How is hemoglobin’s quaternary structure organized?

The quaternary structure of human hemoglobin is organized as a tetramer, meaning it consists of four individual polypeptide subunits. Specifically, this structure is composed of two $\alpha$-chains and two $\beta$-chains.12

These four subunits are held together by noncovalent interactions, specifically hydrogen bonds and hydrophobic interactions. Because these subunits are so closely associated, the entire assembly is referred to as a single molecule, even though no covalent bonds link the individual peptide chains to one another. This arrangement allows the hemoglobin molecule to incorporate four heme groups, each containing an iron atom capable of binding a molecule of oxygen.1234