kcat/KM (the specificity constant) is a better predictor of an enzyme's ability to use a specific substrate than KM alone because it takes into account both the affinity of the enzyme for the substrate (KM) and the catalytic efficiency of the enzyme (kcat).
KM is a measure of the substrate concentration required to reach half-maximal velocity (Vmax) of the enzymatic reaction. It is an indicator of the enzyme's affinity for the substrate, but it does not take into account the enzyme's catalytic efficiency, or the rate at which it can convert substrate to product.
kcat/KM, on the other hand, represents the efficiency of the enzyme in converting substrate to product. It is a measure of the turnover number (kcat) divided by the Michaelis constant (KM) and is a more comprehensive indicator of the enzyme's ability to use a specific substrate. A higher kcat/KM value indicates that the enzyme has a higher catalytic efficiency, which means that it can convert substrate to product more efficiently at a given substrate concentration.
Thus, kcat/KM is a better predictor of an enzyme's ability to use a specific substrate than KM alone because it takes into account both the enzyme's affinity for the substrate and its catalytic efficiency. In other words, kcat/KM provides a measure of how efficiently the enzyme can use the substrate to produce product, which is ultimately what determines the enzyme's biological function.