iHerp Australia Issue 6 - Page 57

Because of the diversity in toxin type and activity, as well the plethora of regulatory molecules maintaining homeostatic processes, identical pathologies could be induced by totally different toxins acting on different molecular targets. For example, neurotoxicity could be induced by a three-finger toxin (3FTx) blocking a given nerve cell receptor, or by a phospholipase (PLA 2 ) breaking down the structural lipids of skeletal muscle cells – two very different toxins with different precise targets, but the same pathological outcome: paralysis. Different roads being driven by different cars, but arriving at the same destination. A biochemical weapon. While the role of venom varies between taxa, the primary function of venom in snakes is for use in predation. A snake’s venom does not just have one type ‘it is common for a venom to be composed of dozens , if not hundreds , of different toxin types ’ of toxin. While there are many species whose venom is dominated by a specific toxin family, it is common for a venom to be composed of dozens, if not hundreds, of different toxin types belonging to over a dozen protein families. These toxins work together, synergistically, to facilitate efficient prey capture. It is important to note here that successful predation is reconciled by prey capture – not prey death. A snake is probably not perturbed if its meal still has a heartbeat, so long as that heartbeat is not powering a defensive attack or a bid for freedom. It just so happens that inducing rapid death is often an outcome of meeting these criteria. Toxins are thus likely to be selected for on this basis: the ability to efficiently immobilise prey, where efficiency is measured by both predation success and energy conservation. At a molecular level, efficiency is dictated by the affinity of toxins for biochemical targets within prey and their efficacy in disrupting physiological pathways. The physiological parameters between different animals’ bodies can vary substantially, depending on the organism and its environment. Blood chemistry, body temperature, and neurone configuration will differ substantially between a lizard, a bird, and a fish, for example. This means that the specific cell types, receptors, and proteins servicing these body processes will also differ. It is therefore probable that positive selection would favour toxins with high affinity and efficacy for their preferred prey’s molecular physiology. When looking at the proteome (protein composition) of snake venoms, there is an observable trend emerging to suggest that venom composition is indeed evolving to become more specific to the physiology of the prey taxa which comprise respective snake species’ diet. Snake venoms often vary intraspecifically as an apparent reflection of differences in diet and feeding ecology. For example, it is not uncommon for the diet - and venom - of a given snake species to vary between populations, according to the prey items that are locally available. This is often observed in species whose distribution covers a Above: the King Cobra (Ophiophagus hannah) is the longest venomous snake in the world, and has been documented to reach a total length in excess of 5.5m. Despite a fearsome reputation, human envenomations are extraordinarily uncommon, as this species is generally placid and avoids confrontation. Image by Susan Schmitz.