enzymes as Biological Catalysts

Question 1

does an enzyme affect the equilibrium of a reaction?

Answer 1

no, just speeds up the rate at which the reaction reaches equilibrium

Question 2

enzymes are mostly proteins, what is the exception?

Answer 2

some types of RNA - eg. ribozymes

Question 3

how potent and specific are enzymes, generally?

Answer 3

each enzyme only has a limited no. of substrates, therefore some can distinguish stereoisomers
V. potent - can convert many substrate molecules into product per second

Question 4

how do enzymes speed up reactions, generally?

Answer 4

decrease activation energy by providing alternative pathways, stabilise transition state

Question 5

what is the transition state?

Answer 5

reaction intermediate species which has the greatest free energy (basically the point at which the reaction tips over the edge)

Question 6

what causes glycogen storage disease?

Answer 6

enzyme deficiency that results in failure of glycogen to enter transition “phosphorylated” state meaning glycogen cant be broken down resulting in low glucose

Question 7

what is the most common glycogen storage disease and how does it present?

Answer 7

Von Gierke’s Disease

hypoglycaemia, hepatomegaly, ulcers, infections, bowel problems

Question 8

how is von gierke’s disease treated?

Answer 8

slow release glucose meal

feed little and often

Question 9

what are the 2 types of cofactor?

Answer 9

metal ions (cofactors)
organic molecules (coenzymes)

Question 10

what do cofactors (metal ions) do?

Answer 10

involved in redox reactions and stabilise transition states

form a metal co-ordination centre in the enzyme (metalloprotein

Question 11

what do coenzymes (organic molecules) do?

Answer 11

many involved in redox reactions or group transfer processes (ATP, CoA)
most associate transiently with the enzyme, chaging charge or structure

Question 12

what is a prosthetic group?

Answer 12

tightly bound coenzymes (eg. haem in haemoglobin)

Question 13

give some examples of cofactors and coenzymes

Answer 13

cofactors - zinc, iron, copper

coenzymes - many derived from vitamins (i.e - vitamin deficiencies reflect loss of enzyme activity)

Question 14

name a common coenzyme for redox reactions and describe its action

Answer 14

Nicotinamide adenine dinucleotide (NAD+)
may donate or receive electrons during enzyme catalysis
easily regenerated

Question 15

give an example of an enzyme that works via induced fit

Answer 15

hexokinase

changes shape as glucose binds to active site

Question 16

name the 3 pancreatic serine proteases and their reactive sites

Answer 16

chymotrypsin - hydrophobic pocket binds aromatic amino acids
trypsin - _ve Asp binds +ve Lys or Arg
elastase - active site partially blocked, only amino acids with small or no side chains can bind

Question 17

give an example of different isozymes of an enzyme

Answer 17

lactate dehydrogenase
H (heart) form = promotes aerobic environment
M (Muscle) form promotes anaerobic environment

Question 18

what is a clinical use of isozymes? give an example

Answer 18

relative amounts of an isozyme in a specific tissue are useful for diagnostic purposes
E.g - creatine kinase (CK)
- M type in skeletal muscle
- B form in Brain
Heart produces both types (MB)
appearance of brain type in blood = stroke or tumour
appearance of heart type = heart attack

Question 19

how are enzymes regulated by phosphorylation?

Answer 19

reversible covalent modification
Via protein kinases
can convert enzyme to active or inactive form

Question 20

what is a zymogen?

Answer 20

inactive precursor of an enzyme

Question 21

how are zymogens activated, give examples

Answer 21

irreversible cleavage of a covalent bond
E.g - trypsinogen and chymotrypsinogen formed in pancreas and cleaved by enteropeptidase in small intestine to give active chymotrypsin

Question 22

give some examples of enzymes that are regulated by partial proteolysis

Answer 22

digestive enzymes (trypsin etc)
Blood coagulation enzymes (VII, IX, X, XI, XIII, thrombin etc)
Blood clot dissolving enzymes (plasminogen etc)
Programmed development enzymes (chitin synthetase, cocoonase, collagenase)