se-

calins,” form the major storage proteins in rye (Gellrich et al.

2003). They have unusual amino acid compositions with high

glutamine/glutamate and proline contents and low levels of ba-

sic and acidic amino acids (Dexter and Dronzek 1975, Gellrich

et al. 2003, Preston and Woodbury 1975). The secalins are highly

polymorphic and can be classified into three groups on the ba-

sis of their amino acid compositions: S-rich, S-poor, and high

molecular weight (HMW) secalins (Fig. 34.1). Some researchers

also purified secalins of low molecular weight (LMW) (10–16 k)

(Charbonnier et al. 1981; Preston and Woodbury 1975), which is

in the same range as that of wheat low molecular weight gliadin

and barley A hordein.

The S-rich orγ-secalins are the most abundant secalins and

consist of two major groups of polypeptides: 40kγ-secalins

(26% of total secalin fraction) and 75kγ-secalins (45% of total

secalin fraction) (Gellrich et al. 2003, Shewry et al. 1982). The

molecular weights of 40kγ- and 75kγ-secalins are 40 k and

75k, respectively, as determined by SDS-PAGE (sodium dodecyl

sulfate–polyacrylamide gel electrophoresis), hence their names

(Field et al. 1983, Gellrich et al. 2003, Shewry et al. 1982),

and 33 k and 54 k, respectively, as determined by sedimentation

equilibrium ultracentrifugation (Shewry et al. 1982) or mass

spectrometry (Gellrich et al. 2001, 2003). The two groups differ

in amino acid composition, with 75kγ-secalins having higher

contents of glutamine/glutamate and proline and lower contents

of cysteine than the 40kγ-secalins (Gellrich et al. 2003, Shewry

et al. 1982, Tatham and Shewry 1991).

Theγ-secalins contain two structural domains: anN-terminal

domain rich in glutamine and proline and consisting of repetitive

sequences (Gellrich et al. 2001, 2004a; Kreis et al. 1985; Shewry

et al. 1982), and aC-terminal domain with a nonrepetitive struc-

ture, which has lower glutamine and proline content and is rich

in cysteine (Kreis et al. 1985). TheN-terminal sequences of the

40kγ- and 75kγ-secalins are identical at 16 or 17 of the first

20 positions (Gellrich et al. 2003, 2004a; Shewry et al. 1982).

TheN-terminal repetitive domains probably have a rod-like con-

formation (Shewry and Tatham 1990), whereas theC-terminal

nonrepetitive domains probably have a compact globular confor-

mation (Shewry and Tatham 1990) rich inα-helical structures

(Tatham and Shewry 1991). The 75kγ-secalins have a lower

content ofα-helical structures than the 40kγ-secalins (Tatham