Final answer:
The item that is not part of a nucleotide is 'b. enzyme'. Nucleotides are composed of a phosphate group, a pentose sugar, and a nitrogen-containing base, not enzymes.
Step-by-step explanation:
The component that is not part of nucleotides is b. enzyme. Nucleotides are the building blocks of nucleic acids, and each nucleotide is comprised of three fundamental components: a phosphate group, a pentose sugar, and a nitrogen-containing base. The pentose sugar can either be ribose in RNA or deoxyribose in DNA. The nitrogenous bases found in nucleotides are adenine, cytosine, guanine, thymine (in DNA), or uracil (in RNA).
Enzymes, on the other hand, are proteins that catalyze biochemical reactions and are not components of nucleotides. Thus, enzymes do not make up the structure of nucleotides, which are essential for forming the nucleic acids of DNA and RNA, and for creating the energy compound adenosine triphosphate (ATP).