A simple method is to mix the protein with cyanogen bromide-activated Sepharose, all in 0.1 M sodium bicarbonate. CNBr-activated Sepharose reacts with lysine residues on the surface of the protein, forming covalent bonds. This can lead to some loss of activity, however.

You can also bind the protein to an ion-exchange resin, such as QAE-Sepharose. This is an ionic interaction, which can be reversed by a high-salt wash to recover the protein. Some loss of activity of the bound enzyme may occur.

If the protein has a His tag or other affinity tag, you can bind the protein to the corresponding affinity resin. This procedure is less likely to cause a reduction in catalytic activity than either of the above methods.

 Ice?

You did say " easiest"

Thanks Charles for reply,

The protocols for enzymes immobilization are diverse related to the enzyme, the purpose and other factors. I need practical informations for "easy" protocol that can be realized at laboratory level as preleminary tests before passing to others.  

I have not tried to bind proteins to silica gels for HPLC yet.   However, avidin-bounded silica gel column is very robust for me (please see file; Ovomucoid gel and Netherlands biotin).

Further, it seems that spacer may be important. 

Jun Haginaka, Tokiko Murashima, Chikako Seyama (1994) Retention and enantioselectivity of 2-arylpropionic acid derivatives on an avidin-bonded silica column Influence of base materials, spacer type and protein modification. Journal of Chromatography A, 677, 229-237.

Simply adsorb enzyme (in solution form) on to silica or celite by incubating the suspension for about two h at room temperature/ 4 c. wash the silica/ celite with 50 mM Phosphate buffer (pH 7.2) and exposed the silica-enzyme/ celite-enzyme to 1% formaldehyde or glutaraldehyde. This approach allows binding/ adsorption and crosslinking of enzyme molecules creating a stable biocatalyst.