Enzymes are good catalysts

Why???

1. Enzymes have reaction specificity for a particular substrate(Many enzymes also have stereo specificity)

2. Reactions with enzymes are typically 103 to 1020 timesfaster than without enzymes.

3. Enzymes are biologically relevant because they functionin mild conditions (aqueous solution, pH~7, etc.)

4. Enzymes are good at coupling different reactions.

CO2 + H2O H2CO3

Hydrolyzes peptide bonds on carboxyl side of aromatic and aliphatic residues

What do we want to know about enzymes??

1. What reaction do they catalyze?

2. How does the enzyme work. (mechanism)

3. Is the enzyme regulated or controlled?

4. What happens if the enzyme stops functioning?

5. As in chapter 4, what are the structural features of the enzyme.(e.g., complex or simple, 1o, 2o, 3o, 4o etc.)

6. Does the enzyme require a cofactor for activity?

Note: not all enzymes are proteins, some are RNA based

Apoenzyme + Cofactor Holoenzyme

(Protein only)Inactive

(Active)

Cofactors and the vitamin in supplements

The six major classes of enzymes

1. Oxidoreductases

2. Transferases

3. Hydrolyases

4. Lyases

5. Isomerases

6. LigasesDistribution of known enzymes

1. Oxidoreductases catalyze oxidation-reduction reactions

C is more reduced(1 H and 1 O bonded)

C is more oxidized(2 bonds to oxygen)

Recall: Carbon is more reduced when more H’s are bonded, while carbon is more oxidized with more O’s bonded.

Compare CH4 vs CO2

When something is oxidized, something else must be reducedElectrons MUST be transferred. (NAD+ is reduce to NADH)

NAD+ - nicotinamide adenine dinucleotide

oxidizedreducedElectron are stored

2. Transferases catalyze group transfer reactions-These enzymes usually require a coenzyme be present.

-coenzyme: complex organic molecule (often a vitamn)needed for catalysis

ammonium group transfer

3. Hydrolyases catalyze hydrolysis.-A special class of transferase- water is the key acceptorof the group transferred.

Water is added to break a one of the-P-O-P- bonds.

4. Lyases catalyze lysis of a substrate. A double bond isgenerated.

-The reverse reaction is called an addition reactionthe enzymes are synthases.

CO2 was removed (a double bond was formed)

5. Isomerase catalyze structural changes within a single molecule-Simply an isomerization reaction

Ammonium group switched places

6. Ligases catalyze ligation or joining of two substrates-These reactions are usually coupled with a second reaction. (chemical potential energy, or stored energy, from the second reaction is used to drive the first reaction.)

Two substrateswere ligated

The second reaction: ATP ADP + Pi

Stored chemical energy

DGo’-The Standard Free Energy at pH = 7

- A measure of where equilibrium lies for a reaction- If DGo’ < 0, [products] > [reactants]

- If DGo’ > 0, [reactants] >[products]

- If DGo’ = 0, [products] = [reactants]

- Many biochemical reactions are near equilibrium.

- Some biochemical reactions are irreversible or “spontaneous” and is largely dependent on conc.

- THE ENZYME DOES NOT ALTER FREE ENERGY

DG╪ IS a measure of Enzymatic “Speed”The energy of activation

Enzymes help tolower the barrieras the reactionprogresses

Active sites may include distant amino acid residues

The transition state isenergetically more favorableonce the enzyme-substratecomplex is formed.

- Active site is small

- Active sites are uniqueenvironments

- Substrate binds via non-covalent interactions

Figure 6.4

Figure 6.6- Induced-fit model of enzyme-substrate binding

Specificity of binding depends on arrangement and type of

atoms in active site

The enzymes structure changes as the substrate binds

Glucose binding to the enzyme hexokinase

The Induced Fit

Assignment

Read Chapter 6Read Chapter 7

Topics not covered:Details of ΔG equations on pages 97 and 98