Enzymes as Biological Catalysts

Question 1

What is the role of enzymes?

Answer 1

Enzymes catalyse the many chemical reactions which together make up the process of metabolism.
An enzyme speeds up the rate at which a reaction reaches equilibrium.
E.g. CO2 + H2O - H2CO3
This reaction is catalysed by the enzyme carbonic anhydrase.
Enzymes do not affect the equilibrium position of reaction.

Question 2

What is the definition of metabolism?

Answer 2

Metabolism is the sum of all catabolic and anabolic reactions in the body.

Question 3

What are properties of enzymes?

Answer 3

Catalysts.
Enzymes are mostly proteins - exception: some types of RNA-ribozymes-are catalysts
Efficient
- work at body temperature, in aqueous solution, near neutral pH
- Can increase the rate of a reaction by a factor of up to 10 to the power of 20.
Specific
- each enzyme has a limited range of substrates
- Some can distinguish stereoisomers
Potent
- each enzyme molecule can convert many substrate molecules into product per second..

Question 4

What are Stereoisomers?

Answer 4

Stereoisomers are molecules that are mirror images of each other and cannot be superimposed but in every other respect are identical.

Question 5

What is the activation energy barrier?

Answer 5

A reaction must overcome the activation energy barrier in order for a reaction to take place.
The maximum at which the reactants interact with each other is called the transition state.
Transition state is the least stable state

Question 6

What is the general concept of enzyme activity?

Answer 6

Enzymes specifically bind and stabilise the transition state.
The transition state is the reaction intermediate species which has the greatest free energy.
Enzymes reduce the activation energy by providing alternative reaction pathways.
(Enzymes bring molecules really close together, forcing them to react, lowering the activation energy)

Question 7

What is Glycogen storage disease?

Answer 7

This is where there is an enzyme deficiency that results in failure of glycogen to enter transition “phosphorylated” state.
This affects glycogen’s ability to enter the many pathway’s that it is involved in.
Defective glycogen synthesis/breakdown in muscle, liver and kidney.
11 variants arising from defects in 12 glycogen or glucose metabolising enzymes.
The most common is Von Gierke’s Disease
Symptoms
- Hypoglycaemia
- Hepatomegaly (liver swelling)
- Skin and mouth ulcers
- Bacterial and fungal infection
- Bowel inflammation and irratibility
Treatment
- Slow release glucose meal (e.g. corn -starch)
- Feed little anfd often = “mimic” glycogen conversion to glucose

Question 8

What are Coenzymes and Cofactors and how do they interact and affect enzymes?

Answer 8

Catalyit activity of many enzymes depends on the presence of small molecules, called cofactors or coenzymes.
2 Types:
- metal ions (inorganic, termed cofactors)
- Organic molecules (organic origin, termed coenzymes)
Metal cofactors form a metal coordination centre in the enzyme.
The enzyme may be referred to as a “metalloprotein”.
Coenzymes mostly associate with the enzyme only transiently.
Coenzymes change charge or structure during the course of the reaction, but are regenerated.
Tightly bound coenzymes are called prosthetic group - e.g. haem in haemoglobin and cytochromes
Enzyme without cofactor is called apoenzyme.
Enzyme with cofactor is called haloenzyme.
Apoenzyme + cofactor = haloenzyme.

Question 9

What are some examples of Cofactors?

Answer 9

Metal ions
- e.g. zinc, iron, copper
- involved in redox reactions
- stabilise transition states
Coenzymes
- many are derived from vitamins
- many involved in redox reactions NAD+, FAD
- Others involved in group transfer processes CoA (=Coenzyme A) transfers acetyl groups. ATP transfers phosphate groups

(The recycling of theses cofactors allows anaerobic respiration to take place)

Question 10

What is the role of coenzymes as vitamins?

Answer 10

Most vitamins function as coenzymes
- symptoms of vitamin deficiencies reflect the loss of specific enzyme activities. May be dietary or functional (e.g. drug or disease-induced deficiencies).

Question 11

How does a Substrate bind to an enzyme and what is an active site?

Answer 11

Substrate binds to an active site
- often a cleft or a crevice of the enzyme
- Contains amino acids essential for catalytic activity
- Contains amino acids for highly specific interactions
Lock-and-key model.
Active site of unbound enzyme is complementary to the shape of the substrate.
Induced fit model.
Binding of substrate induces a conformational change in enzyme, results in complementary fit.
e.g. Hexokinase. It changes shape as the glucose binds to the active site. Co-substrate is ATP.

Question 12

What are some examples of active sites?

Answer 12

3 pancreatic serine proteases contain reactive serine residue and catalyse hydrolysis of peptides at specific sites

• Chymotrypsin: hydrophobic pocket binds aromatic amino acids
• Tripsin: negatively charged Asp interacts with positively charged Lys or Arg.
• Elastase: active site partially blocked, only amino acids with small or no side chains can bind.

Question 13

Temperature and pH affect enzyme activity

Answer 13

Relationship of enzyme activity to environment
Up until the optimum temperature (the temperature at which the enzyme works most efficiently) as the temperature increases the activity of the enzyme increases. Beyond this point as the temperature increases, the enzyme denatures and the activity begins to decrease.
The same pattern arises with pH in regards to enzymes.

Question 14

What are Isoenzymes, when are they created and where are they present in the body?

Answer 14

Isoenzymes are isoforms of enzymes, they catalyse the same reaction but have different properties and structures (and sequence).
Different isozymes can be
- synthesised during different stages of foetal and embryonic development
- present in different tissues
- present in different cellular locations
(Because they have slightly different properties they require different conditions for reactions to take place)

Question 15

How are isozymes relevant in lactate dehydrogenase?

Answer 15

Example: Lactate Dehydrogenase has 2 subunits:
H(Heart) - promotes aerobic metabolism
M(Muscle) - promotes anaerobic metabolism.
LDHA subunit is made in response to hypoxia.
LDHB is made in response to an oxygenated environment.
Depending on how hypoxic or oxygenated the environment affects what isozyme is created and so affects whether aerobic metabolism, anaerobic glycolysis or an intermediate activity is going to take place.

Question 16

What is a clinical use for isozymes?

Answer 16

Relative amounts of isozymes in tissue are useful for diagnostic purposes.
With LDHB and LDHA being hypoxia sensitive it could indicate that there is an issue.
Reasons for hypoxia could be
-stroke
-heart disease
-cancer
Relative amounts of isozymes in blood are useful for diagnostic purposes.
Creatine kinase (CK) is a dimeric protein which binds to the muscle sarcomere
- M form is produced in skeletal muscle (MM)
- B form is produced in brain (BB)
- Heart produces both types, forms a heterodimer (MB)
- Appearance of brain type in blood suggests stroke or tumour
- Appearance of heart type suggests heart attack
More than 20 enzymes are typically used in the clinical lab to diagnose disease.

Question 17

How is enzyme activity regulated by phosphorylation?

Answer 17

An example of reversible covalent modification
- side groups of serine, threonine and tyrosine can form phosphate esters
Protein-OH + ATP = Protein-O-PO3(2-) + ADP (reversible reaction)
Can convert enzyme to active or inactive form
- e.g. glycogen phosphorylase
- phosphorylation converts inactive a form to active b form
Phosphorylation reactions are carried out by protein kinases
- very fast, reversible and found in all cells, especially involved in energy generating processes.

Question 18

How does irreversible covalent modification regulate enzyme activity?

Answer 18

Result in activation of enzymes.
Zymogens - inactive precursors of an enzyme - are irreversibly transformed into active enzymes by cleavage of a covalent bond
- in pancreas: trypsinogen and chymotrypsinogen, inactive precursors, are formed
- in small intestine: enteropeptidase cleaves trypsinogen to form active trypsin which cleaves chymotrypsinogen to form active chymotrypsin
Other examples
- digestive enzymes, blood-clotting enzymes, clot-dissolving enzymes.
(Tryppsinogen and chymotrypsinogen are formed in the pancreas and cause damage if active as they are proteolytic enzymes).

Question 19

Nicotinamide adenine dinucleotide (NAD+) is a common coenzyme for redox reactions. What is i’s role?

Answer 19

NAD+ = NADH, this reaction is reversible.

NAD+ may donate or receive electrons during enzyme catalysis and is easily regenerated.

Question 20

What are some examples of irreversible proteolytic enzymes?

Answer 20

Some enzymes are regulated by partial proteolysis.
Digestive enzymes
- Trypsin
- Chymotrypsin
- Carboxypeptidase A and B
- Elastase
- Pepsin
- Phospholipase
Blood coagulation Enzymes
- Factors VII, IX, X, XI and XIII
- Kallikrein
- Thrombin
Enzymes involved in dissolving blood clots
- Plasminogen
- Plasminogen activator
Enzymes Involved in programmed development
- Chitin synthetase
- Cocoonase
- Collagenase

Question 21

At the end of the powerpoint there are some questions to help study this topic

Answer 21

LOOK AT THEM!!!!!!

Question 22

What does transiently mean?

Answer 22

Lasting for a short time/Impermanent