1

ENZYMESOutline

DefinitionCharacteristics of enzymes

Types of enzymesFactors effecting enzyme activity

The definition & Characteristics of enzymes

• A protein that acts as a catalyst, speeding the rate at which a biochemical reaction proceeds. a type of protein

Characters:

• a catalyst• Effective in smaller quantities• efficient and specific• reaction can be reversed• activities affected by surroundings• need helpers – cofactors/prostethic grps• involve in multiple steps of biochemical pathways

Classification of enzymes

6 main classes according to International Union of Biochemistry and Molecular Biology (IUBMB):

1. oxidoreductase2. transferase

3. hydrolase4. lyase

5. isomerase6. ligase

• Function: catalyzes oxidation-reduction reactions• e.g. alcohol dehydrogenase

• Other e.g. Biliverdin reductase; Glucose oxidase

2

• Function: catalyzes reactions involving transfer of functional groups

• e.g. Hexokinase

• Other e.g. Glycoaldehyde transferase; DNA nucleotidylexotransferase

• Function: catalyzes hydrolytic reactions involving use of water mol.

• e.g. Triacylglycerol lipase

• Other e.g. -amino acid esterase; Oxaloacetase

H2O

• Function: catalyzes cleavage of C-C, C-O, C-N and other bonds by other means than by hydrolysis or oxidation

• e.g. Lysine decarboxylase

• other e.g.: threonine aldolase [EC 4.1.2.5]; cystine lyase

• Function: catalyzes intramolecular arrangement• e.g. Maleate isomerase

• Other e.g. Inositol-3-phosphate synthase; Maltose epimerase]

• Function: catalyzes the joining of two molecules with concomitant hydrolysis of the diphosphate bond in ATP or a similar triphosphate

• e.g. Pyruvate carboxylase

• Other e.g. GMP synthase; DNA ligase

Enzyme as protein

• exhibits characteristics like other proteins• primary structure

amino acid sequence

e.g.: human pancreatic lipase (467 amino acids)N-Met1-…-Ser171-...-Asp194-...-His281-…-Cys467-C

human trypsin (247 amino acids) N-Met1-…-His63-…-Asp107-…-Ser200-…-Ser247-C

3

Lysozyme’s tertiary structure

Anti-parallel

-sheet

(3)

-helix

(5)

Aspartate carbamoyltransferase’s quartenary structure

2 catalytic trimers

3 regulatory dimers

How enzyme reacts

E + S ES E + P

Enzyme reaction…hypothesis

• lock and key hypothesis: proposed by Emil Fischer (1894)

perfect match like specific key & lock

4

• induced-fit hypothesis: proposed by Daniel Koshland

(1958) shows elasticity/flexibility

at active site well-accepted

e.g.: hexokinase reaction

Factors affecting enzyme activity

• enzyme concentration

• substrate concentration

• pH

• temperature

• inhibitors

5

A

B

Change in pH effects the pull/push force of polar/nonpolar intramolecules which change the enzyme shape as well as active site – in acidic conditions, basic grps are protonized while in basic conditions, acidic grps are deprotonized.

AC

B

6

• substances which bind to enzyme & disrupt the enzyme activity by

blocking the production of ES-complex or E + P

• reversible & irreversible

• involves the noncovalent links between inhibitor and enzyme

• 2 types:

competitive inhibitor

noncompetitive inhibitor

uncompetitive inhibitor

Reversible inhibitors

competitive inhibitor

mol. similar to substrate

compete with substrate for active site

e.g.: succinate dehydrogenase (E); succinate (S); malonate (I)

noncompetitive inhibitor

mol./ion attaches to second site (other than active site) at enzyme surface

e.g.: prostaglandin synthase (E); arachidonate (S); aspirin (I)

uncompetitive inhibitor

binds to ES complex, forming an inactive ESI complex

e.g..: polymerase (E); nucleic acid (S); nevirapine (I) - HIV

• covalently bonded – react with functional grp at active site, blocking active site from substrate rendering the enzyme inactive

• mostly are toxic substances

• e.g.: (see attached list)

Irreversible inhibitors

7

8

9

THANK YOUTHANK YOU