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2.1.4 Enzyme-Controlled Reactions (Inhibitors (Competitive (initial rate…
2.1.4 Enzyme-Controlled Reactions
Enzyme-controlled Reactions
Prosthetic Groups
part of the enzyme structure, and others form temporary associations with the enzyme; a cofactor that is permanently bound to enzyme molecules via covalent bonds
e.g. Zn2+ as prosthetic group for carbonic anhydrase (enzyme found in erythrocytes)
Co-enzymes
e.g. vitamins as source of coenzymes
cobalamin coenzymes from B12 (anaemia)
tetrahydrofolate from folic acid (anaemia)
NAP, NADP from B3 (pellagra)
coenzyme A from B6 (high blood plasma fat levels)
thiamine pyrophosphate from B1 (beriberi)
organic non-protein molecules that bind temporarily to active site of enzyme molecules
are chemically changed during a reaction so need to be recycled to original state by different enzyme
Cofactors
e.g. Cl- as cofactor for amylase (enzyme that digests starch into maltose)
a substance that has to be present to ensure that an enzyme-catalysed reaction takes place at the appropriate rate; a cofactor may not be permanently bound to enzyme molecules
may act as co-substrates; form correct shape with substrate to bind to the active site
may change charge distribution on surface of substrate molecules or active site; temporary bonds in enzyme-substrate complex easier to form
Inhibitors
Inhibitor: substance that reduces/stops activity of an enzyme
Role of Product Inhibition
example of negative feedback
enzyme cannot form more of the product than the cell needs
product molecules stay tightly bound to enzyme molecule
Competitive
initial rate of reaction directly proportional to substrate concentration
prevents formation of enzyme-substrate complex
inhibition of an enzyme where the inhibitor molecule has a shape similar to that of the substrate molecule
inhibitor competes with substrate for binding of enzyme active site
forms enzyme-inhibitor complex
increase substrate concentration decreases effect of inhibitor; substrate more likely to bind to active site before inhibitor; max rate of reaction the same
Non-Competetive
prevents formation of enzyme substrate complexes
inhibition of an enzyme where the competitor inhibitor molecule attaches to enzyme in allosteric region, not active site
disrupt tertiary structure, and change its shape
active site is no longer complementary to the shape of the substrate, so it can no longer bind
increase substrate concentration; increase rate of initial reaction, but doesn't reach maximum as without inhibitor
Non-reversible
usually non-competetive
effects of inhibitor cannot be reversed
once bound to enzyme, inhibitor cannot unbind (enzyme is essentially denatured)
Reversible
effects of inhibitor can be reversed
usually competetive
inhibitor is only attached by weak forces, and become unattached
Effects of Inhibitors
Medicinal Drugs
Protease inhibitors
inhibit protease; treat some viral infections by preventing replication of virus in host cells
ATPase inhibitors
inhibits sodium potassium pump in cell-membranes in heart; allow more calcium ions to enter cells; increased muscle contraction; strengthens heartbeat
ACE inhibitors
inhibit CE, normally in metabolic pathway increasing blood pressure; lower blood pressure, treat heart failure
aspirin
salicylic acid inhibits enzymes catalysing formation of prostaglandins, which are involved in pain signalling to the brain, and inflammation; results in pain relief, and reduced inflammation, and reduced blood clots in blood vessels
Metabolic Poisons
cyanide
inhibits aerobic respiration; KCN is hydrolysed to produce HCN; CN- ions bind irreversiblt to enzyme found in mitochondira
snake venom
inhibits enzyme AChE, important at neuromuscular synapse; ACh stays attached to receptors on muscle membrane, and muscles stay contracted; paralysis
Sammer Sheikh