Editing Calcium (section)

===Binding===
Calcium ions may be complexed by proteins through binding the [[carboxyl group]]s of [[glutamic acid]] or [[aspartic acid]] residues; through interacting with [[phosphorylation|phosphorylated]] [[serine]], [[tyrosine]], or [[threonine]] residues; or by being [[chelation|chelated]] by γ-carboxylated amino acid residues. [[Trypsin]], a digestive enzyme, uses the first method; [[osteocalcin]], a bone matrix protein, uses the third.{{sfn|Hluchan|Pomerantz|2005|pp = 489–94}}

Some other bone matrix proteins such as [[osteopontin]] and [[bone sialoprotein]] use both the first and the second. Direct activation of enzymes by binding calcium is common; some other enzymes are activated by noncovalent association with direct calcium-binding enzymes. Calcium also binds to the [[phospholipid]] layer of the [[cell membrane]], anchoring proteins associated with the cell surface.{{sfn|Hluchan|Pomerantz|2005|pp = 489–94}}