Mcat biochem Enzymes PDF

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Professor: Dr. Heather Tienson-Tseng
Biochem Enzymes...

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4.1 Enzymes contd. Tuesday, October 12, 2021

12:20 PM

Enzyme Inhibition ◊

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Competitive Inhibition- molecules compete with a substrate for binding at the active site ○ They can be overcome by adding more substrate, which outcompetes the inhibitor ○ Vmax is not affected, it just takes more substrate to get there ○ Km is increased ○ Corresponds to a roughly linear curve Noncompetitive inhibition- molecules bind at an allosteric site ○ Cannot be overcome by adding more substrate ○ Vmax is decreased ○ Km is not altered § Substrate can still bind to the active site, it just can't catalyze properly ○ Corresponds to a curve that looks similar to uninhibited reaction but lower Uncooperative inhibition- inhibitor is only able to bind to the enzyme-substrate complex, not the enzyme unbound to substrate ○ Bind to allosteric sites ○ Vmax is decreased ○ Km is also decreased § Affinity is affected because the enzyme cannot dissociate properly from substrate ○ Corresponds to a curve that starts out higher than the uninhibited reaction but levels out lower Mixed type inhibition- an inhibitor can bind to the unoccupied or occupied enzyme ○ Vmax decreases ○ Km can increase or decrease § If enzyme has a greater affinity for the inhibitor when it is unbound, Km increases □ It takes more substrate (similar to comp. inhibition) § If enzyme has a greater affinity for the inhibitor when it's complexed, Km decreases □ Similar to uncompetitive inhibition

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If it's equal affinity in both forms, it is a noncompetitive inhibitor

Enzymes for the MCAT Class

Function

Examples

Hydrolase

Hydrolysis of chemical bonds

Proteases, ATPases, lipase

Isomerase

Rearrangement of bonds in a molecule to form an isomer

Topoisomerase - alters DNA's structure

Ligase

Ligating - forming a chemical bond

DNA ligase

Lyase

Breaking of chemical bonds by means other than oxidation or hydrolysis

Pyruvate decarboxylate

Kinase

Transfer of a phosphate group to a molecule PFK - phosphofructokinase from high energy carrier (ATP)

Oxidoreductase Redox reactions

Oxidases, reductases, dehydrogenases

Polymerase

Polymerization - making polymers

DNA polymerase

Phosphatase

Removing a phosphate group

Countering kinases in signal transduction

Phosphorylase

Transferring phosphate group to a molecule Glycogen phosphorylase from inorganic phosphate (phosphorylating)

Protease

Hydrolysis of peptide bonds

Trypsin, pepsin, etc.

- Most enzymes are protein, a few are RNA ○ Most catalyze their own splicing - Two theories for how enzymes bind to substrates ○ Active site model § Aka. Lock and key hypothesis § Substrate has a complementary active site on its enzyme ○ Induced fit model § The substrate and active site differ slightly in structure and the binding of the substrate induces conformational change in the enzyme - Many proteases have a serine residue in the active site ○ Recognition pocket- a pocket near the active site of an enzyme

that attracts certain residues on substrate polypeptides - Enzymes that act on hydrophobic substrates have hydrophobic residues in the active site (like bonds to like) - Cofactors- metal ions or small nonprotein molecules that are required for activity of enzymes ○ Often vitamins ○ If they are organic molecules, they are called coenzymes § Often bind to the substrate during a catalyzed reaction

Enzyme Regulation - Enzymes must be regulated (activated and deactivated) or reactions will be occurring in the body when they should not - Regulation is done by: ○ Covalent modification § Covalent addition of a group that modifies enzyme's activity or lifespan □ Ex. Phosphoryl group from ATP by a kinase ® Added onto threonine, tyrosine, serine residues □ Phosphorylation by phosphorylate proteins using freefloating Pi in the cell ○ Proteolytic cleavage § Enzymes are synthesized in inactive zymogen form and then activated by proteases ○ Association with a subunit § Some enzymes require the addition of a subunit to function or the removal of a subunit to function ○ Allosteric regulation § Modification of active site shape through binding to allosteric sites - Feedback Inhibition- negative feedback where the end product of an enzyme-catalyzed reaction shuts off an enzyme's activity - Feedforward stimulation- positive feedback where the end product of an enzyme catalyzes reaction or a molecule in that pathway stimulates the enzyme's activity - Enzymes can act as valves to regulate the flow of substrates into products ○ Controlling how much of each intermediate/compound is in the cell at a time...