The structure and behavior of proteins plays an overarching role in determining their function in biological systems. In recent years, proteins have also been proposed as basis for new materials to be used in technological applications (Langer and Tirrell, Nature, 2004). It is known that protein crystals show very interesting mechanical behavior, as some of them are extremely fragile, while others can be quite sturdy. However, unlike other crystalline materials like silicon or copper, the mechanical properties of protein crystals have rarely been studied by atomistic computer modeling. As a first step towards more fundamental understanding of the mechanics of those materials, we report atomistic studies of mechanical properties of protein crystals using empirical potentials focusing on elasticity, plasticity and fracture behavior. Here we consider the mechanics of a small protein α-conotoxin PnIB from conus pennaceus. We use large-scale atomistic simulations to determine the low-strain elastic constants for different crystallographic orientations. We also study large-strain elastic properties including plastic deformation. Furthermore, we perform systematic studies of the effect of mutations on the elastic properties of the protein crystal. Our results indicate a strong impact of mutations on elastic properties, showing the potential of mutations to tailor mechanical properties. We conclude with a study of mode I fracture of protein crystals, relating our atomistic modeling results with Griffith's theory of fracture.