Puroindolins

For the baking process, it is expected that the puroindolins protect the foam-like texture of the dough from destabilization by lipids. 

Of the enzymes present in cereal kernels, those which play a role in processing or are involved in the reactions which are decisive for the quality of a cereal product will be described. 

The wheat endosperm contains two basic, cysteine-rich proteins, puroindolin a and b (PIN-a and -b). The name is derived from the presence of tryprophan-rich segments in the amino aicd sequences Trp-Arg-Trp-Trp-Lys-Trp-Trp-Lys in PIN-a and Trp-Pro-Thr-Trp-Trp-Lys in PIN-b. PIN-a consists of a peptide chain with 115 residues (Mr 12,479) and five disulfide bridges. The peptide chains of PIN-a and PIN-b are homologous to an extent of 60%. It has been shown that the puroindolins are identical to the basic friabilins which have been discovered on 

The pH optimum of a-amylase in germinating rye lies in a range similar to that of a-AII of wheat. Therefore, a-amylase is partially inhibited by the decrease in pH in sour dough (cf. 15.4.2.2). 

These enzymes occur in various concentrations in all cereals. Carboxylester hydrolase, readily isolated from wheat germ, is not considered a lipase 

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The foam-negative effects of lipids can also be counteracted with the addition of a lipid-binding protein, wheat puroindoline (PIN), to beer. The PIN may bind the residual free lipids in such a way that they can no longer destabilize the foam. (Adapted from Cooper et ah, 2002)... 

Several other proteins that bind emulsifiers follow the general trends of this model. For example, the properties of the lipid binding protein from wheat called puroindoline has broadly similar properties [15]. 

The molecular genetic picture that has emerged to describe grain hardness can be summarized as follows. Endosperm softness is the dominantly inherited trait (i.e., the soft phenotype is the wild type). When both puroindoline a and puroindoline b are expressed, they work together to impart softness to the endosperm. When both are absent (as in durums), the endosperm is very hard. When only one is present, an intermediate degree of hardness is found. Another way in which an intermediate degree of hardness is obtained is when one of the puroindolines is present and the other has been altered as a result of mutation. 

A number of different hardness alleles have been identified. The gene products of these alleles differ either by the null expression of one of the puroindolines or by a change in a single amino acid in the molecule. One of the earliest mutations to be discovered involved a change in the amino acid at position 46 of puroindoline b from glycine to serine. In an excellent review... 

Table 4.1 Puroindoline a and b Grain Hardness Alleles, Kernel Phenotype, and Molecular Changes in Expressed Proteins Resulting from Mutations...

Pina-Dla Pinb-Dle Hard Puroindoline b null, Trp-39 to stop codon... 

The research efforts discussed in the previous sections of this chapter have steadily elucidated the genetic/molecular basis for variations in endosperm hardness. What is still not very well understood at this stage is the physical mechanism that causes soft wheat to become hard as a result of changes in the puroindoline proteins. In order to make progress in this challenging problem, the science of adhesion needs to be invoked. 

In the high water content of the immature kernel, proteins and lipids compete for the starch granule surface. The granule surface would be expected to be less polar (more hydrophobic) than the aqueous medium. Puroindolines are hydrophobic proteins and would therefore tend to adsorb at the starch granule surface. By the mass action law, their amounts would be greatest in the wild phenotype (i.e., with both puroindolines present). 

The other factor to consider is the molecular weight. Puroindolines are rather small proteins. Other proteins in the kernel can reach very high molecular weights (e.g., glutenins) and thus have high free energies of adsorption. They could therefore replace pmoindolines at the starch surface. 

Gautier, M.-R, P. Cosson, A. Guirao, D. Marion, and P. Joudier. 1994. Triticum aesti-vum puroindolines, two basic cysteine-rich seed proteins cDNA analysis and development gene expression. Plant Molecular Biology 25 43-57. 

Morris, C. F. 2002. Puroindolines The molecular genetic basis of wheat grain hardness. Plant Molecular Biology 48 633-647. 

Rahman, S., G. J. Jolly, J. H. Skerritt, and A. Wallosheck. 1994. Gloning of a wheat 15 kDa grain softness protein (GSP). GSP is a mixture of puroindoline-like polypeptides. European Journal of Biochemistry 223 917-925. 

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