WEEK 10 (Enzyme structure, classification & mechanism)

Question 1

What is the importance of enzymes?

Answer 1

• play an important role in METABOLISM, DIAGNOSIS & THERAPEUTICS
• all biochemical reactions are enzyme catalysed
• level of enzyme in blood provides diagnostic importance
• can be reused

Question 2

What are enzymes?

Answer 2

Proteins that increase the rate of reaction by lowering the energy of activation and are not altered or consumed during the reaction

Question 3

What does the Lock-and-Key Model explain?

Answer 3

• enzyme specificity
• loss of activity when enzymes denature

Question 4

What are the properties of the Lock-and-Key model?

Answer 4

• the active site has a rigid shape
• only substrates with the matching shape can fit
• the substrate is a key that fits the lock of the active site

Question 5

What is the difference between Coenzymes and Prosthetic group?

Answer 5

COENZYME = The non-protein component, loosely bound to apoenzyme by non-covalent bond. Coenzymes or cosubstrates only transiently associate with the enzyme and dissociate from the enzymes in an altered state.
[LARGE ORGANIC MOLECULE & LOOSELY BOUND TO APOENZYME]

PROSTHETIC GROUP = The non-protein component, tightly bound to the apoenzyme by covalent bonds; it is permanently associated with the enzyme
[SMALL INORGANIC MOLECULE OR ATOM & TIGHTLY BOUND TO APOENZYME]

Question 6

What are the properties of the Induced-fit model?

Answer 6

• active site is flexible & not rigid
• shapes of the enzyme, active site and substrate adjust to maximise the fit, improving catalysis

Question 7

What affects enzyme activity?

Answer 7

• Environmental conditions
• Cofactors and coenzymes
• Enzyme inhibitors

Question 8

What is the Active site?

Answer 8

The area on the enzyme where the substrate or substrates attach to

Question 9

What is a substrate?

Answer 9

The reactant at the beginning of the biochemical reaction process on which an enzyme begins its action

Question 10

What is the difference between Apoenzyme and Holoenzyme?

Answer 10

APOENZYME is inactive and is when it doesn’t contain its non-protein moiety (cofactor)

HOLOENZYME is an active enzyme with its non-protein component

Question 11

What is a Co-factor?

Answer 11

A non-protein chemical compound that is bound, either tightly or loosely, to an enzyme and is required for catalysis

Question 12

What are the two different types of Cofactors?

Answer 12

Coenzymes & Prosthetic groups

Question 13

What are the characteristics of enzyme specificity?

Answer 13

• enzymes have varying degrees of specificity for substrates
• enzymes can recognise and catalyse: a SINGLE SUBSTRATE, a GROUP of SIMILAR SUBSTRATES & a PARTICULAR TYPE OF BOND

Question 14

What are the different types of enzyme specificity?

Answer 14

Absolute = catalyse one type of reaction for a single substrate

Group = catalyse one type of reaction for similar substrates

Linkage = catalyse one type of reaction for a specific type of bond

Question 15

What happens in enzyme catalysed reactions?

Answer 15

1) When a substrate fits properly in an active site, an ENZYME-SUBSTRATE COMPLEX is formed
2) Within the active site of the Enzyme-substrate complex, the reaction occurs to convert substrate to product
3) Products are release, allowing another substrate molecule to bind to the enzyme

Question 16

Which environmental conditions affect enzyme activity?

Answer 16

• Extreme temperatures (high temp may denature the enzyme)
• pH
• substrate concentration

Question 17

How do cofactors and coenzymes affect enzyme activity?

Answer 17

Inorganic substances (Zinc, iron) and vitamins are sometimes needed for proper enzymatic activity

e.g Iron must be present in haemoglobin in order for it to pick up oxygen

Question 18

Explain the graph between rate of reaction and concentration of substrate

Answer 18

The rate of reaction increases as substrate concentration increases, at constant enzyme concentration. However, the graph plateaus and maximum activity occurs when the enzyme is saturated and all the enzyme binding sites are taken up.

Question 19

What is the difference between the two types of enzyme inhibitors?

Answer 19

COMPETITIVE INHIBITORS have the same shape as substrates therefore compete for the same site; can be overcome with an increase in substrates

NON-COMPETITIVE INHIBITORS bind to a different site on the enzyme and cause the active site to change shape thus no longer allowing the substrate to bind; no amount of extra substrates can overcome this & the effect is similar to no enzymes

Question 20

What are the properties of a competitive inhibitor?

Answer 20

• structure similar to substrate
• competes with the substrate for the active site
• has its effect reversed by increasing substrate concentration

Question 21

What are the properties of a non-competitive inhibitor?

Answer 21

• structure different than the substrate
• distorts the shape of the enzyme, which alters the shape of the active site
• prevents the binding of the substrate
• cannot have its effect reversed by adding more substrate

Question 22

What are the six functional classes that enzymes are classified into?

Answer 22

EC 1 = OXIDOREDUCTASES
EC 2 = TRANSFERASES
EC 3 = HYDROLASES
EC 4 = LYASES
EC 5 = ISOMERASES
EC 6 = LIGASES

Question 23

Each enzyme has a CLASSIFICATION NUMBER consisting of four digits, what do the digits mean?

Answer 23

first digit is CLASS
second digit is SUBCLASS
third digit is SUB-SUB CLASS
fourth digit is SPECIFIC NAME

Question 24

What is the function of Oxidoreductases?

Answer 24

Catalyse oxidation/reduction reactions and act on many chemical groupings to add or remove hydrogen atoms

Question 25

What is the function of Transferases?

Answer 25

Transfer functional groups between donor and acceptor molecules

Question 26

What is the function of Hydrolases?

Answer 26

Catalyse the hydrolysis of various bonds and add water across a bond

Question 27

What are the functions of Lyases?

Answer 27

• Cleave various bonds by means other than hydrolysis and oxidation
• Add water, ammonia or carbon dioxide across double bonds or remove these elements to produce double bonds

Question 28

What are the functions of Isomerases?

Answer 28

• Catalyse isomerisation changes within a single molecule
• Carry out many kinds of isomerisation including L to D isomerisations and Mutase reactions (shifts of chemical groups)

Question 29

What is the function of Ligases?

Answer 29

Join two molecules with covalent bonds and catalyse reactions in which two chemical groups are joined with the use of energy from ATP

Question 30

How does Plasma enzyme level lead to the prognosis of a disease?

Answer 30

Diseases cause tissue damage including the RUPTURE of plasma membranes and LYSIS of cells in the tissue. As a result, damaged cells release their contents into the blood plasma causing an increase in concentration of enzymes. The level of specific enzymatic activity in the plasma correlates with the extent of tissue damage thus making the elevation of particular enzyme activity in the blood plasma useful in evaluating EXTENT of TISSUE DAMAGE, RESPONSE to therapies and PROGNOSIS for the patient.

Question 31

What limits the diagnostic value of enzymes in the blood plasma?

Answer 31

Increases in plasma levels of enzymes with a wide tissue distribution which provides a less specific indication of the site of cellular injury and limits their diagnostic value