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Biochemistry for Medics

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Mechanism of action of enzymes

Presented by: Hurnaum karishma Roll number 32

Table of contents• Introduction of enzymes

• Active site

• Thermodynamic changes

• Covalent catalysis

• Acid base catalysis

• Catalysis by bond strain

• Catalysis by proximity and orientation

• Biological catalysts

• Neither consumed nor permanently altered • All enzymes are proteins in nature except ribozymes

which are RNA in nature

• Highly efficient

• Act as selective catalysts

Introduction of enzymes

Site where actual reaction occurs

Substrate –bound by weak interaction

Specificity of enzyme depend on arrangement

of atom in active site

Active site

1

• Thermodynamic changes

2• Processes at the active site

Mechanism of action of enzyme

The catalytic efficiency of enzyme is explained by 2 perspectives:

Thermodynamic changes

Substrate products

• acquire a transitional state.

• The difference in energy level of transitional state and substrate is called activational barrier.

Only a few substrate can cross this barrier to be converted to products.

That is why rate of uncatalysed reaction is much slow.

When enzyme is present it provides an alternative pathway for conversion of substrate into products.

Enzymes accelerate reaction rate by providing transition states with low activational energy for formation of products

Hence reaction rate is enhanced by many folds in the presence of enzymes

The total energy of the system remains the same and equilibrium state is not disturbed

Processes at the active site

Pro

cess

es

at

act

ive s

ite

Covalent catalysis

Acid base catalysis

Catalysis by bond strain

Catalysis by proximity & orientation

• Enzyme form covalent linkages with substrate forming transient enz-subs complex with very low activational barrier.

• Enzyme is then released unchanged and unconsumed and substrate is converted into products.

Covalent catalysis

• This process is mostly undertaken by transferases and hydrolases.

• The hydrolytic enzyme especially proteolytic enzyme works in this manner for cleavage of peptide bonds in proteins.

• Proteolytic enzymes mostly have serine at their active site so OH goup of serine makes a transient complex with COOH group of peptide bond with subsequent cleavage by water.

Acid base catalysis

• Mostly undertaken by oxido-reductases.

• Mostly at the active site, either histidine is present which act both as a proton donor and a proton acceptor.

• Sometimes aspartic acid, glutamic acid and cysteine residues are also present which participate in Hydrogen transfer reaction.

pyruvate

NAD+ NADH + H+

LactateLDH

Catalysis by bond strain

• Mostly undertaken by lyases.

• The enz-subs binding causes reorientation of the structure of substrate in the site due to being in a strain condition.

• Thus transitional state is readily acquired and enzyme maintains that transitional state where the bond is in the unfavourable state and is eventually broken.

• So enzyme induces a strain in the bond which is required to be broken.

Catalysis by proximity and orientation

• This mechanism is mostly undertaken by ligases.

• The rate of reaction is ↑ by bringing substrate closer to each other at the a.site.

• A region of high substrate conc is produced at the a.site.

• The substrate molecule is placed at bond forming distances.

• Since substrate is placed at optimal distances.

• The probability of collision ↑ and substrate is eventually converted into products.

• This mechanism involve the condensation of substrate molecule.

References :-

• Class notes• Biochemistry for

medics• Internet• Harper’s

Thank u!