Biochem2 Lecture 5 Enzymes pt 2

winniesmith1's version from 2017-03-20 18:50

Section 1

Question Answer
Describe the equilibirum constant (equilibrium rate)the rate (v) of the forward reaction (A>B) is by definition equal to that of the reverse or back reaction (B>A)
What is the algebraic symbol for equilibrium constantVforward = Vreverse. [B]/[A] = k+1/k-1 = Keq
Go over lecture 5 pg 1/2**equilibrium equations and graphs
What does S meanconcentration of subsrates
What does Vmax meanMaximum reaction rate
What does V1 meanreaction rate (V1= Vmax/2)
What does Km meanMichaelis-Menton constant (Km).
What is the Michaelis-Menten equationused to demonstrate that at the substrate concentration that produces exactly half of the maximum reaction rate, i.e.,1/2 Vmax, the substrate concentration is numerically equal to Km
How do you work out Km and what does it tell youWhen the initial rate = ½ the maximal rate then [S] = Km. these two features tell us that the [S] at this point is sufficient to ensure that half the enzymes’ sites are ‘occupied’ at any one moment - giving the ½ maximal rate • the higher [S] has to be, the weaker the association between substrate and enzyme.
What are the problems with Michaelis-Menton equationThis is ‘idealised’ • determination of Vmax is a problem • asymptote • conditions change with [S] very high
How can you overcome problems with m-m equationlinewaver-burke plot

Section 2

Question Answer
What does Vmax mean/showthe turnover number in molessubs per moleenz per second
What does Km show/meanthe affinity in molessubs per litre (molar)
D-glucose + ATP --->D-glucose-6-P +ADP
How do we show specificity?Use kinetics, look at the rate of formation of product (G-6-P)
(How do we show specificity) Do other substrates demonstrate kinetics? if you replace D glucose with D-fructose/galatose (same conc). Rate of reactions for both are lower than D-glucose.

Section 3

Question Answer
What is a lineweaver-burke plota way of studying kinetics. Rearranged the Michaelis-Menton equation to avoid curvlinear analysis.
How do you use a lineweaver-burke plotVmax is taken when (glucose) is infinitely large and all active sites filled or saturated (saturation kinetics)
How can you tell 'competition' from a L-B plotVmax UNALTERED (when swamped with glucose). Km INCREASED (need raised (glucose) to half occupy the active sites (affinity of enzyme lowered for glucose) when galactose is also present • suggests galactose competes with glucose for the same active site
How can you tell 'competitive inhibition' from a L-B plotVmax UNALTERED (When swamped with glucose). Km INCREASED (affinity of enzyme lowered for glucose when 1-deoxyglucose is also present • suggests 1-deoxyglucose competes with glucose for the same active site, but without reacting - i.e. blocks the site)
How can you tell 'non-competitive inhibition' on a L-B plotVmax LOWERED (even when swamped with glucose). Km UNCHANGED (affinity of enzyme for glucose unchanged by presence of compound “x” • suggests compound “x” does not compete with glucose for the same active site, but reduces the number of sites available)
How can you tell 'allosteric effects' Excitatory molecule or cofactorVmax altered RAISED. Km UNCHANGED.

Section 4

Question Answer
Models of inhibition: Non-competitive inhibitionSite obscured
Models of inhibition: Competitive inhibitionSite occupied
Models of inhibition: Allosteric (non-competitive inhibition)binding elsewhere on enzyme & altering structure as a result
What is the allosteric sitefurther site for recognition • adjusts enzyme structure • alters enzyme function • does not catalyse another reaction
What is non-competitive activationActive site changed: induces site, adjusts to enhance binding, locks in active state. Therefore substrate can bind and react.
Why do we need allosteric effectorse.g. cAMP, inhibition, activation. Act through allosteric sites (separate from the active site).
What do allosteric sites doadjust the nature of the active site (by changing enzyme structure/ internal charges) and so change activity of the active site.

Section 5

Question Answer
Describe multi-site enzymes2 or more active sites per enzyme • typically each ‘housed’ on a sub-unit • multi-site enzymes exhibit allosteric control strongly: • one site active - ‘lock’ all sites into active state • one site inhibited - ‘lock’ all sites into inhibited state
Example of multi-site enzyme?Haemoglobin
What are cofactorsPresent in the active site or • act through allosteric effects (bound elswewhere on enzyme) • ‘compulsory’ for enzyme activity • e.g. Mg2+, Ca2+ etc
What are zymogens• Site obscured • by part of enzyme molecule • prevents enzyme activity • protective • holds capacity till needed • activated by proteolytic enzyme • zymogens removes peptide chain ‘blocking’ active site • typical of digestive enzymes