Catalyzes the reduction of succinate semialdehyde to succinyl-CoA. The enzyme is specific for succinate semialdehyde and succinyl-CoA, and only shows low activity with palmitoyl-CoA. There is no activity with NAD(+) as cosubstrate. CoA + NADP(+) + succinate semialdehyde = H(+) + NADPH + succinyl-CoA Optimum pH is 7.0 for the reduction reaction and 8.5 for the oxidation reaction. Homodimer.