lecture 5

enzymes

biological catalysts that speed up the rate of the chemical reaction without being consumed in the reaction

2 types of enzymes

proteinsenzymes and ribozymes

Proteinsenzymes

are globular proteins

Ribozymes

ribo nucleic acids enzymes that catalyze specific reactions

Enzymology

the study of enzymes

enzymologists

the scientists who study or work on enzyme

enzyme kinetics

is the study of the rates of enzyme catalyzed reactions.

characteristics of enzymes

1. enzyme speed up the reaction by lowering the activation energy

2. they speed up the rate of the chemical reaction without being consumed in the reaction

3. does not affect the nature and properties of the end products

4. highly specific in their action

Substrate (S)

reactant which binds to enzyme form an enzyme-substrate complex (ES)

Product(P)

end result of reaction

active site

a region on an enzyme (E) that binds substrate.a chang-+* in the shape of protein affect the shape of the active site and function of the enzyme

activation energy

the minimum amount of free energy needed to activate or energize the atoms of the reactants

the transition state

transitory of molecular structure in which the molecule are no longer a substrate but not yet a product

transition state complex for the reaction (ES)

enzymes increase reaction rates by decreasing the amount of energy required to form a complex of reactants(ES) that is competent to produce reaction products

enzyme activity

the amount of the enzyme that will convert a certain amount of S to P in a specified period of time under conditions of constant temprature and pH

standard unit of enzyme activity

The International Unit (IU)

IU

the amount of enzyme that can convert one mole of substrate into product per minute at 25C

Specific activity

the number of enzyme units per milligram of protein

Katal

the number of moles of substrate transformed into product per second at 25C

Turnover number

the number of moles of substrate transformed per minute per mole of enzyme

theories of enzyme substrate binding

lock and key theory

induced fit theory

lock and key theory

the active site of the enzyme has a shape that is complementary to the substrate

induced fit theory

the active site is not fully formed . the active site of the enzyme is not complementary to the substrate.

types of reaction they catalysed

1. oxidoreductases

2. transferases

3. isomerases

4. lyases

5. hydrolases

6. ligases

Oxidoreductases

add or remove hydrogen atoms or electrons (oxidation-reduction reaction)

Transferases

transfer a functional groups from one molecule to another

Isomerases

Catalyze conversion of one isomer to another by transferring a functional group from one position to another position within the same molecule

Lyases

Breaks chemical bond without adding water

Hydrolases

Catalyze hydrolysis of substrate by addition of water

Ligases

Catalyze the synthesis of new chemical bond, using ATP. Link two substrates together; forming C-C,C-S,C-N

Types of protein enzyme

1. Simple enzyme

2. Complex ( conjugated enzyme)

Simple protein

Formed of protein only

Complex (conjugated enzyme)

Require an additional chemical component

complex protein

1. Protein part: Apo enzyme

2. Non-protein part : cofactor

3. The whole complex enzyme is Holoenzyme

Two types of Cofactor

1. Coenzyme

2. Prosthetic group

Coenzyme

An organic molecule, loosely attached to enzyme. They are mainly vitamin B derivatives

Example of Coenzyme

NAD and FAD

Prosthetic group

Can be either metal or an organic molecule that is tightly bound to the enzyme molecule either covalent or non-covalent

Factors affecting the enzyme

1. Temperature

2. pH

3. Enzyme concentration

4. Substrate concentration

5. Inhibitors

6. Positive and Negative Allosetric Effectors

Effects of temperature

Rate of reaction increases gradually with the rise in temperature until it reach a maximum at a certain temperature called optimum temperature they become inactive at a very low temperature and denature at very high temperature

Effect of pH

Change of pH above or below optimum pH decreases rate of enzyme action

Optimum pH of pepsin

1.5-2

Optimum pH of pancreatic

7.5-8

Optimum pH of salivary amylase

6.8

Effect of enzyme concentration

The rate of enzyme action is directly proportional to the concentration of enzyme, provided that there are sufficient supply of substrate and constant conditions

Effect of substrate concentration

The rate of reaction increases as the substrate concentration increases up to a certain point at which their action rate is maximum

Vmax

The enzyme is completely saturated with the substrate. Any increase in substrate concentration doesn’t affect the reaction rate.

Michaelis constant (Km)

The michaelis menten constant which is the substrate concentration at half of the vmax

Michaelis-Menten Kinetics

The rate of an enzymatic reaction will increase as substrate concentration increases

Michaelis-Menten equation

V= Vmax[S] /Km+[S]

Inhibitors

1. reversible

2. Irreversible

Reversible inhibitions

1. Competitive

2. Uncompetitive

3. Non competitive

Irreversible

Binds via interaction, string covalent bonding, prevents catalytic activity of enzyme

Competitive inhibitors

The inhibitors compete with the substitute for the active site. The inhibitor resemble the substrate, therefore taking its place and binding to the active site. Inhibitor can be reversed by increasing the concentration of the substrate.

Non competitive inhibitors

Bind as a site other than the active site of the enzyme, causing the enzyme to change its shape which interns alters the active site. Can be reversed when inhibit leave.

Uncompetitive inhibitors

Binds to the enzyme substrate complex, not to enzyme, can be reversed when the inhibitor leaves enzyme substrate complex

Allosteric Effectors

substance bind at allosteric site of enzyme and regulates regulates enzyme activity

Allosteric site

Additional site other than the active site

Positive Allosteric effectors

Enzyme activity is increased

Negative Allosteric effects

Enzyme activity is decreased

Salivary amylase

1. Mouth

2. Source = saliva

3. Substance = starch

4. Optimum pH= 6.7

5. Product = maltose

Pepsin

1. Stomach

2. Source=gastric glands

3. Substrate=protein

4. Optimum pH=1.6-2.4

5. Products=shorter polypetides

Pancreatic amylase

1. Duodenum

2. Source = pancreatic juice

3. Substrate = starch

4. Optimum pH = 6.7-7.0

5. Products = maltose, maltriose, and oligosaccharides

Trypsin, Chymotrypsin, Carboxypeptidase

1. Small intestine

2. Source = pancreatic juice

3. Substrate = polypeptides

4. Optimum pH = 6.7-7.0

5. Products = Amino Acids, dipeptides and tripeptides

Pancreatic lipase

1. Small intestines

2. Source = pancreatic juice

3. Substrate = Triglycerides

4. Optimum pH = 8.0

5. Products = fatty acids and monoglycerides

Maltase

1. Small intestines

2. Source = brush boarder of epithelial cells

3. Substrate = maltose

4. Optimum pH = 5.0-7.0

5. Products = glucose + glucose

Sucrase

1. Small intestines

2. Source = brush boarder of epithelial cells

3. Substrate = sucrose

4. Optimum pH = 5.0-70

5. Products = glucose + fructose

Lactase

1. Small intestines

2. Source = brush boarder of epithelial cells

3. Substrate = lactase

4. Optimum pH = 5.8-6.2

5. Products = glucose + galactose

Aminopeptidase

1. Small intestine

2. Source = brush boarder of epithelial cells

3. Substrate = polypeptides

4. Optimum pH= 8.0

5. Products = amino acids, dipeptides, tripeptides