Enzymes are biomolecules that catalyze chemical reactions and lower their activation energy. They have high catalytic power, increasing reaction rates by factors of 108 to 1020. Enzymes also exhibit specificity, only catalyzing particular reactions and binding intended substrates. Regulation of enzyme activity ensures metabolic reactions proceed at appropriate rates. The International Union of Biochemistry and Molecular Biology established a systematic enzyme classification system based on reaction type. Enzymes are highly specific and can distinguish between stereoisomers, catalyze single reactions, and preferentially bind certain substrates. They exist as proteins, sometimes with tightly bound cofactors, and their tertiary structures are important for catalytic function.
2. ENZYMES
Definitions--
A biomolecule either Protein or RNA, that catalyse a
specific chemical reaction, enhance the rate of a
reaction by providing a reaction path with a lower
activation energy
3. Fundamental Properties
1) Catalytic power-speeding up reactions 108 to
1020 fold.
They speed up reactions without being
used up.
2) Specificity
a) for substrate - ranges from absolute to relative
b) for reaction catalyzed
3) Regulated-- some enzymes can sense metabolic
signals.
4. Catalytic Power
Catalytic Power is defined as the Ratio of the Enzyme-Catalyzed Rate of a
Reaction to the Uncatalyzed Rate
e.g. Urease-
At 20°C, the rate constant for the enzyme-
catalyzed reaction is 3 X 104/sec
the rate constant for the uncatalyzed hydrolysis
of urea is 3 X 1010/sec
1014 is the ratio of the catalyzed rate to the
uncatalyzed rate of reaction
5. Specificity
Defined as the Selectivity of Enzymes for the Reactants Upon which They Act
In an enzyme-catalyzed reaction, none of the substrate is diverted into
nonproductive side reactions, so no wasteful by-products are produced.
6. The substances upon which an enzyme acts are
traditionally called- substrates
The selective qualities of an enzyme are collectively
recognized- specificity
The specific site on the enzyme where substrate binds and
catalysis occurs is called- active site
7. Regulation
Regulation of Enzyme Activity Ensures That the Rate of Metabolic Reactions Is
Appropriate to Cellular Requirements
essential to the integration and regulation of metabolism
Achieved by various ways
Inhibitor
Activator
Hormonal
Rate of synthesis
8. 2. Names bearing little resemblance to their activity
e.g. catalase - the peroxide-decomposing enzyme
Proteolytic enzymes (proteases) of the digestive tract
Trypsin- Gr. Word Tryein means to wear down
Pepsin- Pepsis means digestion
9. IUB nomemclature
1956 - to create a systematic basis for enzyme nomenclature
4 digit numbered code
first digit - major class
Second digit - sub class
third digit - sub sub class
final digit - specific enzyme
10. 2.7.1.1
ATP: glucose phosphotransferase
2- class name (transferase)
7- subclass name (phosphotransferase)
1- sub sub class (hydroxyl group as acceptor)
1- specific enzyme (D- glucose as phosphoryl group
acceptor)
11. Enzyme classification
Six classes
1. Oxidoreductase- transfer of reducing equivalents from
one redox system to another
e.g. Alcohol Dehydrogenase
Lactate dehydrogenase
cytochrome oxidase
13. 3. Hydrolase
cleave C-O, C-N, C-S or P-O etc bonds by adding water
across the bond
e.g. lipase
acid phosphatase
(important in digestive process)
14. 4. Lyases
cleave C-O, C-N, or C-S bonds but do so without addition of
water and without oxidizing or reducing the substrates
e.g. aldolase
fumarase
Carbonic anhydrase
15. 5. Isomerase
catalyze intramolecular rearrangements of functional
groups that reversibly interconvert to optical or
geometric isomers
e.g. Triose isomerase
phosphohexose isomerase
mutase
16. 6. Ligase
catalyze biosynthetic reactions that form a covalent bond
between two substrates utilizing ATP-ADP
interconversion
e.g. glutamine synthetase
DNA- ligase
17. Specificity
highly specific compared to other catalyst
catalyzes only specific reaction
3 types
1. Stereospecificity/ optical specificity
2. Reaction specificity
3. Substrate specificity
18. Optical specificity
able to recognise optical isomers of the substrate
Act only on one isomer
e.g. enzymes of amino acid metabolism (D & L Amino acid
oxidase)
Isomerase do not exhibit stereospecificity
19. Reaction Specificity
catalyze only one specific reaction over substrate
e.g. amino acid can undergo deamination, transamination,
decarboxylation and each is catalysed by separate
enzyme
20. Substrate specificity
specific towards their substrates
e.g. glucokinase and galactokinase- both transfer phophoryl
group from ATP to different molecule
3 types
a. Absolute
b. Relative substrate
c. broad
22. Relative substrate specificity
act on structurally related substrates
Further divide into
i. Group dependent- act on specific group e.g. trypsin-
break peptide bond between lysine and arginine,
Chymotripsin act on aromatic AA
ii. Bond specificity- act on specific bond e.g. proteolytic
enzyme, glycosidase
24. Chemical Nature &
Properties of Enzyme
Protein or RNA
Tertiary structure and specific conformation- essential
for catalytic power
Holoenzyme- functional unit
Apoenzyme & coenzyme
25. Prosthetic group Coenzyme/cofactor
Non protein molecule Non protein molecule
Tightly (covalently)
bound
Loosely bound
Stable incorporation Dissociable
Cannot be dissociated Seperable by dialysis
etc
26. Monomeric Enzyme- made of a single
polypeptide e.g. ribonuclease, trypsin
Oligomeric Enzyme- more than one
polypeptide e.g. LDH, aspartate
carbamoylase
Multienzyme complex- specific sites to
catalyse different reactions in sequence.
Only native conformation is active not
individual e.g. pyruvate dehydrogenase
27. Multienzyme Complexes and
Multifunctional Enzymes
In a number of metabolic pathways, several
enzymes which catalyze different stages of
the process have been found to be
associated noncovalently, giving a
multienzyme complex.
Examples: Pyruvate Dehydrogenase Complex;
Electron Respiratory Chain
In other cases, different activities may be
found on a single multifunctional polypeptide
chain. The presence of multiple activities is
on a single polypeptide chain is usually the
result of a gene fusion event
