Bovine Pancreatic Trypsin Inhibitor
Beta - Loop - Beta
or Beta-Hairpin

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I. Introduction

Here is the structure of amino acids 17-35 of bovine pancratic trypsin inhibitor. This proteins binds trypsin, an endoprotease that cleaves proteins after Lys and Arg residues. In this segment, two antiparallel beta strands are connected by a loop. 

For more information see Biochemistry Online: Chapter 2C - Understanding Protein Conformation

II. General Structure

Backbone and sidechains
Cartoon showing hydrogen bonds
Cartoon and Wireframe
Dot surface and Wireframe