M. Cristina Añón

Globulin-p and 11S-Globulin from Amaranthus Hypochondriacus : are Two Isoforms of the 11S-Globulin

Amaranth is an ancient crop with a high content of good quality proteins. Globulins are some of the most abundant storage proteins of amaranth grain. They contain two fractions distinguishable according to their different solubility: the salt-soluble 7S and 11S-globulins and the globulin-p soluble in mild-alkaline, low-ionic-strength solutions. As part of the amaranth proteins characterization, in this work we investigated the structural characteristics responsible for the different physicochemical properties of these globulins. We studied certain conformational parameters of the purified aggregates (AMGp) and individual molecules (IMGp) of globulin-p and of the partially purified globulin (ppGb) and compared the AMGp polypeptide sequences with the sequence of the 11S-globulin propolypeptide from Amaranthus (gi|122726601). The results indicated that the AMGp aggregates are responsible for the different solubility of globulin-p. Subtle conformational differences as determined by fluorescence spectroscopy and urea sensitivity were found between the molecules studied: The AMGp showed some surface differences from the IMGp and the ppGb; the AMGp also had a lower affinity for the hydrophobic fluorescent probe 1,8-aniline-naphthalene-sulfonate and a higher ionic charge than the ppGb and the IMGp, characteristics that might cause their lower solubility. In addition, we have demonstrated differences between the AMGp polypeptide sequences and that reported for amaranth 11S-globulin. These differences suggest that the globulin-p and 11S-globulin are two 11S-globulin isoforms comprised of polypeptides coming from different legumin-gene subfamilies.

Structural modifications of amaranth proteins during germination.

Amaranth storage proteins begin to be hydrolyzed immediately following the completion of germination. Albumins and globulins (7S globulin, 11S-globulin and globulin-p) were formerly modified, and glutelins, the most aggregated fraction, later. Globulins mobilization starts with the proteolysis of the 7S like-globulin polypeptides and the propolypeptide and acid (A) polypeptides of 11S-globulin and globulin-p. This pattern of 11S-globulin mobilization is accounted by the structural model with propolypeptide and A polypeptides exposed to the outside. Amaranth globulin molecules showed minor changes in their sizes in spite of having some of their polypeptides cleaved. Although globulin-p is more aggregated than 11S-globulin, it showed greater conformational changes. Considering the high susceptibility of the propolypeptide to enzymatic hydrolysis, the higher content of this polypeptide in globulin-p molecules might explain their higher structural changes. According to the results, the order of mobilization of storage proteins depends on the combination of two structural characteristics, the state of aggregation and the presence on the surface of polypeptides susceptible to cleavage.

Large-scale mapping of bioactive peptides in structural and sequence space

Health-enhancing potential bioactive peptide (BP) has driven an interest in food proteins as well as in the development of predictive methods. Research in this area has been especially active to use them as components in functional foods. Apparently, BPs do not have a given biological function in the containing proteins and they do not evolve under independent evolutionary constraints. In this work we performed a large-scale mapping of BPs in sequence and structural space. Using well curated BP deposited in BIOPEP database, we searched for exact matches in non-redundant sequences databases. Proteins containing BPs, were used in fold-recognition methods to predict the corresponding folds and BPs occurrences were mapped. We found that fold distribution of BP occurrences possibly reflects sequence relative abundance in databases. However, we also found that proteins with 5 or more than 5 BP in their sequences correspond to well populated protein folds, called superfolds. Also, we found that in well populated superfamilies, BPs tend to adopt similar locations in the protein fold, suggesting the existence of hotspots. We think that our results could contribute to the development of new bioinformatics pipeline to improve BP detection.