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In page Hemoglobin:

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In adult humans, the most common hemoglobin type is a tetramer (which contains four subunit proteins) called hemoglobin A, consisting of two α and two β subunits non-covalently bound, each made of 141 and 146 amino acid residues, respectively. This is denoted as α2β2. The subunits are structurally similar and about the same size. Each subunit has a molecular weight of about 16,000 daltons,[9] for a total molecular weight of the tetramer of about 64,000 daltons (64,458 g/mol).[10] Thus, 1 g/dL=0.1551 mmol/L. Hemoglobin A is the most intensively studied of the hemoglobin molecules.[citation needed]