Enzymes

Enzymes:

• Are proteins (with few exceptions)

• Can be denatured and still retain full activity

• Have names always ending in “-ase"

• Are also referred to as “coenzymes"

Answer explanation

With the exception of a few classes of catalytic RNA molecules, enzymes are proteins.

Some enzymes require an inorganic ion for catalytic function. When this inorganic ion is very tightly or covalently bound by the enzyme it is called a(n):

• Apoenzyme

• Prosthetic group

• Holoenzyme

• Catalyst

Answer explanation

A coenzyme or metal ion that is very tightly or even covalently bound to the enzyme protein is called a prosthetic group.

What does an enzyme change relative to an uncatalyzed reaction?

• The equilibrium constant

• The rate of the reaction

• The pH

• The free energy change of the reaction

Answer explanation

The role of enzymes is to accelerate the interconversion of S and P. The enzyme is not used up in the process, and the equilibrium point is unaffected. However, the reaction reaches equilibrium much faster when the appropriate enzyme is present because the rate of the reaction is increased.

How does the induced fit mechanism of enzyme catalysis work?

• The enzyme assumes a conformation identical to the

       substrate

• The enzyme undergoes a conformational change to

       maximize weak interactions to the substrate

• The substrate binds the active site of the enzyme

• The enzyme undergoes entropy reduction to accommodate

       substrate

Answer explanation

The enzyme usually undergoes a change in conformation when the substrate binds, induced by multiple weak interactions with the substrate, a mechanism referred to as induced fit. Induced fit brings specific functional groups on the enzyme into the proper position to catalyze the reaction and permits formation of additional weak bonding interactions in the transition state.

An enzyme accepts H+ from hydronium and transfers it to an amine group of the substrate. The result is an increase in the rate of release of the product. This is an example of:

• Metal ion catalysis

• General acid catalysis

• Covalent catalysis

• Specific acid catalysis

Answer explanation

General acid-base catalysis refers to proton transfers mediated by weak acids and bases other than water. Thus, an enzyme-mediated transfer of a H+ from hydronium to an amine group of a substrate is an example of general acid catalysis.

Michaelis-Menten kinetics:

•  Can be applied to reactions with multiple substrates

• Determine the V0 at ½ maximal [S]

• Can be used to analyze inhibitor action

• Apply to all regulatory enzymes

Answer explanation

The Michaelis-Menten equation can be adapted and applied to competitive inhibitors, uncompetitive inhibitors, mixed inhibitors, and noncompetitive inhibitors.

Allosteric enzymes:

• Are never regulated by substrate binding

• Have their activity changed by changes in intersubunit interactions

• Exhibit Michaelis-Menten kinetics

• Always have both inhibitory and activating modulators

Answer explanation

Allosteric proteins are those having “other shapes” or conformations induced by the binding of modulators. Conformational changes induced by one or more modulators interconvert more-active and less-active forms of the enzyme.