Protein catabolism is the same as protein digestion, it is also called proteolysis, where large polypeptide proteins are broken down into single amino acids.
These amino acids are used for absorption.
Protein Catabolism involves the conversion of larger proteins into small lengths of proteins then it is followed by amino acid degradation where the small chain of amino acids is degraded by cleaving peptide bonds between them and forming single amino acid molecules.
Proteins are the polymers of amino acids.
So, the first product of protein catabolism is ammonia, followed by urea and carbon dioxide, amino acids are the end products.