Button D K
Institute of Marine Science and Department of Chemistry and Biochemistry, University of Alaska, Fairbanks, Alaska 99775, USA.
Microbiol Mol Biol Rev. 1998 Sep;62(3):636-45. doi: 10.1128/MMBR.62.3.636-645.1998.
The abilities of organisms to sequester substrate are described by the two kinetic constants specific affinity, a degrees, and maximal velocity Vmax. Specific affinity is derived from the frequency of substrate-molecule collisions with permease sites on the cell surface at subsaturating concentrations of substrates. Vmax is derived from the number of permeases and the effective residence time, tau, of the transported molecule on the permease. The results may be analyzed with affinity plots (v/S versus v, where v is the rate of substrate uptake), which extrapolate to the specific affinity and are usually concave up. A third derived parameter, the affinity constant KA, is similar to KM but is compared to the specific affinity rather than Vmax and is defined as the concentration of substrate necessary to reduce the specific affinity by half. It can be determined in the absence of a maximal velocity measurement and is equal to the Michaelis constant for a system with hyperbolic kinetics. Both are taken as a measure of tau, with departure of KM from KA being affected by permease/enzyme ratios. Compilation of kinetic data indicates a 10(8)-fold range in specific affinities and a smaller (10(3)-fold) range in Vmax values. Data suggest that both specific affinities and maximal velocities can be underestimated by protocols which interrupt nutrient flow prior to kinetic analysis. A previously reported inverse relationship between specific affinity and saturation constants was confirmed. Comparisons of affinities with ambient concentrations of substrates indicated that only the largest a degreesS values are compatible with growth in natural systems.
生物体螯合底物的能力由两个动力学常数来描述，即比亲和力（a°）和最大速度（Vmax）。比亲和力源自底物分子在底物亚饱和浓度下与细胞表面通透酶位点碰撞的频率。Vmax源自通透酶的数量以及被转运分子在通透酶上的有效停留时间（τ）。结果可用亲和力图（v/S对v，其中v是底物摄取速率）进行分析，该图可外推得到比亲和力，且通常呈上凹形。第三个推导参数亲和力常数KA与KM相似，但与比亲和力而非Vmax进行比较，定义为使比亲和力降低一半所需的底物浓度。它可以在不测量最大速度的情况下确定，并且对于具有双曲线动力学的系统，它等于米氏常数。两者都被用作τ的度量，KM与KA的偏差受通透酶/酶比率的影响。动力学数据的汇编表明，比亲和力的范围为10^8倍，Vmax值的范围较小（10^3倍）。数据表明，在动力学分析之前中断营养物质流动的实验方案可能会低估比亲和力和最大速度。先前报道的比亲和力与饱和常数之间的反比关系得到了证实。将亲和力与底物的环境浓度进行比较表明，只有最大的a°S值才与自然系统中的生长相兼容。
