pH is a measure of how acidic/basic water is. The range goes from 0 to 14, with 7 being neutral. pHs of less than 7 indicate acidity, whereas a pH of greater than 7 indicates a base.

The most favorable pH value – the point where the enzyme is most active – is known as the optimum pH. This is graphically illustrated in Figure 14. Extremely high or low pH values generally result in complete loss of activity for most enzymes.

pH: Each enzyme has an optimum pH range. Changing the pH outside of this range will slow enzyme activity. Extreme pH values can cause enzymes to denature. Enzyme concentration: Increasing enzyme concentration will speed up the reaction, as long as there is substrate available to bind to.

The effect of pH Enzymes are also sensitive to pH . Changing the pH of its surroundings will also change the shape of the active site of an enzyme. This contributes to the folding of the enzyme molecule, its shape, and the shape of the active site. Changing the pH will affect the charges on the amino acid molecules.

Changes in pH affect the chemistry of amino acid residues and can lead to denaturation. Protonation of the amino acid residues (when an acidic proton H + attaches to a lone pair of electrons on a nitrogen) changes whether or not they participate in hydrogen bonding, so a change in the pH can denature a protein.

When the pH of the solution containing a protein is changed dramatically, the acid or base will change the charge of the protein. If the pH of a protein solution becomes very low, the protein molecules will become polyanions.

Decreasing the pH by adding an acid converts the –COO- ion to a neutral -COOH group. In each case the ionic attraction disappears, and the protein shape unfolds. Various amino acid side chains can hydrogen bond to each other. Changing the pH disrupts the hydrogen bonds, and this changes the shape of the protein.

The pH-dependence of the degree of hydrogen-bonding between a base and its conjugate acid is considered. This represents sharp pH-dependence.

The disulfide bonds are strong, with a typical bond dissociation energy of 60 kcal/mol (251 kJ mol−1). However, being about 40% weaker than C−C and C−H bonds, the disulfide bond is often the “weak link” in many molecules.

Classical theory suggests that disulfide bonds stabilize proteins by reducing the entropy of the denatured state. More recent theories have attempted to expand this idea, suggesting that in addition to configurational entropic effects, enthalpic and native-state effects occur and cannot be neglected.

Disulfide bonds function to stabilize the tertiary and/or quaternary structures of proteins and may be intra-protein (i.e., stabilizing the folding of a single polypeptide chain) or inter-protein (i.e., multi-subunit proteins such as antibodies or the A and B chains of insulin).

Disulfide bonds are a second post-translational modification that is unique to proteins synthesized in the ER. Their formation is possible due to the presence of an oxidizing redox buffer that is more similar to the extracellular environment where most of these proteins will perform their function.

Disulfide bonds are found predominantly in secreted extracellular proteins. Disulfide bonds rapidly form outside of the cell in the presence of oxygen.

Researchers have successfully demonstrated that disulfide bridge patterns can be identified by mas spectrometry (MS) analysis, following protein digestion either under partial reduction12,13,16,17 or nonreduction conditions. Partial reduction is a widely accepted approach for the determination of disulfide bonds.

Disulfide bonds, covalent linkages between the sulfur-containing side chains of cysteines, are much stronger than the other types of bonds that contribute to tertiary structure. They act like molecular “safety pins,” keeping parts of the polypeptide firmly attached to one another.

Disulfide cross-linkages may be located by cleaving a protein between half-cystinyl residues to give peptides that contain only one disulfide bond. The molecular weights of these peptides are determined by fast atom bombardment mass spectrometry (FAB-MS) and related to specific segments of the parent protein.