1. How enzymes adapt: lessons from directed evolution
Frances H Arnold, Patrick L Wintrode, Kentaro Miyazaki, Anne Gershenson 
Trends in Biochemical Sciences 
Volume 26, Issue 2, Pages (February 2001)
DOI: /S (00)

2. Fig. 1
Divergent evolution leads to adaptation to different environments. Although evolutionary distances can be large (today's enzymes can differ at hundreds of amino acid positions), three-dimensional structures remain very similar. This is illustrated here for a subunit of citrate synthase isolated from a thermophilic (red) and a psychrophilic (blue) organism, which differs at 223 amino acid positions between the two organisms 3,4. Today's citrate synthases (depicted by colored ovals) from organisms living over a range of temperatures are all derived from an ancient enzyme of the same structure. However, much evolution is either neutral or nearly neutral, and many sequence changes probably only lead to small, if any, changes in functional properties. This can confound comparative studies that aim to uncover adaptive mechanisms.
Trends in Biochemical Sciences  , DOI: ( /S (00) )

3. Fig. 2
Homologous enzymes adapted to different temperatures show a trade-off between catalytic activity at low temperatures (high for enzymes from psychrophilic organisms but generally low for enzymes from thermophiles) and thermostability (high for thermophilic enzymes but low for enzymes from psychrophiles). These naturally occurring enzymes lie in the darker shaded area of the figure, bounded on one side (pink area) by the minimal stability and activity that are required for biological function, and on the other side (lighter shaded area in top, right-hand quadrant) by enzymes that are both highly thermostable and highly active at low temperature, which are generally not found in nature. However, laboratory experiments demonstrate that this region of the function space is accessible to evolution, and so we can conclude that active, stable enzymes are physically possible, but not biologically relevant.
Trends in Biochemical Sciences  , DOI: ( /S (00) )

4. Fig. 3
Directed evolution of a psychrophilic enzyme, subtilisin S41, from the Antarctic bacterium TA41 (Ref. 12). (a) Rapid screening for thermostability and activity is performed by taking replicas from a master plate that contains individual clones. One replica plate is assayed for activity at room temperature (RT), and the second is incubated at high temperature (HT) before activity is measured. The ratio of the residual activity (Ar), calculated from the HT plate, to the initial activity (Ai), from the RT plate, provides a measure of thermostability for this irreversibly inactivated enzyme. (b) Activities and stabilities of random S41 mutants 13. The distribution of wild-type clones measured under the same conditions (red dots inside ellipse) shows the reproducibility of the screen. Mutants distribute well outside this region. Most improvements in activity come at the cost of stability, and vice versa. (c) Progression of the evolution of S41 thermostability, as measured by the half-life of enzyme activity at 60°C, in 1 mM CaCl2. One of the mutants discovered in the first generation contained two amino acid substitutions in a loop region. These were subjected to saturation mutagenesis, and the best mutant was recombined with other random mutants. Further rounds of random mutagenesis and screening produced 3-2G7, which has seven amino acid substitutions.
Trends in Biochemical Sciences  , DOI: ( /S (00) )

5. Fig. 4
Temperature dependence of the specific activities (in arbitrary units) of evolved thermophilic enzymes 12,16,17. (a) p-Nitrobenzyl esterase: wild type (green), 4th (red), 6th (cyan) and 8th (pink) generation mutants; (b) subtilisin S41: wild type (green) and 5th generation variant, 3-2G7 (cyan). Activities of mesophilic subtilisin SSII (red) are shown for comparison.
Trends in Biochemical Sciences  , DOI: ( /S (00) )

6. Fig. 5
Room temperature phosphorescence lifetimes for wild-type (WT), pNBE and thermostable mutants (described by increases in melting temperature) 17. The red and green lines represent the short (∼10 ms) and long (∼100 ms) lifetime components of the phosphorescence, respectively. Data are normalized to wild type. Inset: relative contributions of different lifetimes to the total observed phosphorescence for wild type (pink line) and 8G8 (cyan line). The two major components are clearly visible and, although there is a trend towards longer lifetimes (decreased local flexibility), substantial increases in stability can be accompanied by significant decreases in the phosphorescence lifetimes (indicating greater local flexibility). This is particularly notable in the progression from variant 53H5 to variant 6SF9.
Trends in Biochemical Sciences  , DOI: ( /S (00) )