Elsevier

Journal of Cereal Science

Volume 44, Issue 3, November 2006, Pages 272-286
Journal of Cereal Science

Review
Kafirin structure and functionality

https://doi.org/10.1016/j.jcs.2006.05.004Get rights and content

Abstract

The structural and functional properties of kafirins are reviewed. Three classes of kafirin: the α, β and γ forms have been identified at the protein level and one, the δ, has been identified only at the gene and transcript levels. All forms show high homology with the equivalent zein proteins. By analogy with the zeins it is believed that the α-kafirins probably have an extended hairpin structure in solution, comprising elements of α-helix, β-sheet and turns folded back on itself. Kafirins are the most hydrophobic of the prolamins as shown by their solubility, and calculated hydration free energies. The proteins exhibit extensive cross-linking by disulphide bonds and on cooking form indigestible aggregates which are not solubilised by reduction of disulphide bonds. In spite of continuing studies, the reasons for the low digestibility of the protein remain uncertain and there may be several factors involved. Other research has shown that kafirins may have non-food uses and may be used to form films.

Section snippets

Properties of kafirins

In common with other cereals, the major storage proteins (kafirins) in the grain of sorghum are soluble in alcohol–water mixtures and therefore defined as prolamins. The name prolamin was coined by Osborne (1924) to reflect their high contents of proline and amide nitrogen, derived from glutamine, and these two amino acids together account for over 30% of the total residues in total kafirin fractions (Table 1).

Although Osborne (1924) used aqueous ethanol to extract prolamins, we now know that

Groups of kafirins

The closest cultivated relative of sorghum is maize, which is classified in the same subfamily of grasses (Panicoideae). The massive volume of research on maize prolamins (zeins) has therefore provided a framework for the analysis of kafirins while comparison of the prolamins from the two species is of interest in relation to determining the molecular basis for the unique properties of sorghum prolamins.

Sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) of total kafirin

Kafirin hydrophobicity

The results of a calculation of the free energies of hydration of kafirins based on sequence data and amino acid analyses are given in Table 4. Data are normalised to a molecule consisting of 100 residues by multiplying the hydration energy for each residue by its abundance expressed in mol%. In this way, molecular weight effects of individual proteins are removed and values can be compared directly. For comparison, the hydration energy of an “average” protein (Shewry et al., 2003) is −164 

Cross-linking of kafirins

The precise patterns of cross-linking of the kafirins would be expected to influence the properties of the proteins during processing and their digestibility and biological value. El Nour et al. (1998) determined the distributions of kafirin subunits in oligomers extracted with 60% (v/v) t-butanol. Analysis of the extracts by SDS-PAGE (non-reduced and reduced) (Fig. 1) and SE-HPLC showed that about 70% of the fraction comprised oligomers of α- and γ-kafirin subunits. Only trace amounts of β

Conformation of kafirins

Little work has been performed on kafirins but early spectroscopic studies (circular dichroism (CD) and infrared) indicated that the major components have similar secondary structure contents to the α-zeins (Wu et al., 1971), with about 40% α-helix when dissolved in 60% (v/v) t-butanol. In a more recent study (Gao et al., 2005), using Fourier transform-infrared (FT-IR) spectroscopy, the ratio of α-helix to β-sheet conformations was reported as 1.39 to 1. The sample used was a freeze-dried

Effects of cooking and digestion

The low protein digestibility of sorghum in comparison with other cereals has been the subject of extensive research applying in vitro protein digestibility methods. These methods are based on common basic steps but differ in conditions such as the time of digestion, the extraction time and the type of enzyme used. They are summarised in the review of Duodo et al. (2003).

In addition, Duodo et al. (2003) reviewed the factors that may affect sorghum protein digestibility, dividing them in two

Kafirin films

Kafirins may be used to make biodegradable plastic films with properties which are similar to those of zeins (Buffo et al., 1997; Da Silva and Taylor, 2005; Emmambux et al., 2004; Gao et al., 2005). However, the precise nature of the protein–protein interactions that are responsible for the formation of films are not known. Gao et al. (2005) compared methods of extracting kafirins for film formation showing that methods which maintain the greatest proportion of α-helical structure gave the best

Envoy

There is still much about kafirin proteins that remain to be understood. Very little work has been performed on the structure of these proteins and most of our understanding comes from analogy with homologous proteins, notably from maize. However, it is clear that there are significant differences in the behaviour of kafirins and the much more studied zeins of maize. This is particularly true of their responses to cooking where there are marked differences in the digestibility of the cooked

Acknowledgements

Rothamsted Research receives grant-aided support from the Biotechnology and Biological Sciences Research Council of the United Kingdom. The authors wish to thank Dr. M.R. Bugs and Dr. L.A. Colnago (Embrapa, São Paulo, Brazil) for providing Fig. 6C and Dr. A. Nunes for providing Fig. 9, Fig. 10. The authors are indebted to Dr. Rudolph Jung (Pioneer Hi-Bred International, Johnson, Iowa, USA) for discussions and for providing access to information from his unpublished studies of kafirins.

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