Activity as a Function of pH

Gastric lipase displays its maximum lipolytic activity at acidic pH values (3 to 6, see Fig. 10.3) whereas pancreatic lipase is not significantly active below pH 5.0 and displays its maximum activity at pH 7.0-7.5 (Fig. 10.3). In the presence of bile salts, the maximum activity of HPL is shifted towards lower pH values. HGL and HPL are thus well adapted to the pH conditions recorded in vivo, both in the stomach and in the small intestine (Fig. 10.6). 

B Variation with NaTDC concentration in the specific activity of HPL using tributyrin as substrate, in the presence or absence of colipase. Adapted with permission from Thirstrup et al. (1993). 

All main bile salts (glyco- and taurocholate, deoxycholate and chenodeoxycho-late) have the same effects on HGL activity which increases up to a bile salt concentration of around 3 mM for a single species and 4 mM for a mixture of bile salts. However, it is worth noting that synthetic detergents (Triton X-100, Tween, benzalkonium chloride) that dramatically decrease surface tension ( 8mN/m), actually inhibit HGL. 

Castrointestinal Lipolysis of Test Meals in Healthy Human Volunteers 

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Fig. 1. Prostatic acid phosphatase activity as a function of pH ( ) phenyl phosphate (O) p-nitrophenyl phosphate and (A) /8-glycerophosphate. Buffers Ac, acetate Cit, citrate and tris. From Nigam et al. (88).

This chapter deals with three aspects of the cellulolytic enzyme system of Thermoactinomyces sp. the location of the CM-cellulase, Avicelase, and / -glucosidase (cellobiase) activities in the culture, the multiplicity of the extracellular enzyme system, and the stability of the different activities as a function of pH, temperature, and time. The results are discussed with reference to saccharification of cellulosic materials. 

If one measures the enzyme activity as a function of pH or temperature using a soluble substrate, such as hydroxyethyl cellulose, one obtains curves characteristic of many enzymes. The temperature curve follows an Arrhenius dependence at temperatures leading up to the optimum, then drops sharply at inactivating temperatures (Fig. 1). The pH curve is roughly a bell shape, with the optimum spanning 1 to 3 pH units (Fig. 2). This exercise gives a first estimate of the pH and temperature curves, but often the behavior in specific applications is quite different. 

Figure 5. Recovery of YADH activity as a function of pH. Optimum pH for recovery is in the range 4.8-5.2. Phase volume ratio was 1.38. Because of the rapid denaturation of YADH at pH values below 5, all subsequent affinity extractions were carried out at a pH between 5.0 and 5.1.

Activities of monomeric aluminum species calculated in this manner can then be plotted on a graph of log activity as a function of pH and aging time. An example (for solution H) of such a plot is illustrated in... 

Consider the three barbituric acid derivatives thiopental, secobarbital, and barbital with respective pKa of 7.6, 7.9, and 7.8. These drugs are very weak acids. On the basis of their ionization constants we would expect very little difference in their absorption rates from the stomach, yet the drugs are absorbed at very different rates. The reason becomes apparent when the partition coefficients between chloroform and water are considered. Thiopental s value is over 100, whereas the values of secobarbital and barbital are 23 and 0.7, respectively. Now which would one predict to be the least rapidly absorbed and which the most By considering only one physicochemical parameter and excluding others, erroneous conclusions can result. Figure 1-2 illustrates a hypothetical relationship of biological activity as a function of pH only. 

Figure 5 shows the effect of pH at varying temperatures on the expressed activity of glucose isomerase isolated from Bacillus coagulans and immobilized on MPS. It may be noted that the optimum pH (measured at room temperature) is not affected by varying the temperature and that the per cent change of activity as a function of pH remains relatively constant with changing temperatures. Figure 6 is an Arrhenius plot of the pH 7.5 data in Figure 5. Using the formula In K = In A - E /RT an activation energy of 19.6 K cal/mole is obtained.

The graph of enzyme activity as a function of pH is somewhat similar to the behavior as a function of temperature (see > Figure 10.8). Notice in Figure 10.8 that an enzyme is most effective in a narrow pH range and is less active at pH values lower or higher than this optimum. This variation in enzyme activity with changing pH may be due to the influence... 

Fenelon, A.M., and C.B. Breslin. 2004. Polyaniline-coated iron Studies on the dissolution and electrochemical activity as a function of pH. Surf Coat Technol 190 (2-3) 264. 

Figure 15.7 Resulting specific surface area, copper area, and activity as a function of pH and temperature of the precipitation [50,51].

The properties of the enzyme have an impact on the design of the conditions for the recovery process, especially characteristics like temperature and pH stability, solubility of the enzyme protein, and hydrophobicity. The enzyme activity as a function of pH and temperature also influences the choice of process conditions. When processing (e.g., proteases), it can be advantageous to select process conditions where the enzyme has a low activity, to avoid self-digestion (auto proteolysis) of the enzyme. 

The treatment above is essentially equivalent to the treatment of the pH dependence of a polyprotic acid (see Chapter 1). In our case, the enzyme is considered to be a diprotic acid. Increases and decreases in activity as a function of pH simply mirror the increases and decreases in the concentration of the catalytically active species EH (Fig. 6.2). Notice how the bell-shaped pattern for activity as a function of pH (darker lines) corresponds to the net increase and decrease in EH concentration. 

Figure 2. Fluorohydrolase activity as a function of pH for dimethyl pimelimidate crosslinked CCMP. Activity was measured using 5 mM PMSF as the substrate at 30°C. Each activity is the average of two separate measurements obtained by HPLC and fluoride electrode methods described in the text. 

Figure 1. Extent of acetic anhydride inhibition of electron transport activity as a function of pH.- Chloroplasts were treated with acetic anhydride in reaction media containing 0.1 M sucrose, 50 mM KCl,...

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