A-gamma globin ($\mathrm{A}\gamma$)

A-gamma globin is the human HBG1 gene product, a form of gamma-globin with alanine at residue 136. Its name contrasts with G-gamma globin, whose corresponding residue is glycine. Both belong to the gamma-chain class used in fetal hemoglobin.^[ref] A-gamma is not hemoglobin A: the latter is the adult ${\alpha }_2{\beta }_2$ assembly containing beta-globin.

A single changed amino acid residue can supply a useful analytical signature without changing which general globin class is being measured. Replacing glycine with alanine adds the elemental composition $\mathrm{C}{\mathrm{H}}_2$, since their side chains are hydrogen and methyl. In a deliberately nominal-mass calculation this adds about 14 unified atomic mass units. If an otherwise identical diagnostic peptide produces ions with charge number $z$, the expected separation in mass-to-charge ratio is approximatelyFor $z=2$, that is about seven mass-to-charge units. This is a proposed analytical discrimination, not the experimental method used in the historical paper. Actual mass spectrometry must use the appropriate exact masses and account for other sequence changes, isotope peaks and modifications.

A sequence-selective amount measurement also requires a response model. In an original two-channel example, suppose one pmol of each form produces detector responses $k_A=2$ and $k_G=1$ in arbitrary signal units. Signals $I_A=40$ and $I_G=60$ correspond to amounts 20 and 60 pmol, giving an A-gamma fractionUsing the raw signal fraction would instead give 0.40. A distinct molecular signature identifies a channel; it does not guarantee equal analytical response between channels.

The 1968 primary study reported the additional alanine-containing form at position 136 in human fetal hemoglobin components.^[ref] That observation motivates distinguishing sequence forms experimentally, while HBG1 supplies their present gene-level identification. Neither an A-gamma amount nor its ratio to G-gamma alone establishes the oxygen-binding properties of every assembled tetramer.