Solubility effects are important in determining the tertiary structure of globular proteins in aqueous solution. The nonpolar, hydrophobic groups push away from the water toward the interior of the protein while the polar groups pull outwards towards the surface where they are solvated by the water. The hydrophobic interior space formed is often essential for catalytic or binding function. For example, in myoglobin the only polar residues on the inside are two histodines. Like a given amount of charge on a capacitor without dielectric has a higher voltage, being surrounded and protected by nonpolar moeities activates the polarity of the histidine residues, which is essential for oxygen binding. This structural motif, is known as the 'globin fold', is also found in hemoglobin α and β chains and in which a nested heme group transports or stores oxygen reversibly.