We report on a comparison of isolated hard photon production in hadronic Z decays with the predictions of a next-to-leading order matrix-element calculation. We constrain the quark electroweak couplings to the Z boson with a simultaneous fit to three direct photon distributions, and combine this result with an independent constraint from our measurement of the total hadronic width of the Z, obtaining c u = 0.92 ± 0.22 and c d = 1.63 ± 0.15 where c u,d = 4( g V 2 + g A 2 ) u,d . Our results are consistent with standard model predictions.