Each enzyme has a different optimum pH, which is the ideal pH for the enzyme to perform its job successfully.  As can be seen above, the optimum pH for the enzyme Salivary Amylase is around 7. The closer the pH is to 7, the higher the reaction rate. As the pH distances from the optimum, however, the reaction rate decreases because the shape of the enzyme's active site begins to deform, until it becomes denatured and the substrate can no longer fit the active site.
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The pH scale is used to measure the acidity or alkalinity of a sample and describes how many hydrogen ions or hydroxides are present in the sample. The change of pH will lead to the ionization of amino acids atoms and molecules, change the shape and structure of proteins, thus damaging the function of proteins. Enzymes are also proteins, which are also affected by changes in pH. Very high or very low pH will lead to the complete loss of the activity of most enzymes. The pH value at which the enzyme is most active is called the optimal pH value.
pH Effects Enzyme Activity The structure of the enzyme has a great influence on the activity of the enzyme. In other words, changes in the structure of the enzyme affect the rate of chemical reactions. When the pH value of the reaction medium changes, the shape and structure of the enzyme will change. For example, pH can affect the ionization state of acidic or basic amino acids. There are carboxyl functional groups on the side chain of acidic amino acids. There are amine-containing functional groups in the side chain of basic amino acids. If the ionized state of amino acids in the protein is changed, the ionic bonds that maintain the three-dimensional shape of the protein will change. This may lead to changes in protein function or inactivation of enzymes. pH Effects Substrates PH not only affects the activity of the enzyme, but also affects the charge and shape of the substrate, so that the substrate cannot bind to the active site, or cannot be catalyzed to form a product. In a narrow range of pH, the structural and morphological changes of enzymes and substrates may be reversible. However, if the level of pH changes significantly, the enzyme and substrate may be denatured. In this case, the enzyme and the substrate do not recognize each other, so there will be no reaction. Optimal pH All enzymes have an ideal pH value, which is called optimal pH. Under the optimum pH conditions, each enzyme showed the maximum activity. For example, the optimum pH of an enzyme that works in the acidic environment of the human stomach is lower than that of an enzyme that works in a neutral environment of human blood. When the pH value deviates from the ideal conditions, the activity of the enzyme slows down and then stops. The enzyme has an active site at the substrate binding site, and the shape of the active site will change with the change of pH value. Depending on the extreme extent of the enzyme and pH changes, these changes may permanently "destroy" the enzyme, or once the conditions return to the desired range of the enzyme, the enzyme will return to normal. Related Services Enzyme Kinetics To discuss more service details, please contact us. In the same way that every enzyme has an optimum temperature, so each enzyme also has an optimum pH at which it works best. For example, trypsin and pepsin are both enzymes in the digestive system which break protein chains in the food into smaller bits - either into smaller peptide chains or into individual amino acids. Pepsin works in the highly acidic conditions of the stomach. It has an optimum pH of about 1.5. On the other hand, trypsin works in the small intestine, parts of which have a pH of around 7.5. Trypsin's optimum pH is about 8.
If you think about the structure of an enzyme molecule, and the sorts of bonds that it may form with its substrate, it isn't surprising that pH should matter. Suppose an enzyme has an optimum pH around 7. Imagine that at a pH of around 7, a substrate attaches itself to the enzyme via two ionic bonds. In the diagram below, the groups allowing ionic bonding are caused by the transfer of a hydrogen ion from a -COOH group in the side chain of one amino acid residue to an -NH2 group in the side chain of another. In this simplified example, that is equally true in both the substrate and the enzyme. Now think about what happens at a lower pH - in other words under acidic conditions. It won't affect the -NH3+ group, but the -COO- will pick up a hydrogen ion. What you will have will be this: You no longer have the ability to form ionic bonds between the substrate and the enzyme. If those bonds were necessary to attach the substrate and activate it in some way, then at this lower pH, the enzyme won't work. What if you have a pH higher than 7 - in other words under alkaline conditions. This time, the -COO- group won't be affected, but the -NH3+ group will lose a hydrogen ion. That leaves . . . Again, there is no possibility of forming ionic bonds, and so the enzyme probably won't work this time either. At extreme pH's, something more drastic can happen. Remember that the tertiary structure of the protein is in part held together by ionic bonds just like those we've looked at between the enzyme and its substrate. At very high or very low pH's, these bonds within the enzyme can be disrupted, and it can lose its shape. If it loses its shape, the active site will probably be lost completely. This is essentially the same as denaturing the protein by heating it too much.
The rates of enzyme-catalysed reactions vary with pH and often pass through a maximum as the pH is varied. If the enzyme obeys Michaelis-Menten kinetics the kinetic parameters k0 and kA often behave similarly. The pH at which the rate or a suitable parameter is a maximum is called the pH optimum and the plot of rate or parameter against pH is called a pH profile. Neither the pH optimum nor the pH profile of an enzyme has any absolute significance and both may vary according to which parameter is plotted and according to the conditions of the measurements. If the pH is changed and then brought back to its original value, the behavior is said to be reversible if the original properties of the enzyme are restored; otherwise it is irreversible. Reversible pH behavior may occur over a narrow range of pH, but effects of large changes in pH are in most cases irreversible. The diminution in rate as the pH is taken to the acid side of the optimum can be regarded as inhibition by hydrogen ions. The diminution in rate on the alkaline side can be regarded as inhibition by hydroxide ions. The equations describing pH effects are therefore analogous to inhibition equations. For single-substrate reactions the pH behavior of the parameters k0 and kA can sometimes be represented by an equation of the form \[ k = \dfrac{k_{opt}}{1 + \dfrac{[H^+]}{K_1} + \dfrac{K_2}{[H^+]}} \label{eq1}\] in which k represents k0 or kA, and \(k_{opt}\) is the value of the same parameter that would be observed if the enzyme existed entirely in the optimal state of protonation; it may be called the pH-independent value of the parameter. The constants K1 and K2 can sometimes be identified as acid dissociation constants for the enzyme. substrates or other species in the reaction mixture. The identification is, however, never straight forward and has to be justified by independent evidence. The behavior is frequently much more complicated than represented by Equation \(\ref{eq1}\). It is not accidental that this section has referred exclusively to pH dependences of k0 and kA. The pH dependence of the initial rate or, worse, the extent of reaction after a given time is rarely meaningful; the pH dependence of the Michaelis constant is often too complex to be readily interpretable.
When using Representative Method 13.1 to determine the concentration of creatinine in urine, we follow the reactions kinetics using an ion selective electrode. In principle, we can use any of the analytical techniques in Chapters 8–12 to follow a reaction’s kinetics provided that the reaction does not proceed to any appreciable extent during the time it takes to make a measurement. As you might expect, this requirement places a serious limitation on kinetic methods of analysis. If the reaction’s kinetics are slow relative to the analysis time, then we can make our measurements without the analyte undergoing a significant change in concentration. When the reaction’s rate is too fast—which often is the case—then we introduce a significant error if our analysis time is too long. One solution to this problem is to stop, or quench the reaction by adjusting experimental conditions. For example, many reactions show a strong pH dependency, and may be quenched by adding a strong acid or a strong base. Figure 13.7 shows a typical example for the enzymatic analysis of p-nitrophenylphosphate using the enzyme wheat germ acid phosphatase to hydrolyze the analyte to p-nitrophenol. The reaction has a maximum rate at a pH of 5. Increasing the pH by adding NaOH quenches the reaction and converts the colorless p-nitrophenol to the yellow-colored p-nitrophenolate, which absorbs at 405 nm. Contributors and Attributions |
Was the reaction rate higher at ph 6 or ph 7?
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