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Biosynthesis of (a) methionine: Asp is a valid precursor; Thr is not because it requires an energetically unfavorable conversion; (b) arginine: Asp lies two enzymatic steps away from Arg, whereas Pro is five and Glu is six enzymatic steps removed; and (c) leucine: Val is not a valid precursor of Leu because both are products of pyruvate in different enzymatic pathways.
