Download enzymes lecture 4

Enzymes
Presented By
Dr. Salwa Abo El-khair
Catalytic Proteins 2: Enzymes
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Enzyme Specificity
1- Relative, low or bond specificity
The enzyme acts on substrates that are similar in
structure and contain the same type of bonds e.g.
 Amylase acts on α 1-4 glycosidic, bonds in
starch, dextrin and glycogen.
 Lipase hydrolyzes ester bonds in different
triglycerides.
2- Moderate, structural or group specificity
The enzyme is specific not only to the type of bond
but also to the structure surrounding it.
 Pepsin is an endopeptidase that hydrolyzes
central peptide bonds in which the amino group
belongs to aromatic amino acids e.g. phenyl
alanine, tyrosine and tryptophan.
 Trypsin is an endopeptidase that hydrolyzes
central peptide bonds in which the amino group
belongs to basic amino acids e.g. arginine, lysine
and histidine.
 Chymotrypsin is an endopeptidase that
hydrolyzes central peptide bonds in which the
carboxyl group belongs to aromatic amino acids.
 Aminopeptidase is an exopeptidase that
hydrolyzes peripheral peptide bond at the amino
terminal (end) of polypeptide chain.
 Carboxypeptidase is an exopeptidase that
hydrolyzes peripheral peptide bond at the
carboxyl terminal of polypeptide chain.
Specificity
3- Absolute, high or substrate specificity
The enzyme acts only on one substrate e.g.
 Uricase acts only on uric acid.
 Arginase acts only on arginine.
 Carbonic anhydrase acts only on carbonic acid.
 Lactase acts on lactose.
 Sucrase acts on sucrose.
 Maltase acts on maltose.
4- Optical or stereo-specificity
the enzyme is specific not only to the substrate but
also to its optical configuration e.g.
 L amino acid oxidase acts only on L amino
acids.
 D amino acid oxidase acts only on D amino
acids.
 α- glycosidase acts only on α- glycosidic bonds,
which are present in starch, dextrin and glycogen.
 β- glycosidase acts only on β- glycosidic bonds
that are present in cellulose.
5- Dual specificity
There are two types of dual specificity:
A) The enzyme may act on two substrates by
one reaction type. e.g.
xanthine oxidase enzyme acts on xanthine and
hypoxanthine (two substrates) by oxidation
(one reaction type).
xanthine oxidase
Hypoxanthine
xanthine
xanthine oxidase
Xanthine
uric acid
B) The enzyme may act on one substrate by two
different reaction types e.g.
isocitrate dehydrogenase enzyme acts on isocitrate
(one substrate) by oxidation followed by
decarboxylation (two different reaction types).
isocitrate dehydrogenase
Isocitrate
α-ketoglutarate
CO2
NAD
NADH+H
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Ask Questions!
AP Biology
2007-2008
Activity
 1- An enzyme that hydrolyzes protein will not act upon
starch. This fact is an indication that enzymes are
(A) hydrolytic
(B) specific
(C) catalytic
(D) synthetic
 2- Which of the following enzymes would digest a fat?
(A) sucrase
(B) protease
(C) Ligase
(D) lipase
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Activity
 3- Amylase has :
A) Relative specificity.
B) Moderate specificity.
C) Absolute specificity.
D) High specificity
4- enzymes of low specificity:
a) Amylase & lactase
b) Amylase & lipase
c) Lactase & lipase
d) Lactase & sucrase
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 1- Enumerate the levels of enzyme specificity
 2- Discuss with 3 examples group (moderate )
specificity.
 3- Discuss with 2 examples dual specificity
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