Properties of Enzymes

Enzymes are catalystsWhat properties wouldideal catalysts have?

Enzymes are catalystsWhat properties wouldideal 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

CO2 was removed

5.

5.

Amino group switched places

6.

6.

Two substrates were ligated together

Enzyme Kinetics

E = enzymeS = substrateP = productES = enzyme-substrate complexk = rate constant

The rate of reaction is dependent on enzyme concentration[Enzyme] <<< [substrate]

Figure 5.2

Velocity,or how fastthe reaction

is going

Concentration of enzyme

When measuring ratesof enzyme-catalyzed

reactions, initialvelocity (vo) is

measured.

Figure 5.3

[S] – substrate concentration

Vo – initial velocity of a reaction. A significant amount of substrate has not yet been converted to product.

Vmax – maximal velocity of a reaction. Addition of moresubstrate will not increase the rate of the reaction.

Km – The concentration of substrate at which the rate of the reaction is half-maximal

Enzyme kinetics terminology

Michaelis-Menten equationPage 141

The rate of reaction is dependent on substrate concentration

Figure 5.4

Km values are often just above the substrate concentrationin a cell. Rates of reaction are sensitive to small changes

in cellular substrate concentrations.

kcat is a measure of the number of substrate moleculesconverted 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 andinterferes with its activity. Many drugs are enzyme inhibitors.

An inhibitor is characterized by an inhibition constant (Ki).

No Inhibitor

Figures 5.8 and 5.9

No Inhibitor

Many competitive inhibitorsare 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) asa substrate. (b) is a possible drug for the treatment of arthritis.

Figure 5.13

F-

DFP inactivates serine proteasesby covalently modifying anactive site serine residue.

Figure 5.15

Irreversible Enzyme InhibitionSome inhibitors are compounds

that form a stable covalentbond 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 isregulated by reversible phosphorylation.

Example: pyruvate dehydrogenase

Figure 5.22