Even I can see that that circuit is just a touch silly looking, if you don't mind. You can use a current source to power the TL431, the datasheet (that or the LM334 one) show the LM334 being used to provide a supposedly better current to the 431 than a resistor would, but as FvM has said - those components (R3, D5, Q1) are an "unusual configuration" for a current source. The Op Amp is serving as an error amplifier, so, again, you can just use the TL431 to serve that same purpose unless this is some high precision voltage regulator.