School of Life Sciences, University of Hyderabad, 500 134, Hyderabad, India.
Photosynth Res. 1994 Feb;39(2):115-35. doi: 10.1007/BF00029380.
Three to four families of nuclear genes encode different isoforms of phosphoenolpyruvate (PEP) carboxylase (PEPC): C4-specific, C3 or etiolated, CAM and root forms. C4 leaf PEPC is encoded by a single gene (ppc) in sorghum and maize, but multiple genes in the C4-dicot Flaveria trinervia. Selective expression of ppc in only C4-mesophyll cells is proposed to be due to nuclear factors, DNA methylation and a distinct gene promoter. Deduced amino acid sequences of C4-PEPC pinpoint the phosphorylatable serine near the N-terminus, C4-specific valine and serine residues near the C-terminus, conserved cysteine, lysine and histidine residues and PEP binding/catalytic sites. During the PEPC reaction, PEP and bicarbonate are first converted into carboxyphosphate and the enolate of pyruvate. Carboxyphosphate decomposes within the active site into Pi and CO2, the latter combining with the enolate to form oxalacetate. Besides carboxylation, PEPC catalyzes a HCO3 (-)-dependent hydrolysis of PEP to yield pyruvate and Pi. Post-translational regulation of PEPC occurs by a phosphorylation/dephosphorylation cascade in vivo and by reversible enzyme oligomerization in vitro. The interrelation between phosphorylation and oligomerization of the enzyme is not clear. PEPC-protein kinase (PEPC-PK), the enzyme responsible for phosphorylation of PEPC, has been studied extensively while only limited information is available on the protein phosphatase 2A capable of dephosphorylating PEPC. The C4 ppc was cloned and expressed in Escherichia coli as well as tobacco. The transformed E. coli produced a functional/phosphorylatable C4 PEPC and the transgenic tobacco plants expressed both C3 and C4 isoforms. Site-directed mutagenesis of ppc indicates the importance of His(138), His(579) and Arg(587) in catalysis and/or substrate-binding by the E. coli enzyme, Ser(8) in the regulation of sorghum PEPC. Important areas for further research on C4 PEPC are: mechanism of transduction of light signal during photoactivation of PEPC-PK and PEPC in leaves, extensive use of site-directed mutagenesis to precisely identify other key amino acid residues, changes in quarternary structure of PEPC in vivo, a high-resolution crystal structure, and hormonal regulation of PEPC expression.
三种到四种核基因家族编码不同的磷酸烯醇丙酮酸（PEP）羧化酶（PEPC）同工型：C4 特异型、C3 或黄化型、CAM 和根型。高粱和玉米的 C4 叶型 PEPC 由单个基因（ppc）编码，但在 C4-双叶型鸭跖草 Flaveria trinervia 中有多个基因。C4- 质体中 ppc 的选择性表达被认为是由于核因子、DNA 甲基化和独特的基因启动子。C4-PEPC 的推导氨基酸序列精确指出了 N 端附近可磷酸化的丝氨酸、C 端附近 C4 特异的缬氨酸和丝氨酸残基、保守的半胱氨酸、赖氨酸和组氨酸残基以及 PEP 结合/催化位点。在 PEPC 反应中，PEP 和碳酸氢盐首先转化为羧基磷酸和丙酮酸的烯醇盐。羧基磷酸在活性部位内分解为 Pi 和 CO2，后者与烯醇盐结合形成草酰乙酸。除了羧化作用外，PEPC 还催化 PEP 的 HCO3（-）依赖性水解，生成丙酮酸和 Pi。PEPC 的翻译后调控是通过体内的磷酸化/去磷酸化级联反应和体外的酶可逆寡聚化来进行的。酶的磷酸化和寡聚化之间的关系尚不清楚。PEPC-蛋白激酶（PEPC-PK）是负责磷酸化 PEPC 的酶，已被广泛研究，而能够去磷酸化 PEPC 的蛋白磷酸酶 2A 则只有有限的信息。C4 ppc 已在大肠杆菌和烟草中克隆和表达。转化的大肠杆菌产生了功能性/可磷酸化的 C4 PEPC，转基因烟草植物表达了 C3 和 C4 同工型。ppc 的定点突变表明 His(138)、His(579)和 Arg(587)在大肠杆菌酶的催化和/或底物结合中以及 Ser(8)在高粱 PEPC 的调节中很重要。C4 PEPC 进一步研究的重要领域包括：在叶片中 PEPC-PK 和 PEPC 的光激活过程中光信号转导的机制、定点突变的广泛应用以精确鉴定其他关键氨基酸残基、体内 PEPC 四级结构的变化、高分辨率晶体结构以及激素对 PEPC 表达的调节。
