W2 Enzymes I Flashcards
Enzymes
Proteins that speed up (catalyse) specific chemical reactions
Enzyme functions
Digestion: carbohydrates, fats, proteins
Blood clotting: fibrin clot catalysed by thrombin
Defence-immune system-activation of complement
Movement: muscle actomyosin is an ATPase
Nerve conduction: membrane ion pumps for Na+, Ca++
Enzyme defects cause disease
Phenylketonuria = cannot convert Phe to Tyr
Glycogen storage disease = enzyme deficiencies for enzymes involved in synthesis/breakdown
Tay-Sachs disease = membrane cerebroside
TSD caused by no hexosaminidase-A (one of 3 proteins needed for hydrolysis of GM2) → GM2 ganglioside accumulates = toxicity
Enzymes are drug targets
Antibiotics = e.g. penicillins inhibit cell wall synthesis
Anti-inflammatory agents = aspirin block prostaglandin
Anticancer drugs = methotrexate is a folate analogue, interferes with synthesis of DNA precursors
Methotrexate = disease modifying anti-rheumatic drug, reduces activity of immune system
Enzyme properties
Increase reaction rate by up to 10 billion fold!
Show specificity
Unchanged at end of reaction
Do not alter reaction equilibrium
Facilitate reaction by decreasing the free energy of activation of the reaction
Evidence for AS
Crystallise w/substrate to see where substrate is binding → put crystal in a beam → X-rays fired at crystal (electrons give off x rays as they loop around the synchrotron) → rays scattered onto electronic detector by crystal → ways of working back from the diffraction pattern
Induced fit/lock and key
L+K:
Emil Fischer 1984 - correct sized key fits into keyhole of lock
Induced fit:
Enzyme partially flexible fits around specific complementary substrate
E-S binding energy used to…
Bring molecules together in active site
Constrain substrate movement
Stabilize positive and negative charges in t-state
Provide a reaction pathway of lower energy: e.g. involving covalent enzyme-substrate intermediates
Strain particular bonds in the substrate- making breakage easier. Substrate distorted on binding to resemble transition state
Use cofactors (non-protein chemical compound or metallic ion required for enzymes activity) = bring new chemistry to active site
Vmax
ROR when enzyme is saturated w/substrate
Km
Michaelis constant
The [substrate] which permits enzyme to achieve half of Vmax
Low Km = saturated
Mechanisms of regulation of enzyme activity
Control of gene expression thus enzyme amount
Compartmentation: sequences in enzyme polypeptide chain target enzyme to ER, mitochondrion, nucleus etc
Allosteric regulation: regulatory molecules control protein shape-increase (or decrease activity)
Covalent modification of enzyme. Change enzyme shape and activity e.g. phosphorylation
Feedback inhibition
Where output of a process is used as an input to control the behaviour of the process itself. (limiting production of more product)
Allows balance of amount of product provided w/amount of product needed
ATCase (Aspartate carbamoyltransferase)-regulated by CTP
CTP is a feedback inhibitor of ATCase as CTP is the end product of pathway that ATCase catalyses
Allosteric enzyme
Enzymes that change their structure upon binding of an effector, which results in an apparent change in binding affinity at a different ligand binding site
Allosteric enzyme properties
Multisubunit complexes (show cooperative
Regulatory sites and catalytic sites on different subunits
Regulation occurs via conformational changes
Exhibit non-Michaelis-Menten kinetics:
V vs S plots are sigmoidal = S curve defined by formula:
S(x) = 1/(1+e^-x) = e^x/(e^x+1)
Involved in feedback inhibition of metabolic pathways
Involved in cell signalling
