Lecture 5

  holoenzyme = protein part  +  non-protein part

apoenzyme + cofactor

Three types of cofactors:

1. coenzyme: small
organic molecule, loosely
held in the complex

2. inorganic metal ion:
examples Mg⁺², Ca⁺², Zn⁺²

Trace elements

3. prosthetic groups:
small organic molecules,
tightly held in the complex

oxidation-reduction reactions

An enzyme combines physically with its
substrate, forming a reactive intermediate.

Just like a key fits into a lock,
a substrate fits into an enzyme.

Induced Strain Mechanism

The substrate attaches at the active site
on the enzyme.

General properties of enzymes:

1. Q₁₀ value

rate at (T + 10)° C
rate at T° C

Q₁₀ ≈ 2 for chemical reactions

Q₁₀ ≈ 2 for enzyme- catalyzed reactions

Enzymes show an optimum temperature
where their activity is at a maximum.

2. Enzymes are sensitive to pH. 
Most enzymes are most active
in a narrow range of pH values. 

The pH at which an enzyme
is most active is its pH optimum.

3. The activity of enzymes can
be affected by enzyme modulators. 
Many modulators are inhibitors.

Antibiotic

Antiseptic

Some inhibitors are reversible;
others are irreversible.

4.Enzymes are able to reverse
the reactions that they catalyze.

A + B ↔ C + D

Names and Characteristics
Of Enzymes:

1. dehydrogenase enzymes are
involved in oxidation- reduction reactions.

Oxidation reactions remove
electrons from molecules.

In biology, when an electron is removed,
a proton goes along with it.

An electron and a proton
is an atom of hydrogen.

2. Decarboxylase enzymes remove
carbon dioxide (CO₂) usually
from a carboxylic acid group.

3. Kinase enzymes add
phosphate their substrate,
usually taking it from ATP.

4. Phosphatase enzymes remove
phosphate from their substrate.

Coupled reactions

Respiration

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

38ADP + 38Pi → 38ATP + 38 H₂O

first page of handout

Acetyl Coenzyme A

Krebs Cycle = Tricarboxylic Acid Cycle
= Citric Acid Cycle

Glycolysis = Embden- Myerhoff
Pathway

Pyruvic acid is the end of glycolysis.