def convert_to_one_letter_code_sing(seq):
    """
    Converts a single amino acid one letter code to the 3 letter code.

    Arguments:
        -seq (str) - one letter AA code.

    """
    conversion = {
        "GLY": "G", "PRO": "P", "VAL": "V", "ALA": "A", "LEU": "L",
        "ILE": "I", "MET": "M", "CYS": "C", "PHE": "F", "TYR": "Y",
        "TRP": "W", "HIS": "H", "ARG": "R", "LYS": "K", "GLN": "Q",
        "THR": "T", "ASP": "D", "ASN": "N", "SER": "S", "GLU": "E"
    }
    n_seq = conversion[seq]
    return n_seq