3 Enzymes

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Georgie

Cards (32)

Rates increase up to a limit. Q10 is generally 2 for biological reactions as the rate doubles for every 10 degree rise in temperature
Taq polymerase is an enzyme that can survive very hot temperatures, it is a thermophile
pH-dependence reflects ionization of groups on S and in the active-site of E
pH can affect both KM and kcat
pH-dependence can provide useful insights into the nature of groups (amino acid side chains) involved in the active site
Angiotensin-converting enzyme (ACE)
Angiotensin-converting enzyme (ACE) inhibitors:
treatment of hypertension, diabetic nephropathy, and some types of chronic heart failure
Enzymes are the targets for many drugs
ACE (a proteinase) inhibitors lower blood pressure by blocking production of a signal promoting blood vessel contraction
Aspirin inhibits an enzyme that produces prostaglandin, blocking the inflammatory response
Penicillin inhibits enzymes which enable bacteria to make stable, robust, cross-linked cell walls. It fixes the enzyme in the ES complex.
Many venom toxins paralyse by inhibiting acetylcholine-esterase, promoting muscle contraction. Some insecticides do the same.
Glyphosate inhibits the production of tryptophan, tyrosine and phenylalanine. It binds in the substrate-binding site of the enzyme and mimics the intermediate complex.
Receptors are also frequent targets - Beta-blockers bind and inhibit the action of noradrenaline on β-adrenergic receptors.
Some inhibitors react covalently with essential active site groups. These irreversible inhibitors lead to permanent inactivation. Examples are:
Organomercurials and nerve gases which, respectively, target Cys residues and reactive Ser residues
Mercury, was used in animal skin curing and as an ‘antidote’ for syphilis! Mercury causes irreversible inhibition of selenoenzymes
Enzyme inactivation by iodoacetamide, a group-specific reagent. Iodoacetamide inactivates enzymes by reacting with critical cysteine residues.
Enzyme inactivation by diisopropylphosphofluoridate (DIPF)
DIPF is a nerve gas; it inactivates acetylcholinesterase by reacting with a highly reactive serine in the active site
Novichok-type agents are nerve gasses
Many compounds bind non-covalently to enzymes, obstructing their activity. These are reversible inhibitors; they can be removed by dilution or dialysis.
Competitive inhibitors act by blocking substrate binding (they compete with S to bind to E) and are often structural homologues of S
Non-competitive inhibitors bind at same time as S, but not in the same site, and act by slowing down conversion to P
Competitive inhibition implies that, at very high [S], S will displace I; therefore, even in the presence of I E will become saturated with S.
Enzyme activity is reduced at low [S] but shows the same Vmax at very high [S].
Competitive inhibition shows a characteristic plot of v vs [S] with altered KM
In Lineweaver Burk plots, data obtained at a series of [I] converge at the same point on 1/v axis = (1/Vmax)The “observed Km “ changes
If the ‘observed’ or ‘apparent’ Km is determined at a series of [I], the data give a measure of how tightly I binds to E (inhibitor affinity)
Ki = Km.[I] / Km obs - Km
Oseltamivir is a competitive inhibitor of neuraminidase. It mimics a natural substrate acetyl sialic acid
Non-competitive inhibitors bind to the ES complex to give an ESI complex.
ESI complex is not able to generate product i.e. ESI is a ‘dead-end’. Therefore, binding of I lowers the catalytic activity towards S
A competitive inhibitor has no effect on Vmax but increases KM.
A non-competitive inhibitor decreases Vmax but has no effect on KM.
Note: there are cases where both parameters are affected, ‘mixed inhibition’.
On a Lineweaver-Burk plot, if the two lines intercept at the y axis it is a competitive inhibitor.
On a Lineweaver-Burk plot, if the two lines intercept at the x axis it is a non-competitive inhibitor.
Glyphosate inhibits synthesis of aromatic amino acids. Ki for glyphosate = 12 μM, Km for PEP = 62 μM