Characteristics of Guanosine Triphosphate Cyclohydrolase Purified from Pseudomonas putida = Pseudomonas putida에서 분리된 Guanosine Triphosphate Cyclohydrolase의 효소구조 및 특성

국문 초록 (Abstract)

Pseudomonas putida에서 GTP를 $H_2$-neopterin triphosphate으로 전환시키는 효소인 GTP cyclohydrolase를 순수 분리하여 그 구조 및 특성에 관하여 연구하였다. Analytical gel filtration 방법을 사용하여 분자량을 측정하였는데, 이 효소는 분자량이 약 460,000의 고분자 형태와 약 120,000의 저분자 형태로 존재함을 발견하였다. 즉, 이 효소를 포함하는 효소용액의 ionic strength 또는 pH가 높아질수록, 혹은 효소의 농도가 낮아질수록 가역적으로 고분자에서 저분자 형태로 전환된다. 효소의 두 형태는 모두 효소활성 을 지니고 있으며, 온도와 pH에 따른 효소활성의 변화양상, 그리고 negative cooperativity의 정도는 서로 비슷하였다. 그러나 열처리 및 pH에 따른 효소의 안정도, 기질에 대한 친화도 등은 차이가 있었다. 고분자와 저분자 형태 사이의 가역적인 상호전환과 negative cooperativity는 Pseudomonas putida에서 GTP cyclohydrolase의 활성조절에 관여 할 것 이라고 사료된다.

다국어 초록 (Multilingual Abstract)

GTP cyclohydrolase, an enzyme that catalyze the first reaction in the pathway for the biosynthesis of the pteridine portion of folic acid, was purified from Pseudomonas putida by 3,000-fold to apparent homogeneity. Its molecular weight is estimated at 460,000. At relatively high concentrations of salt (e.g. 0.3 M KCl) the enzyme can be dissociated into seemingly identical subunits of 120,000 molecular weight. Removal of salt allows reassociation. This conversion to the dissociated form was promoted by increasing the ionic strength or pH of the buffer and by decreasing the concentration of the enzyme solution. GTP at concentration of $30\;{\mu}M$ did not promote the reassociation of the subunits in the presence of 0.3 M salt. When the enzyme was subjected to electrophoresis on polyacrylamide gel under denaturating conditions only one protein band was evident; its molecular weight was estimated at 13,000. Partial amino acid sequence of this enzyme from $NH_2$-terminal was Thr-Leu-Glx-Trp-His-Glx-. These observations suggest that the enzyme consists of four identical subunits and that each subunit contains 8 to 10 identical polypeptide chains. Both the high and low molecular weight forms possess catalytic activity and both showed similar degrees of negative cooperativity. However, two enzyme forms were dissimilar in following properties: dependence of stability on heating and pH, affinity to substrate (The $K_m$ values for GTP are $17\;{\mu}M$ and $3\;{\mu}M$ for the high molecular weight form and the dissociated form, respectively).