Physiology enzymes

Question 1

What is an enzyme

Answer 1

Enzymes are biological catalysts
* Provide a reaction surface (Active Site)
* Provide a suitable environment (e.g. hydrophobic)
* Bring reactants together
* Position reactants correctly for reaction
* Weaken bonds in the reactants
* May provide acid / base catalysis
* May provide nucleophiles
* Do not change equilibrium position of reaction

Question 2

what is required for a reaction to occur (hint Gibbs)

Answer 2

Every chemical reaction in body accompanied by change in free energy, ΔG
* For reaction to proceed it must exhibit a negative ΔG
* ΔG = (free energy of initial state) – (free energy of final state)
* To reach final state reaction must first reach activation energy
* Enzymes decrease reaction energy = increased rate of reaction

Question 3

what is an active site

Answer 3

Active site is a region within an enzyme that fits
the shape of substrate molecules

Question 3

describe how active site binds to substrate

Answer 3

• Amino acid side-chains align to bind the substrate
  through H-bonding, hydrophobic interactions, etc.
• Products are released when the reaction is
  complete (they no longer fit well in the active site)

Question 4

what is the lock and key hypothesis

Answer 4

• Active site has rigid shape
• Fit between substrate and active site is exact
• Temporary structure called enzyme-substrate complex formed
• Products have different shape from substrate, and are released from
  active site
• Enzyme then free to bind another substrate

Question 5

what is the induced fit hypothesis

Answer 5

• Active site is flexible and can change its conformation
• Shape of enzyme, active site, and substrate adjust to maximise fit
• Changed environment results in improved catalysis
• Greater range of substrate specificity, reacting with substrates of similar
  type

Question 6

at which pH do enzymes woek at optimum rate and why

Answer 6

Enzymes are most active at optimum pH (~7.4)
* Amino acids with acidic or basic side-chains have the
proper charges when the pH is optimum
* Activity is lost at low or high pH as tertiary structure is
disrupted

Question 7

Give examples of exceptions to the 7.4 PH

Answer 7

Enzyme Location Substrate pH optima
Pepsin Stomach Peptides 2
Urease Liver Urea 5
Sucrase S. Intestine Sucrose 6.2
Amylase Pancreas Amylose 7
Trypsin S. Intestine Peptides 8
Arginase Liver Arginine 9.7

Question 8

what is the basis for the pH

Answer 8

Changing the pH alters the ionization state of amino acid side chains
* e.g. Lys, Arg, His, Asp and Glu
* Alters ionic bonding, structural stability, shape and functionality of enzyme
* Related to the overall 3-D structure of all enzymes.

Question 9

what is the optimum temprature for enzyme activity

Answer 9

Enzymes are most active at an optimum temperature (usually 37°C)

Question 10

why do enzymes only function at optimum temp

Answer 10

They show little activity at low temperatures
* Activity is lost at high temperatures as denaturation occurs

Question 10

why do enzymes only function at optimum temp

Answer 10

They show little activity at low temperatures
* Activity is lost at high temperatures as denaturation occurs

Question 10

why do enzymes only function at optimum temp

Answer 10

They show little activity at low temperatures
* Activity is lost at high temperatures as denaturation occurs

Question 11

what is a cofactor

Answer 11

Active enzyme / Holoenzyme:
* Polypeptide portion of enzyme (apoenzyme)
* Nonprotein prosthetic group (cofactor)
* Cofactors bound to enzyme to maintain correct configuration of active site
* Metal ions and other inorganic factors

Question 12

what is a coenzyme

Answer 12

• An organic molecule bound to the enzyme by
  weak interactions / Hydrogen bonds
• Most coenzymes carry electrons or small groups
• Many have modified vitamins in their structure

Question 13

wjat is the difference between a cofactor and coenzyme

Answer 13

Coenzyme Cofactor
Meaning
It carries chemical groups between enzymes They bind to an enzyme
Also known as
Cosubstrates Helper molecules
Bind
Coenzyme loosely bound to enzymes
Some cofactors covalently bound the enzyme
Removal
Can be easily removed
It can be removed only by denaturation
Form
Chemical molecule Chemical compound
Characteristic
Organic substances Inorganic substances
Types
It is a type of cofactor
Two types of cofactors: Coenzyme and prosthetic groups
Function
They act as carriers Increase the speed of reaction
Examples
Biotin, Vitamin, Coenzyme A Metal ions such as K+, Zn2+

Question 14

what are the different enzyme classifications

Answer 14

• Oxidoreductases
• Transferases
• Hydrolases
• Lyases
• Isomerases
• Ligases/synthases

Question 15

what do Oxidoreductases catalyse

Answer 15

Catalyse transfer of electrons from donors to acceptors
* Broad class of enzymes that catalyze the many oxidation-reduction
reactions found in biochemical pathways.

Question 16

what do Transferases catalyse

Answer 16

Catalyse transfer of a specific functional group between molecules.
* Kinases transfer phosphate groups, usually from ATP to another molecule
(e.g. protein kinases that phosphorylate protein hydroyxl groups)
* Aminotransferases transfer amino groups, important in amino acid
metabolism
Transferases
* Glycosyltransferases - transfer carbohydrate residues

Question 17

What does Hydrolases catalyse

Answer 17

Catalyse cleavage of bonds by the addition of a water molecule.
* Phosphatases – Hydrolyse phosphoric ester bonds
* Lipases Hydrolyse ester bonds in lipids
* Peptidases – Hydrolyse peptide bonds to form two products

Question 18

what does Lyases catalyse

Answer 18

Catalyse the addition of water, ammonia or CO2 to double bonds, or
remove them to create double bonds
* Cleave bonds by means other than hydrolysis or oxidation.
* Decarboxylases Remove CO2
* Dehydrases Remove H20
* Deaminases Remove NH3

Question 19

what does Isomerases catalyse

Answer 19

Catalyse interconversion of isomeric forms of a molecule by
transferring groups within the same molecule
* Conversion of Cis and Trans molecules
* Conversion of D and L isomers

Question 20

what do ligases catalyse

Answer 20

Catalyse synthesis of new covalent bonds using ATP energy
(aka Synthetases)

Question 21

explain the concept of enzyme specificity

Answer 21

Enzymes have varying degrees of specificity for substrates
* Enzymes may recognize and catalyze:
- a single substrate
- a group of similar substrates
- a particular type of bond

Question 22

what are the different forms of enzyme specificity

Answer 22

Absolute: Catalyses one type of reaction for a single substrate
e.g. Urease catalyses only hydrolysis of urea
Group: Catalyses one type of reaction for similar substrates
e.g. Hexokinase adds phosphate groups to hexoses
Linkage: Catalyses one type of reaction for a specific type of bond
e.g. Trypsin catalyses hydrolysis of peptide bonds

Question 23

explain the concept of isoenzymes

Answer 23

Different forms of an enzyme catalysing same reaction in different tissues
* Slight variations in the AA sequences of subunits in quaternary structure
* Can have different physical attributes and optimal conditions
e.g. lactate dehydrogenase (LDH), which converts lactate to
pyruvate, consists of five isoenzymes

Question 24

explain the concept of isoenzymes

Answer 24

Different forms of an enzyme catalysing same reaction in different tissues
* Slight variations in the AA sequences of subunits in quaternary structure
* Can have different physical attributes and optimal conditions
e.g. lactate dehydrogenase (LDH), which converts lactate to
pyruvate, consists of five isoenzymes

Question 25

what is allosteric regulation of enzyme action and what is its two forms

Answer 25

Effector molecules change activity of an enzyme by binding at second site
– Some effectors speed up enzyme action (positive allosterism)
– Some effectors slow enzyme action (negative allosterism)