The fasta sequence is:
>gi|229893|pdb|1ECD|  Hemoglobin (Erythrocruorin, Deoxy)
LSADQISTVQASFDKVKGDPVGILYAVFKADPSIMAKFTQFAGKDLESIKGTAPFETHANRIVGFFSKII
GELPNIEADVNTFVASHKPRGVTHDQLNNFRAGFVSYMKAHTDFAGAEAAWGATLDTFFGMIFSKM

The number of occurrences of letter A is: 17