Core answer: The statement implies the native protein exists as a heterodimer or multimeric complex, but the exact option depends on whether the two listed amino acids come from two distinct subunits or from two ends of a single subunit. Based on normal interpretation, if one mole of protein yields one mole each of Asp and Val upon N-terminal analysis, that suggests there are two different N-termini present in the native assembly, consistent with a heterodimer. Therefore the correct choice is (C) Hetero-dimer.

Explanation of options

• Monomer (A)

  • If the protein were a single polypeptide chain (monomer) in its native state, N-terminal analysis would yield the N-terminal amino acid of that one chain only, not two different N-termini. Since two different N-terminal residues are observed, a monomer is unlikely.​

• Homo-dimer (B)

  • A homodimer consists of two identical subunits. If two N-termini were identical across both subunits, the N-terminal analysis would typically reflect a single N-terminal residue or a single dominant N-terminus, not two different ones. The observation of Asp and Val separately argues against a simple homodimer unless post-translational processing creates two distinct termini, which is less common.​

• Hetero-dimer (C)

  • A heterodimer comprises two different subunits. If each subunit contributes a distinct N-terminus, analyzing the intact native protein can yield two different N-terminal residues, such as Asp from one subunit and Val from the other. This aligns with the reported result of one mole Asp and one mole Val per mole of protein, supporting a heterodimer model.​

• Tetramer (D)

  • A tetramer has four subunits; unless there are four distinct N-termini in the same ratio, the simple 1:1 Asp:Val observation is less directly indicative of a tetramer. Without additional data showing four distinct termini or a specific subunit arrangement, tetrameric interpretation is less straightforward here.​

Key considerations

• N-terminal analysis is used to identify the amino-terminal residues of protein subunits. In heterodimeric assemblies, different subunits can contribute different N-termini, which would yield more than one type of N-terminal residue upon analysis of the native complex.​

• The classic MCAT-style question often uses “one mole of native protein yields one mole each of two different amino acids at the N-terminus” as a cue for heteromeric subunit composition in the native state.​

Citations

• N-terminal sequence analysis principles and the concept of multiple N-termini arising from distinct subunits:.​

• Background on N-terminal sequencing methods and the interpretation of multiple Nt residues in native protein assemblies:.​