Interaction among subunits: salt bridges in deoxyhaemoglobin

Both the structure of each subunit in deoxyhaemoglobin (tertiary structure) and the interaction among the four subunits (quaternary structure) are stabilised by 8 salt bridges (electrostatic interactions), formed between sidechains from several amino acids and the terminal carboxy and amino groups of the alpha chains:

Salt bridges between the two alpha chains

The terminal amino group in one alpha chain, Val₁, forms a salt bridge with the terminal carboxy group in the other alpha chain, Arg₁₄₁ .

In addition, the guanidinium group (sidechain) of the same Arg₁₄₁ forms a salt bridge with the carboxy group in the sidechain of Asp₁₂₆ from the other alpha chain.

Each type of amino acid in a colour (Val, Asp, Arg)

To see the interactions in detail, let's the model
(α₁Val₁ :: α₂Arg₁₄₁ :: α₁Asp₁₂₆)
highlight the atoms in the salt bridge (always N⁺ group with O^– group)

Salt bridges in beta chains

The terminal carboxy group in one beta chain, His₁₄₆, forms a salt bridge with the amino group in the sidechain of Lys₄₀ from an alpha chain (β₁His₁₄₆ :: α₂Lys₄₀ y β₂His₁₄₆ :: α₁Lys₄₀).

In addition, the same His₁₄₆ forms another salt bridge, by its sidechain, with the carboxy group in the sidechain of Asp₉₄ from its same beta chain. Differently to all the former ones, this is an intrachain bridge.

Each type of amino acid in a colour (His, Asp, Lys)

To see the interactions in detail, let's the model
(α₂Lys₄₀ :: β₁His₁₄₆ :: β₁Asp₉₄)
highlight the atoms in the salt bridge (always N⁺ group with O^– group)

Detail of this salt bridge:

To end, the whole deoxyhaemoglobin molecule with its 8 salt bridges.