Enzymes Flashcards
What structure do enzymes have?
Tertiary and quaternary structure.
Most enzymes are three dimensional globular proteins.
What are ribozymes?
Special RNA species that act as enzymes
What is the active site of an enzyme?
Is a region that binds substrates, co-factors and prosthetic groups.
It contains a residue that helps to hold the substrate.
How much space do active sites occupy?
Usually less than 5%
What about shape of the active site?
It is specific due to tertiary structure of protein.
A change in the shape of a protein affects the shape of active site and function of the enzyme.
What are 2 types of active site?
Binding site: chooses the substrate and bonds it to the active site.
Catalytic site: performs the catalytic action of enzyme.
What are co-factors?
Co-factors are non protein molecules that carry out chemical reactions that cannot be performed by 20 a.a.
How do co- factors function?
They bind to an inactive protein, and by doing so, they activate it, and provide binding sites to which substrates can join.
What are examples of inorganic co-factors?
1) enzyme carbonic anhydrase requires Zn++ for its activity.
2) hexokinase has co-factor Mg++
What is an example of organic co-factor?
Glycogen phosphorylase requires a small organic molecule pyridoxal phosphate.
What are 2 groups of organic co-factors?
1) prosthetic group: tightly bound organic co-factor (Flavins, heme groups and biotin)
2) coenzyme: loosely bound organic co-factor (NAD+)
What is an apoenzyme?
An enzyme with its co-factor removed
What is holoenzyme or holoprotein?
The complete complex of protein with all necessary small organic molecules, metal ions and other components
How are enzymes synthesised?
These are made by ribosomes which are attached to Rer.
Information for synthesis of enzyme is carried by DNA.
Amino acids are bonded together to form specific enzymes according to DNA’s codes.
What are intracellular enzymes?
These are synthesised and used in the cell. They are found in the cytoplasm, nucleus, mitochondria and chloroplast.
Oxydoreductase catalyses biological oxidation.
Enzymes involved in reduction in the mitochondria.
What are extracellular enzymes?
These are synthesised in the cell but are secreted to work externally.
Digestive enzymes produced by pancreas are secreted in the duodenum.
What is a turnover number? (Numero di fatturato)
Number of substrate molecules transformed per minute by one enzyme molecule.
Catalase turnover number= 6x106/min
What are six major classes of enzymes?
Oxidoreductases
Transferases
Hydrolases
Lysases
Isomerases
Ligases
What is reaction type of oxidoreductases?
Redox
What is reaction type of transferases?
Move chemical group
What is reaction type of hydrolases?
Hydrolysis; bond cleavage with transfer of functional group of water.
What is reaction type of lysases?
Non-hydrolytic bond cleavage.
What is reaction type of isomerases?
Intramolecular group transfer (isomerization)
What is reaction type of ligases?
Synthesis of new covalent bond between substrates, using ATP hydrolysis.
By what 2 perspectives the catalytic efficiency of enzymes is explained?
Thermodynamic changes
Processes at the active site
What are 4 processes that occur at the active site?
1) covalent catalysis
2) acid base catalysis
3) catalysis by proximity
4) catalysis by strain
What is covalent catalysis?
Enzymes form covalent linkages with substrate forming transient enzyme-substrate complex with very low activation energy.
What is acid base catalysis?
Happens mostly at the active site, histidine is present which act as both proton donor and proton acceptor.
Mostly undertaken by oxido-reductases enzyme.
What is catalysis by proximity?
In this catalysis molecules must come in bond forming distance.
When enzyme binds: a region of high substrate concentration is produced at active site. This will orient substrate molecules especially in a position ideal for them.
(Substrates join: form bonds)
What is catalysis by bond strain?
Mostly undertaken by lysases.
The enzyme-substrate binding causes reorientation of the structure of site due to in a strain condition. (Enzyme changes shape slightly as substrate binds)
Thus transitional state is required and here bond is unstable and eventually broken.
Who and when proposed lock and key model? (Modello di serratura e chiave)
Emil Fischer in 1894
Who and when proposed induced fit model? (Modello di adattamento indotto)
Danial Kosh Land in 1958
What are 4 types of reversible inhibition?
Competitive
Uncompetitive
Mixed
Non-competitive
What is competitive inhibition?
Inhibitors compete with substrate for the active site
Example: stain drug (lipitor)
What is uncompetitive inhibitor?
It’s the one that binds to an allosteric site of the enzyme and changes shape of the active site.
What is mixed inhibition?
In this type of inhibition both Enzyme inhibitor and Enzyme substrate inhibitor complexes are formed. Both complexes are catalytically inactive.
What is non-competitive inhibitor?
Inhibitor reduces the activity of the enzyme and binds equally well to the enzymes whether or not it has already bound substrate.
What is irreversible inhibition?
This inhibition involves the covalent attachment of the inhibitor to the enzyme. The catalytic activity is completely lost. It can only be restored by synthesising molecules.
Examples: aspirin
What is activation?
Activation is the conversion of inactive form of enzyme to active form which processes the metabolic activity.
What are 2 types of activation?
By co-factors
By conversion of an enzyme precursor
