Thursday, January 30, 2014

Michaelis Constant Chart



                  MICHAELIS CONSTANTS FOR THE METHIONINE PATHWAY


The Michaelis constant (Km) is the half‑maximally saturating concentration of substrate for a given enzyme.  The higher the Km when compared to blood or tissue levels of substrate, the more the enzyme can be "pushed" by increasing levels of substrate.  If the enzyme is already near saturation by normal levels of substrate, increasing substrate levels will not increase enzyme activity.  When two or more enzymes compete for the same substrate, the enzyme with a relatively low Km is favored at low substrate concentrations, and the enzyme with a relatively high Km is favored at high substrate concentrations.


Source                              Enzyme                            Km                           Substrate Levels


Human                           Aminoacyl                                                       Human blood
placenta                         Methionyl:                        50 uM                        Methionine:
                                     Transfer RNA              (Methionine)                   13-37 uM/L
                                      synthetase
                                     EC 6.1.1.10

Human  skin                  Si = 10.8
fibroblasts                    nanomoles/mg protein/hr

This enzyme functions to place methionine on the ribosome during protein synthesis.  It is constitutive, in that it does not respond to increasing levels of methionine.




Human                       Methionine:                                                       Human  plasma 
liver                           Adenosine                         650 uM                      Methionine:        
                                  Transferase                   (Methionine)                13-37 uM/L
                                   EC 2.5.1.6
                                        
                                                                                                 
                                                                                                              Human plasma
Human                           Si = 30-130                                                         SAM levels:
liver                              nanomoles/mg protein/hr                                     38-50 nM/L

Rat liver                        Si = .282
male                              Units/ gm liver

Rat liver                         Si = .465
female                            Units/ gm liver

Function:  This enzyme combines methionine and adenosine to form S‑Adenosyl‑Methionine (SAM), the primary methyl donor in the body.    Inhibited by excess SAH.  Induced by methionine.  Magnesium dependent.  Glutathione dependent.   50% inhibition by androgen in rats.




Source                          Enzyme                                                 Km         Substrate level

All                                S-Adenosyl methionine-                  < 100 uM
tissues                           dependent  Transmethylases

Human                         SAM-Phospholipid                         Km = ?
liver                              Methyltransferase

                                     Si = 14.4 nanomoles/ mg protein/hr
   
Function:  to transfer a methyl group from SAM to a myriad of acceptors.  Leaves SAH.
Inhibited by SAH.   There are more than 100 of these transmethylases.




Human                          S‑Adenosyl‑                                          Human Plasma
Monocytes                    Homocysteine                                            32‑39 uM 
                                      Hydrolase
                                      EC 3.3.1.1                                      S‑Adenosyl‑HCY levels:
                                                                                                    40 nmol/L

                                                                                                      62 uM
                                                                                                   (rat liver)  
                                    Si = ?

Function:  Removes adenosine from SAH, leaving homocysteine.  Inhibited by excess homocysteine.




Human                     Methyl‑THF:                             Human blood                     HCY:
liver                        Homocysteine                                   60 uM        0‑20uM/L (normal)
                               Methyl Transferase                     (Homocysteine)        20‑300+ uM/L
                               EC 2.1.1.13                                                                  (abnormal)   
           
Human                      Si = 1.30                      Human                       Si = 13.0
liver                nanomoles/mg protein/hr    fibroblasts       nanomoles / mg protein / hr

Human                     Si = 2.34                               Human           Si = 1.37
kidney              nanomoles/mg protein/hr             brain        nanomoles / mg protein / hr

Function:  This enzyme remethylates homocysteine back to methionine, with the methyl group obtained from serine, by way of B2, B6, B12, and folate.  Inhibited by excess methionine.




Rat                          Choline                                      5000 uM           Human liver tissue 
liver                      dehydrogenase                           (Choline)                 Choline: 60 uM
                               EC 1.1.99.1                                                               
                                                                                                              Human serum
                                                                                                              Choline: 10 uM
Human                    Si = 78 nmoles / mg wet tissue / hr
liver                           = 84 gm/24 hr liver only

Rat liver                  Si = 1332 nmoles / mg wet tissue / hr    

Function:  Forms betaine aldehyde and trimethylamine from choline.





Source                               Enzyme                            Km                        Substrate levels

Human                           Betaine                                                         Human liver tissue
liver                             aldehyde                           Km = ?                Betaine aldehyde:  ?
                                dehydrogenase                                    Human serum betaine:          
                                   EC 1.2.1.8                                                      18-73 uM/L           

Rat liver                    Si = 5.7 umoles NADH / min / mg  protein

Function:  Forms betaine from betaine aldehyde.





Human                               Betaine:                          2200 uM       Human blood Betaine:
liver                              Homocysteine                     (Betaine)             18-73 uM/L
                                         Methyl                                                  Human blood HCY:
                                       Transferase                          4  uM                        0‑20 uM/L
                                       EC 2.1.1.5              (Homocysteine)             Human liver tissue
                                                                                                                   Betaine: ?
Human                            Si = 32.1                                                     Human liver tissue
liver                   nanomoles/mg protein/hr                                         Homocysteine: ?

Human                          Si =  14.1
kidney                           nanomoles/mg protein/hr

Human                           Si =  0.0                   
brain                              nanomoles/mg protein/hr

Function:  Remethylates homocysteine, with the methyl group obtained from triglycerides, by way of choline and betaine.  Serves as an alternate source of methyl groups when glucose is scarce.  Activated by excess homocysteine.  Inhibited by excess methionine. Forms dimethylglycine. Found primarily in the liver and kidney.
Serves to conserve the sulfur backbone of homocysteine when methionine is scarce.





Rat                              Dimethyl                                                Human serum level
liver                            glycine                   Km = ?                        Dimethylglycine=
                                  dehydrogenase                                               1.4-5.3 uM/L
                                    EC 1.5.99.2

                                          Si = ?

Function: Forms sarcosine from dimethylglycine.





Source                               Enzyme                            Km                      Substrate levels

Rat                                   Sarcosine                                             Human serum glycine:
liver                               dehydrogenase                      Km = ?             147-422 uM/L
                                       EC 1.5.99.1                      (Sarcosine)

                                         Si = ?

Function: Forms glycine from sarcosine.   





Rat                                  SAM:Glycine                            130 uM                  Rat liver
liver                            N-Methyltransferase                   (Glycine)                   Glycine:
                                      EC 2.1.1.20                                                              1 uM/gm

Rat                               Si = 1.87 nmol sarcosine
liver                                     /mg protein/min

Function:  Forms methylated glycine (sarcosine), which is then oxidized to CO2, and glycine.  Serves as an excess methyl group removal pathway.  Activated by excess methionine and SAM.





Human                            Cystathionine             4000 uM           Human blood Serine:
liver                               beta‑synthase                  (Serine)                  95-230 uM/L
                                       E.C. 4.2.1.22                                                                  
                                                                        25000 uM             Human blood HCY:    
Human                            Si = 98                  (Homocysteine)                0-20 uM/L
liver                               nanomoles/mg protein/hr  

Function:  Condenses serine and homocysteine to form cystathionine. Provides sulfur for cysteine formation.  Irreversibly removes serine and homocysteine from the methionine remethylation cycle.  Activated by excess methionine and homocysteine.  Dependent on sufficient serine.





Human                     Cystathionase                    1200 uM            Human blood
liver                          E.C. 4.4.1.1                 (Cystathionine)        Cystathionine:
                                                                                                          0‑1uM/L    
Human                           Si = 126                         
liver                              nanomoles/mg protein/hr         

Function:  Cleaves cystathionine to form cysteine and homoserine.  Activated by excess cystathionine.





Source                           Enzyme                             Km                      Substrate Levels

Rat                              Cysteine                          450 uM                       Human blood
liver                         Dioxygenase                  (Cysteine)                        Total Cystine:
                               E.C. 1.13.11.20                                                       200-361 uM/L
                                                                                                                                  Free Cysteine:                                                                                          12 uM/L


Rat                      Si = 60 nanomoles/mg protein/hr                                           
liver                                  on low protein diet
                           Si = 1820 nanomoles/mg protein/hr
                                        on low protein diet +  5% methionine

Function:  Cleaves cysteine, forming cysteinesulfinic acid,  which either transaminates to sulfite (95%), or is decarboxylated to hypotaurine (5%).  Regulates cysteine levels.  Activated by excess cysteine,  inhibited by cysteine deficiency.





Rat                              Cysteine Sulfinate               Km =  9.4 mM         Human liver: ?
liver                            Transaminase                    (Cysteinesulfinate)
                                   E.C.  2.6.1.1

Rat                               Si = 47.8 mM
brain                                    / mg protein / hr

Rat                              Si =  13.68 mM
liver                                     / mg protein / hr


Function:  Transaminates cysteinesulfinate to b-sulfinylpyruvate, which then nonenzymatically transforms to sulfite and pyruvate.   Also known as Aspartate Aminotransferase.





Human                         Sulfite                                 Km =  17 uM               Human liver
liver                            Oxidase                                   (Sulfite)                       Sulfite:   ?
                                    E.C. 1.8.3.1                                                                              
                                                                                                            Human plasma
                                                                                                           sulfate :  0.3 mM
Human                         Si = 0.66
liver                            nanomoles of sulfate/mg protein/min  

Function:  Oxidizes sulfite to sulfate.  Molybdenum dependent.







Source                            Enzyme                             Km                       Substrate 

                            Cysteinesulfinic acid               Km = 120 uM       Tissue levels = ?       
Human                      Decarboxylase               (Cysteine-sulfinate)
liver                            E.C. 4.1.1.29

Human                        Si = 0.32
liver                                     nanomoles/ mg protein/ hr

Rat                               Si = 468
liver                                      nanomoles/ mg protein/ hr


Function:  Decarboxylates CSA to hypotaurine, which then spontaneously oxidizes to taurine. Vitamin B6 dependent.  Rate limiting for taurine formation in humans.





 Source                             Enzyme                     Km                    Substrate Levels


Human                         Phosphoserine              36 uM                        Human brain
brain                             Phosphatase             (Serine)                       Serine: 80 uM/L
                                       E.C. 3.1.3.3

                                       Si = 600 nanomoles/mg protein/hr
                                      Human brain (1350 gm) =  serine/24 hr

                                       Si in uM/
                                       gm liver/hr for:
                                       chicken= 25                      410 uM               Human plasma
                                       pigeons= 18                      (Serine)         Serine: 95-230 uM/L
                                       rats   = 26
                                      guinea pigs= 18
                                       humans =  ?

            18 uM Serine(M.W.=105) / 1500 gm liver / 24hr = 68 gm/day in man

Function:  Removes phosphate from phosphoserine, formed from glucose, glutamate, and ATP, leaving serine.   Inhibited by excess serine.





Human                            Serine                          6200 uM                     Human brain
brain                               hydroxymethyl             (Serine)                   Serine: 80 uM/L
                                       transferase
                                        E.C. 2.1.2.1

Human                             Si = 32 uM/
brain                                 gm protein/hr = 22 gm serine/ 24 hr

Human                            Si = 1834 uM/                                     Human serum  Serine:
liver                                  gm protein/hr                                            95-230 uM/L

            1834 uM Serine / 300 gm liver protein / 24hr = 1368 gm/day in man

Function:  Removes hydroxymethyl group from serine, leaving glycine, and places the methyl group on Tetrahydrofolate.  Primary source of endogenous methyl groups in the brain. Primary source of methyl groups for purine and pyrimidine synthesis.  Primary source of methyl groups for endogenous synthesis of methionine.  Activated by serine.  Inhibited by excess glycine.  B6 dependent.

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