Lecture 9 Flashcards
What two features are necessary for a reaction to occur? What part of an enzyme does this (how?)?
The two reacting molecules need to be close together and in the right orientation. Enzymes store the substrates within active sites, areas within the 3D structure which have amino acid side chains projecting into it which will bind the substrate with weak forces (the substrate can fall out before reacting).
What two specificity types do enzymes show?
Geometric (the groups on the substrate) and stereospecificity (how the groups are arranged) (the shape of the enzyme determines the specificity).
How does the enzyme-substrate interactions work?
The substrate binds to the active site, stabilised by non covalent interactions which leads to the enzyme substrate complex and then the enzyme product complex (in between which is a transition state), the product is then releases.
What are the two models of enzyme substrate reactions? What is the difference?
The lock and key states that the substrate perfectly fits in the enzyme active site, this helps to ensure the reaction.
The induced fit model suggests that the parts of the substrate may not quite fit together in the enzyme active sites, this causes both the enzyme and the substrate to change shape slightly to fit, this can make the transition state easier to reach and hence lowers the activation energy. This is the more correct model.
What are the catalytic mechanisms?
Acid-base catalysis, covalent catalysis, metal ion catalysis, electrostatic catalysis, proximity and orientation effects, preferential binding of the transition state complex. Enzymes can use a mix of these
What is acid base catalysis? What is it typically done with? What large factor can prevent enzymes of this type working?
Catalysis involving H+ transfer, COOH to COO- and H+ (Glu, Asp) or NH3+ to NH2 and H+ (Lys). pH will heavily affect the effectiveness of these enzymes.
Histidine is also good for this because its pKA is 6.5 (close to physiological pH), this allows it to donate or accept a proton, depending on the need.
What is covalent catalysis?
Involves the formation of a highly reactive, short lived intermediate which is covalently attached to the enzyme typically using nucleophiles like COO-, NH2 or S-, aromatic-O-, CH2-OH (like Ser). Water can be used to remove the nucleophile.
What is metal ion catalysis? Whats an example enzyme?
A catalysis which needs a metal ion, about a third of enzymes require these. The metals provide substrate orientation, binding energy and sites for oxidation-reduction reactions.
Hexokinase uses Mg2+
What is a cofactor?
A factor which helps enzymes catalyse reactions, these could be metal ions or small organic molecules known as coenzymes.
