This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
[{'content': 'True regarding ubiquitin is: \nSelect and explain. \nA. Product of purine metabolism, B. Protein destructions, C. Present in prokaryotes, D. Protein synthesis', 'role': 'user'}, {'content': 'Answer: B \nExplanation: There are two major pathways of protein degradation in eukaryotes. One involves lysosomal proteases and does not require ATP. The other way involves ubiquitin and is ATP-dependent. Ubiquitin is a small, highly-conserved regulatory protein that is ubiquitously expressed in eukaryotes. Ubiquitination (or ubiquitylation) refers to the post-translational modification of a protein by the covalent attachment ( an isopeptide bond) of one or more ubiquitin monomers. The most prominent function of ubiquitin is labeling proteins for proteasomal degradation.', 'role': 'assistant'}]