Which of the following is NOT primarily a protein-based biological molecule?

Steroid hormones such as cortisol, which are synthesized from cholesterol and act through intracellular receptors rather than being composed of amino acids

Hemoglobin, a tetrameric protein that transports oxygen in red blood cells

Many enzymes, which are protein catalysts that increase reaction rates in metabolism

Insulin, a peptide hormone that is synthesized as a protein and regulates blood glucose