3. 1. Catalyze only thermodynamically possible
reactions
2. Are not used or changed during the reaction.
3. Don’t change the position of equilibrium and
direction of the reaction
4. Usually act by forming a transient complex with
the reactant, thus stabilizing the transition state
Common features for enzymes and
inorganic catalysts:
4. Specific features of enzymes:
1. Accelerate reactions in much
higher degree than inorganic
catalysts
2. Specificity of action
3. Sensitivity to temperature
4. Sensitivity to pH
5. Structure of enzymes
Enzymes
Complex or holoenzymes (protein part
and nonprotein part – cofactor)
Simple (only protein)
Apoenzyme (protein
part)
Cofactor
Prosthetic groups
-usually small inorganic
molecule or atom;
-usually tightly bound to
apoenzyme
Coenzyme
-large organic
molecule
-loosely bound to
apoenzyme
6. Example of prosthetic group
Metalloenzymes
contain firmly
bound metal ions
at the enzyme
active sites
(examples: iron,
zinc, copper,
cobalt). Example of metalloenzyme: carbonic
anhydrase contains zinc
7. Coenzymes
• Coenzymes act as group-transfer reagents
• Hydrogen, electrons, or groups of atoms can be
transferred
Coenzyme classification
(1) Metabolite coenzymes - synthesized from
common metabolites
(2) Vitamin-derived coenzymes - derivatives of
vitamins
Vitamins cannot be synthesized by mammals, but
must be obtained as nutrients
8. Examples of metabolite coenzymes
ATP
S-adenosylmethionine
ATP can donate
phosphoryl group
S-adenosylmethionine
donates methyl groups
in many biosynthesis
reactions
10. Vitamin-Derived Coenzymes
• Vitamins are required for coenzyme synthesis
and must be obtained from nutrients
• Most vitamins must be enzymatically
transformed to the coenzyme
• Deficit of vitamin and as result correspondent
coenzyme results in the disease
11. • Nicotinic acid (niacin) an nicotinamide are precursor of
NAD and NADP
• Lack of niacin causes the disease pellagra
NAD+ and NADP+
NAD and
NADP are
coenzymes
for
dehydro-
genases
12. FAD and FMN
• Flavin adenine dinucleotide (FAD) and Flavin
mononucleotide (FMN) are derived from riboflavin (Vit B2)
• Flavin coenzymes are involved in oxidation-reduction
reactions
FMN (black), FAD (black/blue)
13. Thiamine Pyrophosphate (TPP)
• TPP is a
derivative of
thiamine (Vit B1)
• TPP participates
in reactions of:
(1) Oxidative
decarboxylation
(2) Transketo-
lase enzyme
reactions
14. Pyridoxal Phosphate (PLP)
• PLP is derived from Vit B6 family of vitamins
PLP is a coenzyme for enzymes catalyzing reactions involving amino
acid metabolism (isomerizations, decarboxylations, transamination)
15. Enzymes active sites
Active site – specific region in the
enzyme to which substrate molecule is
bound
Substrate usually is relatively small
molecule
Enzyme is large protein molecule
Therefore substrate binds to specific
area on the enzyme
16. Characteristics of active sites
Specificity (absolute, relative (group),
stereospecificity)
Small three dimensional region of the protein.
Substrate interacts with only three to five amino
acid residues. Residues can be far apart in sequence
Binds substrates through multiple weak
interactions (noncovalent bonds)
There are contact and catalytic regions in the
active site
17. Active site of lysozym consists of six amino acid
residues which are far apart in sequence
18. Active site contains functional groups (-OH, -NH, -COO etc)
Binds substrates through multiple weak interactions
(noncovalent bonds)
19. Theories of active site-substrate
interaction
Fischer theory (lock and key model)
The enzyme active site (lock) is able to accept only a
specific type of substrate (key)
20. Koshland theory (induced-fit model)
The process of substrate binding induces specific
conformational changes in the the active site region
21. Properties of Enzymes
Specificity of enzymes
1.Absolute – one enzyme acts only on one substrate
(example: urease decomposes only urea; arginase
splits only arginine)
2.Relative – one enzyme acts on different
substrates which have the same bond type
(example: pepsin splits different proteins)
3.Stereospecificity – some enzymes can catalyze
the transformation only substrates which are in
certain geometrical configuration, cis- or trans-
22. Sensitivity to pH
Each enzyme has maximum activity at a particular pH
(optimum pH)
For most enzymes the optimum pH is ~7 (there are
exceptions)
23. -Enzyme will
denature above 45-
50oC
-Most enzymes have
temperature
optimum of 37o
Each enzyme has
maximum activity at a
particular
temperature (optimum
temperature)
Sensitivity to temperature
24. Naming of Enzymes
Common names
are formed by adding the suffix –ase to the name
of substrate
Example:
- tyrosinase catalyzes oxidation of tyrosine;
- cellulase catalyzes the hydrolysis of cellulose
Common names don’t describe the chemistry of the
reaction
Trivial names
Example: pepsin, catalase, trypsin.
Don’t give information about the substrate,
product or chemistry of the reaction
25. Principle of the international classification
All enzymes are classified into six categories
according to the type of reaction they catalyze
Each enzyme has an official international name
ending in –ase
Each enzyme has classification number
consisting of four digits: EC: 2.3.4.2
First digit refers to a class of enzyme, second -
to a subclass, third – to a subsubclass, and
fourth means the ordinal number of enzyme in
subsubclass
26. The Six Classes of Enzymes
1. Oxidoreductases
• Catalyze oxidation-reduction reactions
- oxidases
- peroxidases
- dehydrogenases
