Properties of Enzymes

Enzymes are catalysts What properties would ideal catalysts have?

1.High degree of specificity for their substrates. 2.Accelerate chemical reactions tremendously. 3.Function in mild conditions. 4.Be recycled to participate again.

A Few Definitions Cofactor – additional chemical component needed for catalysis. - often an inorganic metal ion (mineral). Coenzyme – complex organic molecule needed for catalysis. - often a vitamin. Prosthetic group – non amino acid portion of the enzyme needed for catalysis. Often a coenzyme or metal ion. Holoenzyme – complete catalytically active enzyme, with all necessary prosthetic groups. Apoenzyme – The protein part of the holoenzyme. Prosthetic groups are absent.

Six Classes of Enzymes 1.Oxidoreductases 2.Transferases 3.Hydrolases 4.Lyases 5.Isomerases 6.Ligases

Stored electrons One bond to oxygen Two bonds to oxygen

Amino group transferred

Water did chemistry to break a bond

CO 2 was removed

5.

Amino group switched places

6.

Two substrates were ligated together

Enzyme Kinetics E = enzyme S = substrate P = product ES = enzyme-substrate complex k = rate constant

The rate of reaction is dependent on enzyme concentration [Enzyme] <<< [substrate] Figure 5.2 Velocity, or how fast the reaction is going Concentration of enzyme

When measuring rates of enzyme-catalyzed reactions, initial velocity (v o ) is measured. Figure 5.3

[S] – substrate concentration V o – initial velocity of a reaction. A significant amount of substrate has not yet been converted to product. V max – maximal velocity of a reaction. Addition of more substrate will not increase the rate of the reaction. K m – The concentration of substrate at which the rate of the reaction is half-maximal Enzyme kinetics terminology

Michaelis-Menten equation Page 141

The rate of reaction is dependent on substrate concentration Figure 5.4

K m values are often just above the substrate concentration in a cell. Rates of reaction are sensitive to small changes in cellular substrate concentrations.

k cat is a measure of the number of substrate molecules converted to product per second per enzyme molecule

Experimental determination of kinetic constants Figure 5.6

Reversible Enzyme Inhibition An inhibitor is a compound that binds to an enzyme and interferes with its activity. Many drugs are enzyme inhibitors. An inhibitor is characterized by an inhibition constant (K i ).

No Inhibitor Figures 5.8 and 5.9

No Inhibitor

Many competitive inhibitors are substrate analogs Benzamidine is an inhibitor of the enzyme trypsin

Many competitive inhibitors are substrate analogs. Compound (b) designed as an inhibitor of the enzyme purine nucleoside phosphorylase, that utilizes guanosine (a) as a substrate. (b) is a possible drug for the treatment of arthritis. Figure 5.13

F-F- DFP inactivates serine proteases by covalently modifying an active site serine residue. Figure 5.15 Irreversible Enzyme Inhibition Some inhibitors are compounds that form a stable covalent bond with the target enzyme.

Regulation of enzyme activity by metabolite concentration Activator

Regulator ADP binds at an allosteric site, Separate from the active site

The activity of the some enzymes is regulated by reversible phosphorylation. Example: pyruvate dehydrogenase Figure 5.22