Proteins

Understanding enzyme function is vital in modern medicine. Our understanding comes from knowledge of enzyme kinetics, a term used to describe enzyme effects on reaction rates. The rate at which enzyme catalyzed reactions proceed can be calculated from changes in concentration of substrate (chemical reactants) and product. Each specific enzyme substrate complex has a rate constant (kM ), which scientists use to study enzyme kinetics. The substrate concentration at which an enzyme catalyzed reaction proceeds at half its maximum velocity determines the kM value of the enzyme. The Michaelis-Menten equation is used to describe the velocity of an enzyme catalyzed reaction at unlimited substrate concentration. Enzyme function can be analyzed via comparison to kinetics of function under the Michaelis-Menten equation.

The rate at which an enzyme functions depends on temperature, pH and ionic concentration, and the relative concentration of enzyme and substrate. Very high temperatures inhibit many important enzymes due to denaturation or change in protein shape. Each enzyme has an optimal temperature range for maximum activity.

Shifts in pH and ion concentration can change the noncovalent bonding patterns and shapes of proteins. Recall that protein shape beyond the primary polypeptide is the result of reactions between amino acid side chains. Hydrogen and polar bonds are readily broken when pH changes. When the non-polar bonds that form enzyme secondary and tertiary structure break, the substrate may no longer fit the active site.

The relative amounts of enzyme and substrate present also affect the rate of activity. When all of the enzyme active sites become bound to substrate, a system is saturated. That means the chemical reaction being catalyzed is proceeding at the maximum rate and adding more substrate will not change that rate. The Michaelis-Menten equation was derived based on conditions of substrate saturation.

Often the products of enzyme-catalyzed reactions will serve as feedback inhibitors for further enzyme activity. Sometimes enzyme activity is regulated by competitive binding at the enzyme active site. Often a feedback inhibitor will be the product of some metabolic pathway initiated by the enzyme being inhibited.

Allosteric enzyme regulation occurs when binding of a signal molecule at an allosteric site changes the shape of an enzyme. Binding by allosteric inhibitors prevents enzyme-substrate interaction. Allosteric activators bind to allosteric sites to change protein shape and actually produce the key-in-lock enzyme substrate fit. In allosteric activation, the enzyme will not function until the signal molecule changes the enzymes shape. Allosteric enzyme regulation is also a part of many complex metabolic pathways.

Question

Why is pH important to protein function?

A H ions can affect bonding patterns of amino acids.
B H ions act as allosteric inhibitors and activators.
C H ions competitively bind to the enzyme active site.
D H ions act as coenzymes to activate enzymes.

Answer

©2007 ABCTE. All rights reserved.