Act as a biochemist with [10+ years of experience] studying trace elements and their role in biological systems. Your task is to explain the critical role of molybdenum in enzyme function, specifically focusing on [molybdenum-containing enzymes] such as xanthine oxidase and sulfite oxidase. Discuss how molybdenum acts as a [cofactor] in these enzymes, facilitating key metabolic processes like [purine catabolism] and [sulfur amino acid metabolism]. Provide detailed examples of how molybdenum deficiency or excess could impact human health, and suggest [dietary sources] rich in this essential trace element. Your explanation should be clear, scientifically accurate, and accessible to an audience with a basic understanding of biochemistry.