3.1.4.2 Many proteins are enzymes

Enzymes are globular proteins that act as catalysts by altering the rate of a chemical reaction without undergoing permanent changes themselves. They can be reused and are therefore effective in small amounts. They catalyse a wide range of intracellular and extracellular reactions that determine structures and functions from cellular to whole-organism level.

The minimum amount of energy required to activate the reaction is known as the activation energy. For reactions to occur initially (naturally) a number of conditions must be satisfied:

• The substrates must collide with sufficient energy to alter the arrangement of their atoms to form the produce
• The free energy of the products must be less than that of the substrates

The activation energy must be initially overcome before the reaction can proceed. Enzymes lower the activation energy level. A specific region of the enzyme (the active site) is functional. It forms a small depression within the much larger enzyme molecule. Enzymes act upon substrates which fit neatly into the active site forming an enzyme-substrate complex. The substrate is temporarily held in place by temporary bonds between amino acids of the active site and groups on the substrate.

We need to know a bit about the induced fit model of an enzyme. The induced fit model proposes that the active site forms as the enzyme and substrate interact. The proximity of the substrate leads to a change in the enzyme that forms the functional active site. As it changes shape the enzyme puts strain on the substrate molecule. This strain distorts particular bonds in the substrate and consequently lowers the activation energy needed to break the bond.

We need to know about the effects of certain factors on the rate of enzyme controlled reactions. Providing there are no limiting factors the following will occur:

• enzyme concentration
• the more enzymes the more active sites so the faster the reaction
• substrate concentration
• the more substrates the faster the reaction
• concentration of competitive and non-competitive inhibitors
• competitive inhibitors block the active site. The more competitive inhibitors the lower the rate of reaction
• non-competitive inhibitors distort the active site. The more non-competitive inhibitors the lower the rate of reaction
• pH
• A pH far from the enzymes optimum will denature the enzyme and the reaction will cease
• at optimum pH the rate of reaction will be the fastest
• temperature
• A temperature far from the enzymes optimum will denature the enzyme and the reaction will cease
• at optimum temperature the rate of reaction will be the fastest

Okay so we need to be able to calculate pH from hydrogen ion concentration. To do this we use the equation:

pH = - log [H+]