Sandbox 47

'''Bovine Pancreatic Trypsin Inhibitor (BPTI) Mutant with Altered Binding Loop Sequence'''

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BPTI is a single chain polypeptide with 58 amino acids which inhibits the serine protease Trypsin. It's secondary structure includes one <scene name='Sandbox_47/Sheet_ribbon/1'>beta sheet</scene> and <scene name='Sandbox_47/Helices/1'>two alpha helices</scene>. The beta sheet consists of <scene name='Sandbox_47/Sheet_side_chains/1'>two antiparallel strands</scene> (Ile18 to Asn 24 and Leu 29 to Tyr 35) with a <scene name='Sandbox_47/Turn/1'>beta hairpin</scene> consisting of four residues (Ala 25, Lys 26, Ala 27, Gly 28). Alpha helix

<scene name='Sandbox_47/H1/1'>H1</scene> is 4 residues (Asp 3 to Cys 6), and <scene name='Sandbox_47/H2/1'>H2</scene> is 8 residues (Ala 48 to Cys 55). There are three intrachain disulfide bonds in this polypeptide, one between <scene name='Sandbox_47/Ds1/1'>Cys 5 and Cys 55</scene>, one between <scene name='Sandbox_47/Ds2/1'>Cys 14 and Cys 38</scene>, and one between <scene name='Sandbox_47/Ds3/1'>Cys 30 and Cys 51</scene>. These are important for BPTI's stability and its ability to withstand the harsh conditions of the digestive system. This protein has a <scene name='Sandbox_47/Hydrophobic_core/1'>hydrophobic core</scene> (gray) with polar and charged residues (purple) extending mostly on the <scene name='Sandbox_47/Hydrophobic_dist/3'>surface</scene>. The N-terminus and C-terminus are in contact through a salt bridge.

BPTI interacts with 98 <scene name='Sandbox_47/Water/1'>water molecules</scene> as well as 4 <scene name='Sandbox_47/Ligands/1'>sulfate anion ligands</scene>. Ligand <scene name='Sandbox_47/Ligand_61/1'>61</scene> binds the protein through hydrophobic interactions between Glu 7, Lys 7, and Phe 41, as well as through 3 hydrogen bonds to Arg 42. Three water molecules are also interact with this ligand. Ligand <scene name='Sandbox_47/Ligand_62/1'>62 </scene> has hydrophobic interaction with Ala 40, two hydrogen bonds with Arg 20, and one hydrogen bond with Tyr 35. Ligand

<scene name='Sandbox_47/63/1'>63</scene> has hydrophobic interaction with Pro 2 and one hydrogen bond with Arg 1. Ligand <scene name='Sandbox_47/Ligand_64/1'>64</scene> has hydrophobic interaction with Cys 14.

BPTI has both a <scene name='Sandbox_47/Primary_loop/1'>primary</scene> and a <scene name='Sandbox_47/Secondary_loop/1'>secondary loop</scene> involved in the binding and inhibition of Trypsin. This particular mutant of BPTI contains several residue substitutions within these loop, including a substitution at <scene name='Sandbox_47/Lys_mutation/1'>residue 15</scene>, which is is normally Lys but has been replaced by Arg. This basic Lys normally binds to the active site of Trypsin and is therefore important to the inhibition of the protease. This mutant also has <scene name='Sandbox_47/Thr11_sub/1'>Thr11Ala</scene>, <scene name='Sandbox_47/Sub/1'>Pro13Ala</scene>, and <scene name='Sandbox_47/Met_52_sub/1'>Met52Leu</scene> substitutions. Altogether these substitutions reduce BPTI's association constant by approximately 30 times (Czapinska et al,1999).

Resources:

Czapinska, H., Otlewski, J., Krzywda, S., Sheldrick, G.M, Jaskilski, M. (1999) "High-resolution structure of bovine trypsin inhibitor with altered binding loop sequence." J. Mol. Biol. 295:1237