Physiology enzymes Flashcards
(28 cards)
What is an enzyme
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
what is required for a reaction to occur (hint Gibbs)
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
what is an active site
Active site is a region within an enzyme that fits
the shape of substrate molecules
describe how active site binds to substrate
- 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)
what is the lock and key hypothesis
- 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
what is the induced fit hypothesis
- 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
at which pH do enzymes woek at optimum rate and why
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
Give examples of exceptions to the 7.4 PH
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
what is the basis for the pH
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.
what is the optimum temprature for enzyme activity
Enzymes are most active at an optimum temperature (usually 37°C)
why do enzymes only function at optimum temp
They show little activity at low temperatures
* Activity is lost at high temperatures as denaturation occurs
why do enzymes only function at optimum temp
They show little activity at low temperatures
* Activity is lost at high temperatures as denaturation occurs
why do enzymes only function at optimum temp
They show little activity at low temperatures
* Activity is lost at high temperatures as denaturation occurs
what is a cofactor
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
what is a coenzyme
- 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
wjat is the difference between a cofactor and coenzyme
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+
what are the different enzyme classifications
- Oxidoreductases
- Transferases
- Hydrolases
- Lyases
- Isomerases
- Ligases/synthases
what do Oxidoreductases catalyse
Catalyse transfer of electrons from donors to acceptors
* Broad class of enzymes that catalyze the many oxidation-reduction
reactions found in biochemical pathways.
what do Transferases catalyse
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
What does Hydrolases catalyse
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
what does Lyases catalyse
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
what does Isomerases catalyse
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
what do ligases catalyse
Catalyse synthesis of new covalent bonds using ATP energy
(aka Synthetases)
explain the concept of enzyme specificity
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
