Sickle-cell anemia is an inherited blood disorder. It is caused by a change in the amino acid sequence of hemoglobin. This mutation affects hemoglobin protein structure at what level?

Tertiary protein structure

Explanation:

The tertiary structure of the haemoglobin consists of four heme and four polypeptide subunits; two alpha chains and two beta chains, which gives it its function (ability to carry oxygen).

In the case of sickle - cell anemia which is caused by the replacement of an amino acid, glutamate with valine at position 6 of the beta chain of the haemoglobin structure, this disrupts the tertiary structure and hence its oxygen carrying ability.