Metabolic Control Analysis (MCA)

Transcription

1 Metabolic Control Analysis (MCA) The restriction imposed by MCA is that we only study effects of small perturbations: what will happen if we nudge the metabolic system slightly of its current steady state Mathematically, we employ a linearized system around the steady state, thus ignoring the non-linearity of the kinetics. The predictions are local in nature; in general different for each steady state

2 Questions of interest How does the change of enzyme activity affect the fluxes? Which individual reaction steps control the flux or concentrations? Is there a bottle-neck or rate-limiting step in the metabolism? Which effector molecules (e.g. inhibitors) have the greatest effect? Which enzyme activities should be down-regulated to control some metabolic disorder? How to distrub the overall metabolism the least?

3 Coefficients of control analysis The central concept in MCA is the control coefficient between two quantities (fluxes, concentations, activities,... ) y and x: ( ) x cx y y = y x x 0 Intuitively, c y x is the relative change of y in response of infinitely small change to x

4 ɛ-elasticity coefficient ɛ-elasticity coefficient ɛ k i = S i v k v k S i quantifies the change of a reaction rate v k in response to a change in the concentration S i, while everything else is kept fixed. perturbation v 1 v 2 v 3 S 1 S 2?? response?

5 Flux control coefficients The flux-control coefficient (FCC) FCC j k = v k J j J j v k is defined as the change of flux J j of a given pathway, in response to a change in the reaction rate v k. v? 1 v v 3 2 S 1 S 2? v 4? S 3 perturbation

6 Concentration control coefficients The concentration-control coefficient (CCC) CCC i k = v k S i S i v k is defined as the change of concentration S i, in response to a change in the reaction rate v k. perturbation v 1 v 2 v 3 S 1 S 2??

7 Theorems of MCA Unlike the elasticity coefficients, the control coefficients cannot be directly computed from the kinetic parameters of the reactions, even in principle. In order to determine the coefficients we need both some MCA theory and experimental data MCA theory consists of two sets of theorems: Summation theorems make statements about the total control of a flux or a steady-state concentration Connectivity theorems relate the control coefficients to the elasticity coefficients

8 Summation theorems The first summation theorem says that for each flux J j the flux-control coefficients must sum to unity r FCC j k = 1 k=1 Thus, control of a flux is shared across all enzymatic reactions For concentration control coefficients we have r CCCk i = 0 k=1 Control of a concentration is shared across all enzymatic reactions, some exerting positive control, other exerting negative control.

9 Flux control connectivity theorems Connectivity theorems tie elasticity coefficients ɛ v k S i coefficients FCC J j v k, CCC S i v k together. Flux control connectivity is given by r k=1 FCC J j v k ɛ v k S i = 0 In our example we have FCC J 1 ɛ1 S + FCC J 2 ɛ2 S = 0 giving FCC1 J FCC2 J = ɛ2 S ɛ 1 S and control which shows that, everything else remaining constant, an increase in ɛ 2 S needs to be countered with a decrease in FCC J 2 v 1 v 2 P 1 S P 2 J

10 Concentration control connectivity Similar connectivity theorems hold for concentrations, The concentration control connnectivity theorem ties the elasticity of reaction v k with respect to concentration S i to the concentration control of v k over the concentration S h We have for h i, and r k=1 r k=1 CCC S h v k ɛ v k S i = 0 CCC S i v k ɛ v k S i = 1

11 Calculating control coefficients With the help of the summation and connectivity theorems and elasticities for single reactions one can determine values for the control coefficients. For the two step pathway below, we apply the summation theorem FCC1 J + FCC 2 J = 1 and the connectivity theorem FCC1 Jɛ1 S + 2 2ɛ2 S = 0 We obtain FCC J 1 = ɛ2 S ɛ 2 S ɛ1 S, FCC J 2 = ɛ1 S ɛ 2 S ɛ1 S where the elasticity coefficients, computed from reaction kinetics can be substituted. v 1 v 2 P 1 S P 2 J

12 Calculating control coefficients Since typically we have ɛ 1 S < 0 and ɛ2 S > 0 from FCC J 1 = ɛ2 S ɛ 2 S ɛ1 S, FCC J 2 = ɛ1 S ɛ 2 S ɛ1 S we see that both reactions exert positive control over the flux of the pathway v 1 v 2 P 1 S P 2 J

13 Calculating control coefficients The concentration control coefficients fulfill which yields and CCCv S 1 + CCCv S 2 = 0, CCCv S 1 ɛ v 1 S + CCC v S 2 ɛ v 2 S = 1 CCC1 S 1 = ɛ v 2 S ɛv 1 S CCC2 S = 1 ɛ v 2 S ɛv 1 S With ɛ 1 S < 0 and ɛ2 S > 0 we get CCC S v 1 > 0 and CCC S v 2 < 0, that is the rise of first reaction rate rises the concentration of S while rise of the second reaction rate lowers the concentration of S v 1 v 2 P 1 S P 2 J

14 MCA example: simple junction Reaction R0 has constant flux v 0 = 0.1 Reactions R1, R4 and R5 irreversible with mass action kinetics v = k + S Reactions R2 and R3 reversible with mass action kinetics v = k + S k P All kinetic constants equal k + = k = 0.1 Let us perform MCA analysis with given steady state Results computed with the COPASI simulator ( R A R R 4 R 2 C B R 3 R 5 D

15 MCA example: simple junction Elasticities ɛ k i = S i v k v k S i A B C D R R R R R R R A R R 4 R 2 C B R 3 R 5 D

16 MCA example: simple junction Flux control coefficients FCC k J = v k J J v k R0 R1 R2 R3 R4 R5 R R R R R R R A R R 4 R 2 C B R 3 R 5 D

17 MCA example: simple junction Concentration control coefficients CCC k i R0 R1 R2 R3 R4 R5 A B C D = v k S i S i v k R A R R 4 C R 2 B R 3 R 5 D

18 MCA example: predicting the results of perturbation Let us consider optimization of the flux over a linear pathway of four reactions by modulating enzyme concentrations. Assume the following kinetics v i = E i (k i S i 1 k i S i ), initial enzyme concentrations E i = 1 and rate constants k i = 2, k i = 1 and concentrations of external substrates S 0 = S 5 = 1 The steady state flux J = 1 and the flux control coefficients FCC J 1 = 0.533, FCC J 2 = 0.267, FCC J 3 = 0.133, FCC J 4 = can be solved from the above equations.

19 MCA example: predicting the results of perturbation According to MCA, increasing the concentration of a single enzyme E i by p% will increase the flux approximately by i = FCCi J (p/100), giving 1 = , 2 = , 3 = , 4 = On the other hand, the underlying true kinetic model would predict 1 = , 2 = , 3 = , 4 = Thus MCA predicts fairly accurately the results of a small preturbation.

20 MCA example: predicting the results of perturbation Large perturbations would not be equally accurately predicted by MCA. Assume we can double the total enzyme concentration Ei = 4 8. How should the enzyme be allocated for best results? E 1 5E 1 : MCA predicts 1 = = 2.665, kinetic model gives 1 = E 4 5E 4 : MCA predicts 4 = = 0.335, kinetic model 63 The maximal increase of for the flux is obtained by modifying all the enzyme concentrations: E 1 = 3.124, E 2 = 2.209, E 3 = 1.562, E 4 = 1.105

21 The End Course Exam Wednesday am-12pm, in A111 Examined contents: lecture slides and exercises Exam will consist of five questions, each worth 8 points Types of questions: defining concepts, essays as well as technical questions asking for analysis of a given metabolic model