Catalyzes the decarboxylation of 2-ketoarginine, leading to the formation of 4-guanidinobutyraldehyde. 5-guanidino-2-oxopentanoate + H(+) = 4-guanidinobutanal + CO2 Binds 1 thiamine pyrophosphate per subunit. Amino-acid degradation; L-arginine degradation. Induced by L-arginine in the absence of ArgR, via the AruR/AruS two-component regulatory system. Belongs to the TPP enzyme family.