Heme proteins play various important roles in biological systems such as oxygen storage and transport, electron transfer, catalysis and signaling. Although most heme proteins exist as a monomer, homomeric complexes are also observed for some heme proteins in vitro and/or in vivo. This review summarized the progress of dimerization, oligomerization and polymerization of heme proteins, including myoglobin, cytochrome c, cytoglobin, cytochrome b₅, cytochrome b₅₆₂, nitrite reductase, heme-based sensor and heme transport proteins, etc., and focused on the resultant structures and functions, as well as the approaches for rational design of polymers and their applications. These progresses, on one hand, enhanced our knowledge of the structure-function relationship of heme proteins in biological systems, and on the other hand, endowed us an ability of regulating and utilizing heme proteins by functional protein design through polymerization.

Contents
1 Introduction
2 Dimerization of heme proteins
2.1 Structure and function of dimeric myoglobin
2.2 Structure and function of dimeric cytochrome c
2.3 Structure and function of dimeric cytoglobin
2.4 Structure and function of dimeric nitrite reductase
2.5 Structure and function of dimeric heme-based sensor protein
2.6 Structure and function of dimeric heme transport protein
3 Oligomerization of heme proteins
4 Polymerization of heme proteins
4.1 Rational design of heme protein polymer
4.2 Applications of heme protein polymer
5 Conclusion and outlook