How might substrate specificity, a protein’s designed ability to interact only with very specific molecular shapes, play a role in the cell-cell identification process that features glycoproteins and receptors? what are the real-world human health consequences of this specificity

Generation of toxins by a wide variety of organisms ranging from single cells (i.e., bacteria, fungi) to complex organisms of the plant and animal kingdoms (i.e., scorpions, reptiles) is a well-known phenomenon in cell biology, enabling certain organisms to respond to their environment in a defensive or offensive manner. The number of toxins isolated and identified has constantly increased over the years and better insights into their molecular structure and mechanism of action have been achieved. A vast majority of these toxins act by either modulating essential signaling cascades in the eukaryotic cell [1] or by direct interference with the ionic equilibrium maintained by the cell membrane barrier. With respect to the latter, some toxins act by direct insertion into the lipid bilayer to form pores while others act on ion pumps or ion-gated channels responsible for maintaining the ion gradients (Figure 1). The molecular mechanism underlying the interference of a toxin with a signaling cascade is directed at a specific target on the cell membrane where regular signals or second messengers initiate a normal physiological response. Upon interaction with a toxin, the subsequent biochemical chains of events are perturbed, and in most cases, a pathological response is evident. These targets are rather distinct at the membrane level and include diverse groups such as ligand-gated ionotropic receptors, G protein-coupled receptors, tyrosine kinase receptors, integrin receptors and certain lipid species present in the bilayer plasma membrane of the cell. Some targets are localized inside the host cell, and consist of functional organelles, which react specifically to some but not all toxins. Two major mechanisms of changes in the target protein receptor at the plasma membrane level are distinguishable. One mechanism consists of conformational changes such as the binding of the acetylcholine molecule to the alpha subunit of the nicotinic receptor followed by allosteric transition and opening of the channel for ion–induced cell depolarization [2]. A second mechanism of protein modulation involves translocation, as exemplified by the stimulation of the skeletal muscle cell by insulin, which causes contraction-induced glucose transporter translocation from the intracellular vesicles to the outer plasma membrane domain to enable glucose uptake [3]. The changes in membrane lipids brought about by toxins containing hydrolytic enzyme activity such as phospholipase A2 (PLA2) or phospholipase C (PLC) are notorious since they may generate lytic products and restructure the lipid bilayer [3,4,5]. Other toxins act as pores to interfere and collapse the intracellular ionic equilibrium.

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