Hemoglobin subunit composition

The subunit composition of hemoglobin tetramers undergoes complex changes during development. The... 

The form Hb A2, subunit composition a2 d2, is a minor component of normal adult hemoglobin. The a-chains of Hb A and Hb A2 are identical. Thed-chains are b-like chains produced at a slow rate from separate genes. 

In fetal hemoglobin, the subunit composition is with replacement of the Chains by the ychains. The sickle-cell mutation affects the p-chain, so the fetus homozygous for Hb S has normal fetal hemoglobin. 

The terms polypeptide and protein are used interchangeably in discussing single polypeptide chains. The term protein broadly defines molecules composed of one or more polypeptide chains. Proteins having only one polypeptide chain are monomeric proteins. Proteins composed of more than one polypeptide chain are multimeric proteins. Multimeric proteins may contain only one kind of polypeptide, in which case they are homomultimeric, or they may be composed of several different kinds of polypeptide chains, in which instance they are heteromultimeric. Greek letters and subscripts are used to denote the polypeptide composition of multimeric proteins. Thus, an ag type protein is a dimer of identical polypeptide subunits, or a homodimer. Hemoglobin (Table 5.1) consists of four polypeptides of two different kinds it is an hetero-multimer. 

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