Effect of divalent metal ions on the digestibility of concanavalin A by endopeptidases

Structural and biochemical analyses of concanavalin A circular permutation by jack bean asparaginyl endopeptidase

Over 30 years ago, an intriguing posttranslational modification was found responsible for creating concanavalin A (conA), a carbohydrate-binding protein from jack bean (Canavalia ensiformis) seeds and a common carbohydrate chromatography reagent. ConA biosynthesis involves what was then an unprecedented rearrangement in amino-acid sequence, whereby the N-terminal half of the gene-encoded conA precursor (pro-conA) is swapped to become the C-terminal half of conA. Asparaginyl endopeptidase (AEP) was shown to be involved, but its mechanism was not fully elucidated. To understand the structural basis and consequences of circular permutation, we generated recombinant jack bean pro-conA plus jack bean AEP (CeAEP1) and solved crystal structures for each to 2.1 and 2.7 Å, respectively. By reconstituting conA biosynthesis in vitro, we prove CeAEP1 alone can perform both cleavage and cleavage-coupled transpeptidation to form conA. CeAEP1 structural analysis reveals how it is capable of carrying out both reactions. Biophysical assays illustrated that pro-conA is less stable than conA. This observation was explained by fewer intermolecular interactions between subunits in the pro-conA crystal structure and consistent with a difference in the prevalence for tetramerization in solution. These findings elucidate the consequences of circular permutation in the only posttranslation example known to occur in nature.

Modification of drug delivery to improve antibiotic targeting to the stomach

The obstacles to the successful eradication of Helicobacter pylori infections include the presence of antibiotic-resistant bacteria and therapy requiring multiple drugs with complicated dosing schedules. Other obstacles include bacterial residence in an environment where high antibiotic concentrations are difficult to achieve. Biofilm production by the bacteria is an additional challenge to the effective treatment of this infection. Conventional oral formulations used in the treatment of this infection have a short gastric residence time, thus limiting the duration of exposure of drug to the bacteria. This review summarizes the current research in the development of gastroretentive formulations and the prospective future applications of this approach in the targeted delivery of drugs such as antibiotics to the stomach.

Calcium ions make phytohemagglutinin resistant to trypsin proteolysis

To investigate the mechanism of phytohemagglutinin (PHA) susceptibility or resistance to the action of proteolytic enzymes, its in vitro proteolysis by trypsin was studied. It was found that Ca (2+) gives resistance to the native PHA molecule to trypsin proteolysis. In the absence of Ca (2+) trypsin performs a thorough hydrolysis of PHA. At the first stage of trypsin hydrolysis of PHA the formation of a relatively stable high molecular mass product occurs (PHA-T) as a result of non-co-operative proteolysis. At the second stage, the degradation of PHA-T occurs, and this degradation is performed by parallel co-operative proteolysis. This type of proteolysis differs from the action of trypsin on phaseolin, the main storage protein from common bean ( Phaseolus vulgaris L.). The implications of Ca (2+)influence of PHA hydrolysis by trypsin are discussed.

Stability of enzyme inhibitors and lectins in foods and the influence of specific binding interactions

Proteins with actual or potential antinutrient or toxicant activity found in foodstuffs include (1) enzyme inhibitors, especially those specific for serine proteinases and alpha-amylases, and (2) lectins (hemagglutinins). These inhibitors and lectins must be inactivated during processing or food preparation, usually by heat, to avoid possible undesirable effects. Knowledge of their heat stabilities thus helps determine conditions required for their inactivation or denaturation. Many are heat-stable proteins, and their conformations can be stabilized or destabilized by interactions with other constituents present in the food or the digestive tract. Differential scanning calorimetric (DSC) results show that specific binding interactions can lead to substantial increases in kinetic thermal stability of proteins. Examples of such stabilization include serine proteinase-proteinase inhibitor, alpha-amylase-amylase inhibitor, and metal ion-lectin complexes. The extent of thermal stabilization of proteinases in complexes with inhibitors is correlated with the equilibrium association constant. Presence of more than one denaturing unit revealed by DSC in complexes involving multiheaded inhibitors can be interpreted in relation to domain structures of the inhibitors. Basic information on stability of the enzyme inhibitors and lectins is relevant to food processing, quality, and safety.

Dependence on cations for the binding activity of lectins as determined by affinity electrophoresis

The metal ion content of eighteen different lectins was determined. The lectins were demetallized and the binding activity of native and demetallized forms were investigated using non-denaturing polyacrylamide affinity gel electrophoresis. The binding activities of all lectins were dependent on their metal ion content; when the cations were removed the lectins lost their carbohydrate binding activity. There was a marked difference in the strength with which lectins bind divalent cations.

Effects of manganese and calcium on conformational stability of concanavalin A: a differential scanning calorimetric study

The effect of degree of saturation of concanavalin A with Mn2+ or Ca2+, or both, on its thermal denaturation was investigated by differential scanning calorimetry. Acid-demetallized concanavalin A was partly or fully remetallized in acetate buffer (pH 5.0) containing 0.4 to 0.5 M NaCl. Under these conditions, native dimeric concanavalin A is highly stable, undergoing heat denaturation at 101 degrees C, with an enthalpy of denaturation of 7.4 cal/g. Removal of metal ions lowered stability considerably; concanavalin A with 0.06 Mn2+/monomer and 0.23 Ca2+/monomer (mol/mol) was denatured at 74 degrees C with an enthalpy of denaturation of 3.2 cal/g. Added Mn2+ stabilized demetallized concanavalin A, but added Ca2+ alone (up to 2 mol/mol monomer) did not. The Ca2+/ concanavalin A ratio influenced stabilization by Mn2+. In the presence of 1 to 2 Mn2+/ monomer and 0.5 or less Ca2+/monomer (mol/mol), stabilized concanavalin A was denatured at 85-88 degrees C and at 94-97 degress C, indicating presence of two stabilized metallo-concanavalin A species. At 1.0 or more mole each of Mn2+ and Ca2+ per monomer, one endotherm was observed at or above 98 degrees C and the enthalpy of denaturation was increased to 5.3 cal/g from less than 3.6 cal/g at lower metal ion/protein ratios. Stabilization was greater with Mn2+ plus Ca2+ than with Mn2+ alone, consistent with intrasubunit cooperativity in metal ion-induced stabilization of concanavalin A.

Metal ion content of dolichos biflorus lectin and effect of divalent cations on lectin activity

The metal ion content of the Dolichos biflorus seed lectin has been determined by using atomic absorption spectrophotometry. Ca2+, Mg2+, Mn2+, Zn2+, and Cu2+ were all found in the native lectin and could be removed to different degrees by using a variety of techniques such as high ionic strength, low pH, chelating agent, and combinations of these procedures. By use of affinity electrophoresis, the remaining binding capacity of the lectin could easily be determined, and the D. biflorus lectin showed an absolute requirement for divalent cations to be able to bind N-acetyl-D-galactosamine. The association constant (Ka) of the lectin for N-acetyl-D-galactosamine was determined after remetallization of the lectin with each individual metal ion and a combination of them. The Ka of the lectin for the hapten differed depending on which divalent cation had been used for remetallization. Ca2+ ion alone was equal to a combination of all metal ions in its ability to confer the highest binding activity of the lectin for N-acetyl-D-galactosamine. The other cations, Mg2+, Mn2+, Zn2+, and Cu2+, could not restore the binding activity of the lectin to the same degree as Ca2+ or a combination of all ions. The D. biflorus lectin is, therefore, one of the first lectins that has been shown to require only a single cation to fully retain its binding activity in contrast to most lectins that require a combination of Ca2+, Mn2+, Mg2+, or Zn2+. Furthermore, we have shown that there is a selective process of ion uptake into the lectin since the metal ion ratios in the native lectin compared to the whole seed are quite different.