Starting with conformations of calcium-binding sites in parvalbumin and integrin (representative structures of EF-hand and calcium blade zones, respectively) we introduce four new different local Ca2+-recognition units in proteins: a one-residue unit type I (ORI); a three-residue unit type I (TRI); a one residue unit type II (ORII) and a three-residue unit type II (TRII). Based on the amount and nature of variable atoms, the type I and II units theoretically can have four and twelve variants, respectively. Analysis of known "Ca2+-bound functional niches" in proteins revealed presence of almost all possible variants of Ca2+-recognition units in actual structures. Parvalbumin, integrin alpha-IIb and sixteen other proteins with different Ca2+-bound functional niches contain various consecutively joined combinations of OR(I/II) and TR(I/II) units. Such a OR(I/II)+TR(I/II) joint unit forms a tripeptide, which uses three main chain atoms for metal binding: nitrogen(n) (Donor), oxygen, (Acceptor) and nitrogen(n+2) (Donor). Thus, taken together, the described ORI, TRI, ORII and TRII units can serve as elementary blocks to construct more complex calcium recognizing substructures in a variety of calcium binding sites of unrelated proteins. (C) 2017 Elsevier Inc. All rights reserved.