The key enzymes involved in the degradation and turnover of proteins within cells are proteases. Proteases are a diverse group of enzymes that catalyze the hydrolysis of peptide bonds, leading to the breakdown of proteins into smaller peptides or individual amino acids. There are several types of proteases, and they differ in terms of their mechanism and substrate specificity. The major proteases involved in protein degradation within cells are:1. Lysosomal proteases: These enzymes are found within the lysosomes, which are membrane-bound organelles containing hydrolytic enzymes. Lysosomal proteases are responsible for the degradation of extracellular and intracellular proteins that are delivered to the lysosome through endocytosis or autophagy. The main types of lysosomal proteases are: a. Cathepsins: These are cysteine proteases that cleave peptide bonds using a catalytic cysteine residue. Cathepsins have diverse substrate specificities and are involved in the degradation of various proteins. b. Aspartyl proteases: These proteases use two aspartic acid residues in their active site to catalyze peptide bond hydrolysis. Examples include cathepsin D and cathepsin E, which are involved in the degradation of intracellular and extracellular proteins.2. Proteasomes: The proteasome is a large, multi-subunit protein complex found in the cytoplasm and nucleus of eukaryotic cells. It is responsible for the degradation of intracellular proteins that are marked for destruction by the addition of ubiquitin, a small protein modifier. The 26S proteasome is the major proteasome complex involved in protein degradation and consists of a 20S core particle and two 19S regulatory particles. The 20S core particle contains multiple protease subunits, including: a. Caspases: These are cysteine proteases that cleave peptide bonds using a catalytic cysteine residue. Caspases are involved in the degradation of proteins during apoptosis, a form of programmed cell death. b. Threonine proteases: These proteases use a catalytic threonine residue to hydrolyze peptide bonds. The -subunits of the 20S proteasome are threonine proteases, and they have distinct substrate specificities based on the amino acid sequence surrounding the target peptide bond.3. Calpains: Calpains are calcium-dependent cysteine proteases found in the cytoplasm of eukaryotic cells. They are involved in the degradation of cytoskeletal proteins and other substrates in response to changes in intracellular calcium levels. Calpains have diverse substrate specificities and are regulated by calcium binding and interactions with other proteins.4. Serine proteases: These proteases use a catalytic serine residue to hydrolyze peptide bonds. Some serine proteases, such as the protease HtrA, are involved in the degradation of misfolded proteins in the endoplasmic reticulum and mitochondria.In summary, the key enzymes involved in protein degradation within cells are proteases, which differ in their catalytic mechanisms and substrate specificities. Lysosomal proteases, proteasomes, calpains, and serine proteases are the major types of proteases involved in the degradation and turnover of proteins in eukaryotic cells.