Single-atom　catalysts　(SACs)　with　complete　dispersion　of　metal　species　on　substrates　provide　the　ultimate　atom　economy　and　opportunities　for　improved　heterogenous　catalysis　or　to　develop　more　catalyst　formulation.　However,　atomically　dispersed　dopants　can　also　improve　the　catalyst　in　an　indirect　capacity.　Here,　we　report　an　Ir-1/alpha-MoC　SAC　that　exhibits　high　activity　for　water　gas　shift　reaction　with　similar　to　100%　CO　conversion　at　150　degrees　C.　The　reaction　rate　of　this　catalyst　is　orders　of　magnitude　higher　than　previous　Ir-based　catalysts　and　similar　to　4　times　that　of　alpha-MoC.　Experimental　characterizations　combined　with　density　functional　theory　(DFT)　calculations　reveal　that　such　good　performance　originates　from　the　effective　catalysis　by　the　alpha-MoC　substrate　with　Ir-1　serving　as　a　single-atom　promoter.　DFT　calculations　also　establish　a　correlation　between　the　active　Mo　sites　and　the　catalytic　performance　for　water　gas　shift　reaction.　This　work　highlights　the　possibility　of　enhancing　catalysis　by　single-atom　promoters　as　a　promising　strategy　for　designing　effective　SACs.