= A-gamma globin
{c}
{title2=$\mathrm A\gamma$}
\b[<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>.<Human globin gene records in NCBI Gene>[HBG1, Gene 3047, RefSeq Summary, alanine/glycine distinction at residue 136]{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 $\mathrm{CH_2}$, 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 approximately
$$
\Delta(m/z)=\frac{14}{z}.
$$
For $z=2$, 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 fraction
$$
p_A=\frac{I_A/k_A}{I_A/k_A+I_G/k_G}=0.25.
$$
Using 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.<Evidence for multiple structural genes for the gamma chain of human fetal hemoglobin by W. A. Schroeder and others>[p.537, opening paragraph]{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.
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