Understanding the chemical and physical interactions at the interface of protein surfaces and inorganic crystals has important implications in the advancement of immobilized enzyme catalysis. Recently, enzyme inorganic hybrid complexes have been demonstrated as effective materials for enzyme immobilization. The precipitation of phosphate nanocrystals in the presence of enzymes-creates enzyme-Cu-3(PO4)(2)center dot 3H(2)O particles with high surface-to-volume ratios, enhanced activity, and increased stability. Here, we begin to develop a mechanistic understanding of enzyme loading in such complexes. Using a series of enzymes including horseradish peroxidase (HRP), a thermostable alcohol dehydrogenase (AdhD), diaphorase, catalase, glucose oxidase (GOx), and the protein bovine serum albumin (BSA), we identified a correlation between particle synthesis temperature, overall- enzyme charge, and enzyme loading. The model enzyme HRP has a high predicted pI of similar to 7.5 and maintains an overall positive charge under the synthesis conditions, phosphate buffer pH 7.4. HRP loading in HRP-Cu-3(PO4)(2) complexes was enhanced by 4.2-fold when synthesis was carried out at 37 degrees C in comparison with synthesis at 4 degrees C. HRP loading was further enhanced with synthesis at pH 8.0, correlating with a decrease in overall enzyme charge. Proteins with lower predicted pI values and negative overall charge (AdhD), pI of 5.6; diaphorase, pI of 6.8; GO(x), pI of 5.2; catalase, pI of 6.9; and, BSA, pI of 5.7) exhibited higher enzyme loadings with 4 degrees C synthesis, 2.7-, 2.6-, 2.S-, 1.8-, and 1.7-fold'protein loading enhancements, respectively. Using HRP as a model system, we also demonstrate that activity increased concomitantly with enzyme loading, and that particle nanostructure was minimally affected by synthesis temperature. Combined, the results presented here demonstrate the control of enzyme loading in enzyme inorganic particles opening up new possibilities in enzyme and multienzyme catalysis.