Traptavidin 2Y3E

Streptavidin is a tetrameric protein purified from the bacterium Streptomyces avidinii. It has an extraordinarily high affinity for a molecule called biotin (also known as vitamin B7);

Streptavidin is used extensively in molecular biology and bionanotechnology due to the streptavidin-biotin complex's resistance to organic solvents, denaturants (e.g. guanidinium chloride), detergents (e.g. SDS, Triton), proteolytic enzymes, and extremes of temperature and pH. Essentially you can use it to attach different molecules to each other in a specific way like attaching pieces of velcro to things.

Traptavidin (Shown above) is a human engineered mutant of Streptavidin with an even higher binding affinity (10x better) showing that we're now able to improve and modify natures machines for our purposes.

Cystatin C

Cystatin C or cystatin 3 (formerly gamma trace, post-gamma-globulin or neuroendocrine basic polypeptide),[1] a protein encoded by the CST3 gene, is mainly used as a biomarker of kidney function. Recently, it has been studied for its role in predicting new-onset or deteriorating cardiovascular disease. It also seems to play a role in brain disorders involving amyloid (a specific type of protein deposition), such as Alzheimer's disease. [wikipedia]

The image above shows part of the cystatin C crystal obtained for crystallography and actually shows 8 cystatin C molecules in a complex arrangement of symmetries.

In humans, all cells with a nucleus (cell core containing the DNA) produce cystatin C as a chain of 120 amino acids. It is found in virtually all tissues and bodily fluids. It is a potent inhibitor of lysosomal proteinases (enzymes from a special subunit of the cell that break down proteins) and probably one of the most important extracellular inhibitors of cysteine proteases (it prevents the breakdown of proteins outside the cell by a specific type of protein degrading enzymes). Cystatin C belongs to the type 2 cystatin gene family. [wikipedia]

Molybdenum cofactor biosynthesis protein MoaC.

The molybdenum cofactor (Moco) is an essential component of a large family of enzymes involved in important transformations in carbon, nitrogen and sulfur metabolism. The Moco biosynthetic pathway is evolutionarily conserved and found in archaea, eubacteria and eukaryotes. In humans, genetic deficiencies of enzymes involved in thispathway trigger an autosomal recessive and usually deadly disease with severe neurological symptoms. The MoaC protein,together with the MoaA protein, is involved in the first step of Moco biosynthesis. (from paper abstract)

Wuebbens et al. Structure Fold.Des. 8: 709-717