Identification of a α‐helical molten globule intermediate and structural characterization of β‐cardiotoxin, an all β‐sheet protein isolated from the venom of Ophiophagus hannah (king cobra)

Abstract

β-Cardiotoxin is a novel member of the snake venom three-finger toxin (3FTX) family. This is the first exogenous protein to antagonize β-adrenergic receptors and thereby causing reduction in heart rates (bradycardia) when administered into animals, unlike the conventional cardiotoxins as reported earlier. 3FTXs are stable all β-sheet peptides with 60-80 amino acid residues. Here, we describe the three-dimensional crystal structure of β-cardiotoxin together with the identification of a molten globule intermediate in the unfolding pathway of this protein. In spite of the overall structural similarity of this protein with conventional cardiotoxins, there are notable differences observed at the loop region and in the charge distribution on the surface, which are known to be critical for cytolytic activity of cardiotoxins. The molten globule intermediate state present in the thermal unfolding pathway of β-cardiotoxin was however not observed during the chemical denaturation of the protein. Interestingly, circular dichroism (CD) and NMR studies revealed the presence of α-helical secondary structure in the molten globule intermediate. These results point to substantial conformational plasticity of β-cardiotoxin, which might aid the protein in responding to the sometimes conflicting demands of structure, stability, and function during its biological lifetime. PDB Code(s): 3PLC;

Suppression of cardiomyocyte functions by β-CTX isolated from the Thai king cobra (Ophiophagus hannah) venom via an alternative method

“…Among these cells, the major pathway causing cellular death is perforation at cellular and sarcolemma membranes, thereby interfering with Ca 2+ homeostasis in the cytosol [14,15]. However, a previous study suggests that β-CTX does not exert an ionophore activity as the compound contained fewer charges as compared to the other CTXs [2]. The absence of myoblast cytotoxicity of β-CTX may be explained by the lack of the appropriate receptor for β-CTX or that the effective concentration of β-CTX in the myoblast cells was not reached in our study.…”

“…The protein contains 63 amino acids with an estimated molecular weight of 7 kDa [ 1 ]. Like all cardiotoxins (CTXs), the protein is formed by a two beta-pleated structure, containing five small beta strands [ 2 ]. The toxin was previously found in the venom of the king cobra from various regions of Asia, including Malaysia, Indonesia, China, and Thailand [ 3 - 6 ].…”

“…Despite overall structural similarity to conventional cardiotoxins, there are notable differences in both the loop region and overall surface charge distribution of the β-CTX protein that may underlie the relatively low cytosolic activity of this cardiotoxin compared to those more conventional cardiotoxins. Furthermore, the loss of charges distributed in the protein structure completely disregarded its cytolytic property [ 2 ]. β-CTX also shows negative chronotropy, dose-dependently, in both isolated heart and in vivo studies.…”

Cardiovascular Effects of Snake Toxins: Cardiotoxicity and Cardioprotection

“…It is capable of blocking β1 and β2 ARs [ 72 ]. This leads to a decrease in the heart rate in vivo and in vitro without noticeable cytotoxicity, which may be associated with the inability of β-cardiotoxin to directly interact with the membrane, due to some of its structural features [ 73 ]. Later, a cytotoxic effect on cultured smooth muscle cells and no effect on skeletal cells and cardiac myocytes were shown in [ 74 ].…”

Scope of practice and educational needs of infection prevention and control professionals in Australian residential aged care facilities

“…Homodimers, haditoxin (inhibits muscle (α1)2β1 and neuronal α7, α3β2 and α4β2 nAChRs) [148] and fulditoxin (inhibits muscle (α1)2β1ε and neuronal α7, α4β2 and α3β2 nAChRs) [124] (see Table 2), reveal a breadth of nAChR selectivity which is unusual for their monomeric subunit SC-α-3FNTxs that bind only to muscle nAChRs [149]. Similarly, dimerization not only retained α-CbTx's ability to inhibit muscle (α1)2β1 and α7 nAChRs, but extended its capacity to target α3β2 nAChRs [143,145].…”