L11 Properties Of Enzymes And Enzyme Kinetics

Front 1

What are enzymes?

Back 1

• Biological catalysts which speed up the rate ofa reaction, without altering the final equilibriumbetween reactants and products.

• Extremely efficient, e.g. the enzyme catalasecatalyses the breakdown of its substrate H2 O2to water at a rate 1014 times faster than theuncatalysed reaction at 30C.

Front 2

What is the Effect of enzyme on activationenergy of a reaction?

Back 2

Front 3

What is the Effect of an enzyme on a biologicalreaction?

Back 3

Increases the reaction velocity

Lower Km value

Front 4

What are enzymes responsible for?

Back 4

Enzymes are responsible for the controlled“combustion” (oxidation) of foodstuffs

Front 5

What does enzyme specificity mean?

Back 5

Enzymes will usually catalyse only one type of reaction (e.g.alcohol dehydrogenase liver will oxidise primary alcohols toaldehydes) and will act on only a few related molecules(‘group specificity’ e.g. the above enzyme will oxidisemethanol, ethanol and propanol).

Front 6

Will some enzymes only act on one substrate?

Back 6

A few enzymes are so specific they will act on one substrate

Front 7

Can enzymes distinguish between isomers?

Back 7

If a natural compound can exist in two stereoisomer forms e.g.D-glucose and L-glucose, the enzyme concerned with itsmetabolism in the cell will usually act only on one isomer.

Front 8

What is specificity determined by?

Back 8

Specificity is determined by the presence of a groove or cleftof defined shape called the ‘active site’ into which only thesubstrate of the correct shape and charge can fit

Front 9

What can be the consequence of enzyme specificity?

Back 9

A group of enzymes present together in one compartment of acell,

e.g. the cytoplasm of muscle cells, can give rise to acomplex and co-ordinated metabolic pathway in which the initialsubstrate D-glucose is converted through a sequence ofspecific enzyme catalysed reactions to the product lactic acid

Front 10

How are enzymes classified?

Back 10

Specificity of enzymes has led to a systematic classificationscheme, established by the I.U.B. Commission on Enzymes

Front 11

What are the 6 main classes enzymes are divided into?

Back 11

Enzymes are divided into 6 main classes according to thetype of reaction they catalyse e.g. Class 1, oxidoreductases,contains the enzymes catalase and alcohol dehydrogenase.

Front 12

How are the six classes then further divided?

Back 12

Six classes are then further divided into subgroupsaccording to their substrate or source.• Each enzyme is identified by its own individual 4 digitnumber (e.g. catalase is E.C. 1.11.1.6.)

Front 13

What do oxidoreductases do?

Back 13

Oxidoreductases catalyze the transfer ofhydrogen atoms and electrons

Example - Lactate Dehydrogenase

Front 14

What do transferases do?

Back 14

Transferases catalyze the transfer of functionalgroups from donors to acceptors

Example - Alanine aminotransferase

Front 15

What do lyases catalyse?

Back 15

Lyases catalyze the cleavage of C-C, C-O, or C-Nbonds

(addition of groups to double bonds or formation of doublebonds by removal of groups) Example - ATP-citrate lyase

Front 16

What do hydrolases do?

Back 16

Hydrolases catalyze the cleavage of bonds bythe addition of water (hydrolysis)

Example - Trypsin

Front 17

What do isomerases do?

Back 17

Isomerases catalyze the transfer of functionalgroups within the same molecule

Example - Phosphoglucose isomerase

Front 18

What do Ligases do?

Back 18

Ligases use ATP to catalyze the formationof new covalent bonds19

• Example - DNA ligase

Front 19

What type of compound are enzymes?

Back 19

Enzymes are proteins

i.e. they are composed of one ormore polypeptide chains folded into a complex 3-dimensional shape

Front 20

What is enzyme structure stabilised by?

Back 20

Enzyme structure is stabilised by many weak bonds e.g.H-bonds, electrostatic salt links, hydrophobic interactions

Front 21

Are these bonds easily broken?

Back 21

These weak bonds are easily broken e.g. by heating theprotein, giving rise to a disorganised or tangled structure inwhich the enzyme no longer has any catalytic activity(inactive, ‘denatured’ state).

Front 22

What does this property mean for the enzymes?

Back 22

This property makes enzymes very sensitive to changes intheir environment

Front 23

What does the active site contain to stabilise the transition state of the reaction?

Back 23

The active site of the enzyme contains functional groupsthat stabilize the transition state of the reaction

Front 24

What is the “Lock and Key” model for enzyme catalysis?

Back 24

Front 25

What is the induced fit model of enzyme catalysis?

Back 25

Front 26

What is the Effect of temperature on enzymereactions?

Back 26

Front 27

What is the effect of pH on enzyme activity?

Back 27

Front 28

What are some inorganic elements that act ascofactors for enzymes?

Back 28

• Cu2+ — Cytochrome oxidase

• Fe2+/3+— Cytochrome oxidase, catalase, peroxidase

• K+ — Pyruvate kinase

• Mg2+ Hexokinase, G-6-phosphatase, pyruvate kinase

• Ni2+ — Urease

• Se — Glutathione peroxidase

• Zn2+ — Carbonic anhydrase, alcohol dehydrogenase,carboxypeptidase

Front 29

What are some iron containing enzymes?

Back 29

Iron-sulphur centre

e.g flavin enzymes

Heme group

e.g cytochrome oxidase

Front 30

What are some co-enzymes?

Back 30

CH3 CH2 OH + NAD+ ----> CH3 CHO + NADH + H+ alcohol dehydrogenase

succinate + FAD ----> fumarate + FADH2 succinate dehydrogenase

glucose + ATP ----> glucose-6-phosphate + ADP glucokinase

Front 31

What are isoenzymes?

Back 31

Describes enzymes with different protein structureswhich catalyse the same reaction.

Front 32

What are they coded for by?

Back 32

Coded for by different genes

Front 33

Where are different isoenzymes found?

Back 33

Different isoenzymes often found in differentcellular compartments (cytoplasm or mitochondria),or different amounts in different tissues

Front 34

What are some examples?

Back 34

•Distinct biochemical roles

•e.g. hexokinase /glucokinase. Lactatedehydrogenase