Apparently arsenate "inhibits" GAPDH because arsenate is so similar to phosphate that the enzyme incorporates arsenate into G3P rather than phosphate. The arsenate ester that GAPDH creates, however, almost instantaneously hydrolyzes in aqueous solution, so the enzyme ends up consuming G3P without producing the product.
See Crane and Lipmann. 1952. The effect of arsenate on aerobic phosphorylation. J. Biol. Chem. 201: 235. http://www.jbc.org/content/201/1/235.full.pdf+html and the papers it references in the introduction.
