As the name suggests, human cationic trypsin complexed with bovine pancreas trypsin inhibitor (PDB ID: 2RA3, BPTI) is a composite protein formed by two subunits: cationic trypsin from Homo sapiens and pancreatic trypsin inhibitor from Bos taurus (1). The uncomplexed form of human cationic trypsin has been implicated in the pathogenesis of human pancreatitis (2). Many patients with pancreatitis are documented to have one or more mutations at the PRSS1 gene encoding human cationic trypsinogen (2). Additionally, the degradation of human pancreatic trypsin inhibitor SPINK1 by early activation of human mesotrypsin may play another role in human pancreatitis (3). Studies exploring the human trypsins, especially in regards to their interactions with trypsin inhibitors, may offer insight into the pathology of diseases of the pancreas. 

          Human cationic trypsin complexed with BPTI was specifically formulated in order to study the mechanics of human mesotrypsin with polypeptidase trypsin inhibitors. There are three human trypsins isoforms, cationic trypsin, anionic trypsin and mesotrypsin, which are all secreted in the pancreas as digestive zymogens and then activated in the duodenum (3). Typically, trypsin inhibitors are bound to the human trypsins tightly and then cleaved slowly. Human cationic trypsin and anionic trypsin interact with trypsin inhibitors in such a way, but mesotrypsin binds more loosely to polypeptidase trypsin inhibitors to then cleave them quickly. The decreased attraction of mesotrypsin to polypeptidase trypsin inhibitor is attributed to Arg-193. Both cationic trypsin and anionic trypsin (as well as other proteases) have glycine residues at this position (3). In order to better understand the interactions between mesotrypsin and other polypeptidase trypsin inhibitors, mesotrypsin was complexed with BPTI. BPTI is known to form high affinity protein-protein complexes (3). Human cationic trypsin is highly homologous to mesotrypsin, and when complexed with BPTI, it serves as a perfect control group for studies about human mesotrypsin's abnormal binding and cleavage behavior with trypsin inhibitors (2). 

          The BPTI-human cationic trypsin complex exists in crystalline form and was obtained via vapor diffusion hanging drop method. Structural data was collected by X-ray diffraction (1). The complex has a molecular weight of 61263.56 Da and isoelectric points of 7.64 and 9.24 for the trypsin chain and bovine chain, respectively (4). Structurally,the protein complex contains two subunits each of which is composed of two polypeptide chains (1). The secondary structure of human cationic trypsin complexed with BPTI consists of 30% helices (alpha and 3/10) and 62% beta-sheets (1). The human trypsin subunit has two mutations that change the wild-type Ser-195 to alanine (S195A) and an Arg-117 to histidine (R117H) (1). These residues were mutated to better crystallize the BPTI complex (3). The ligands of human cationic trypsin complexed with BPTI are sulfate and calcium ions. Unfortunately, the primary research on this protein does not offer any insight into the role these ligands play in the function or behavior of the protein. 

          Several residues in the BPTI-human cationic trypsin complex are worth mentioning. Arg-17 of BPTI and His-40 of human cationic trypsin bind to one another via hydrogen bonding. This binding site represents stabilizing interactions of the complex. These residues, and subsequent interactions, are not present in mesotrypsin complexed with BPTI, and they may contribute to the inhibitor resistance of mesotrypsin (3). Gly-193 is the residue of human cationic trypsin attributed to its sensitivity to polypeptidase trypsin inhibitors (3). Mesotrypsin has an arginine residue at position 193. Mutagenesis of the active site at position 193 in which arginine was converted to glycine induced an inhibitor sensitivity not seen in the wild-type mesotrypsin (5). By understanding the role that residues play at position 193 in human trypsins, researchers may be able to better study the mechanics of trypsin inhibitor cleavage. Since early cleavage of trypsin inhibitors in the human pancreas have been implicated in diseases like pancreatitis, further study of key residues like Gly-193 in mesotrypsin and Arg-193 in cationic trypsin could offer new insights into complicated diseases. 

          Comparisons can be made between human cationic trypsin complexed with BPTI and other known proteins through comprehensive databases and search tools. PSI-BLAST is one such tool that finds proteins whose primary structure resemble an initial protein query. Each protein listed in the PSI-BLAST results is given an "E-value" which is deemed significant if it is less than 0.5 (6). Dali Server is another useful search engine which finds proteins that have a similar tertiary structures by calculating the difference in intramolecular distances. Results from the Dali Server are given "Z-scores". A Z-score above 2 indicates significant similarity between proteins (7). 

          Using both PSI-BLAST and Dali Server hundreds of proteins similar to human cationic trypsin complexed with BPTI were found. The Bowman-Birk-type inhibitor in ternary complex with porcine trypsin (PDB ID: 3MYW) was one protein I decided to research further in order to compare structure and function with the BPTI-human cationic trypsin complex. The Bowman-Birk-type inhibitor in ternary complex with porcine trypsin had an E-value of 9x10^-119 and a Z-score of 43.1 (6,7). Clearly, the primary and tertiary structure of these two proteins are very similar. In regards to secondary structure the Bowman-Birk-type inhibitor in ternary complex with porcine trypsin has only 10% helices and 60% beta sheets. The quaternary structure of this comparison protein, like human cationic trypsin complexed with BPTI, has two subunits (1). However, while human cationic trypsin complexed with BPTI has two subunits with 2 chains each, the Bowman-Birk-type inhibitor in ternary complex with porcine trypsin has a trypsin subunit with 2 chains and a trypsin inhibitor subunit with one chain (1). This unique quaternary structure allows for an asymmetric protein complex (8). 

          Functionally, the Bowman-Birk-type inhibitor has been shown to bind tightly to the porcine trypsin portion. The porcine trypsin has been observed to slowly cleave the inhibitor much the way human cationic trypsin interacts with BPTI in complex (8). The trypsin-trypsin inhibitor interaction of the Bowman-Birk-type inhibitor and porcine trypsin is very similar to that of human cationic trypsin and BPTI even though the proteins were extracted from very different organisms. The Bowman-Birk-type inhibitor comes from Vigna radiata (mung bean) and the porcine trypsin with which it is complexed is crystallized from Sus scrofa (wild boar). Despite the differences in origin organisms, the human cationic trypsin complexed with BPTI and the Bowman-Birk-type inhibitor complexed with porcine trypsin share the characteristic binding and cleavage behavior. 

          Comparing target proteins to similar structures is an important way to learn more about a given protein. For human cationic trypsin complexed with BPTI it is also useful to look at how the other human trypsins, mesotrypsin and anionic trypsin, react in complex with BPTI. Taking into account the physical and chemical properties of human cationic trypsin complexed with BPTI as well, it is possible to begin to see the functional importance of this protein in human life. By understanding human cationic trypsin complexed with BPTI as its own protein, and in comparison to others researchers may be able to learn more about how mutations of this protein lead to diseases, such as pancreatitis.