4-Aminobutyrate aminotransferase(GABA transaminase)는 lysyl group에 특이한 Bis-PLP나 -SH group에 특이한 DTNB 또는 DPD에 의하여 효소가 불활성화 되었으며 이 불활성화는 효소의 기질인 ${alpha}$-ketoglutarate의 결합부위에 lysyl group과 cysteinyl group이 위치하고 있음을 보여주는 것이다. GABA transaminase의 second catalytic site에 대한 구조적 특성을 밝히기 위하여 native 효소를 $NaBH_4$로 환원시킨 후 몇 가지 아미노산 기질을 이용하여 kinetic parameters를 조사해 본 결과 큰 차이가 없음을 알 수 있었다. 이는 second catalytic site가 구조적으로나 작용기적으로 native 효소의 catalytic site와 같다는 것을 나타낸다. 마지막으로 native 효소에서 cofactor가 떨어졌을 때(apo 효소) 구조적 변화를 가져왔다. 이는 효소가 in vivo에서 합성될 때 불안정한 apo enzyme으로 만들어진 후 cofactor가 binding 함으로써 촉매 기능적으로 또는 구조적으로 안정화 된다고 본다.
4-Aminobutyrate aminotransferase (GABA transaminase) was inactivated by either lysyl or sulfhydryl reagents. The inhibitory effect was protected by ${alpha}$-ketoglutarate, one of the substrate of GABA transaminase. These are powerful evidence that lysyl and cysteinyl residues are located in catalytic domain of the enzyme. The function of the second catalytic site of the enzyme was also examined. After reduction with $NaBH_4$ followed by cofactor reconstitution, the reduced enzyme was capable of catalyzing transamination reaction of the amino substrate, such as 4-aminobutyrate, ${eta}$-alanine or 5-aminovalerate. Since there is no significant difference in the catalytic parameter between the native and reduced enzymes, it seems reasonable to conclude that the additional catalytic site is functionally identical to the catalytic site of the native enzyme. Moreover, apoenzyme undergoes conformational changes upon binding of cofactor pyridoxal-5-phosphate (PLP) which indicates that apoenzyme becomes conformationally stabilized as a consequence of the cofactor binding.
