Trimethyllysines

Enzymatic hydroxylation of biological molecules is often catalyzed by hydroxylases. These types of enzymes are either oxygenases or peroxidases, in which the source of oxygen is O2 or H2O2, respectively. Cytochrome P-450-dependent enzymes represent a common class of enzymes that carry out hydroxylation reactions. L-Carnitine is a metabolite isolated from many organisms and its biosynthesis begins with the enzymatic hydroxylation of trimethyllysine. The intermediate, 3-hydroxyl-e-(A(A(ALtrimethyl)-L-lysine, is further... 

In different organs of the rat [128], Ehrlich ascites tumor cells [144], trout testis [127], calf thymus [145], and carp testis [146], H4 is modified mainly as the N -dimethyllysine, while H3 is modified as N -monomethyllysine, N -dimethylly-sine and N -trimethyllysine with the N -dimethyllysine predominating. Pea seedling H4 is not methylated and H3 exits as N -mono- and N -dimethyllysine with N -trimethyllysine not being detectable [147,148]. 

Figure 13.1 (a) Trimethyllysine 4 and dimethylarginine 2 at histone H3 from X-ray structure [4]. (b) Chemical structure of the methylating agent S-adenosyl-L-methionine. 

P.A., Bmnzelle, J.S. and Trievel, R.C. (2007) Specificity and mechanism of JMJD2A, a trimethyllysine-specific histone demethylase. Nature Structural S, Molecular Biology, 14, 689-695. 

Carnitine, L-3-hydroxy-4-(trimethylammonium)butyrate, is a water-soluble, tri-methylammonium derivative of y-amino-jS-hydroxybutyric acid, which is formed from trimethyllysine via y-butyrobetaine [40]. About 75% of carnitine is obtained from dietary intake of meat, fish, and dairy products containing proteins with trimethyllysine residues. Under normal conditions, endogenous synthesis from lysine and methionine plays a minor role, but can be stimulated by a diet low in carnitine. Carnitine is not further metabolized and is excreted in urine and bile as free carnitine or as conjugated carnitine esters [1, 41, 42]. Adequate intracellular levels of carnitine are therefore maintained by mechanisms that modulate dietary intake, endogenous synthesis, reabsorption, and cellular uptake. 

Monomethyl- and dimethyllysine residues (Fig. 27-29c) occur in some muscle proteins and in cytochrome c. The calmodulin of most species contains one trimethyllysine residue at a specific position. In other proteins, the carboxyl groups of some Glu residues undergo methylation, removing their negative charge. 

While proteins may be modified to favor rapid ubiquitination, others may be altered to protect them from ubiquitination. For example, calmodulin produced from a cloned gene in bacteria is a good substrate for ubiquitination but within cells it appears to be protected by the posttranslational conversion of Lys 115 to trimethyllysine.v... 

Trimethylamine dehydrogenase 782, 784s Trimethylarsonium lactic acid, 387s Trimethyllysine... 

Lysine is not only a constituent of proteins. It can also be trimethylated and converted to carnitine (p. 944). In mammals some specific lysyl side chains of proteins undergo N-trimethylation and proteolytic degradation with release of free trimethyllysine (Eq. 24-30) 278/279 The free trimethyllysine then undergoes hydroxylation by a 2-oxoglutarate-Fe2+-ascorbate-dependent hydroxylase (Eq. 18-51) to form P-hydroxytrimethyllysine, which is cleaved by a PLP-dependent enzyme (Chapter 14). The resulting aldehyde is oxidized to the carboxylic acid and is converted by a second 2-oxoglutarate-Fe2+-ascorbate-dependent hydroxylase to carnitine (Eq. 24-30 see also Eq. 18-50). 

Individuals with HMG-CoA lyase deficiency are particularly susceptible to carnitine deficiency. With restriction of red meats and dairy products, dietary carnitine intake is quite low. Carnitine is also synthesized endogenously from the modified, methylated lysine resides of various proteins free trimethyllysine is released when the protein is degraded. Since the therapy for patients with HMG-CoA lyase deficiency must minimize their endogenous protein catabolism, they also have limited availability of trimethyllysine for carnitine synthesis. 

As shown in Table 13.1, a number of iron-containing hydroxylases share an unusual reaction mechanism in which hydroxylation of the substrate is linked to decarboxylation of 2-oxoglutarate. Proline and lysine hydroxylases are required for the postsynthetic modification of collagen, and proline hydroxylase also for the postsynthetic modification of osteocalcin (Section 5.3.3) and other proteins. Aspartate /3-hydroxylase is required for the postsynthetic modification of protein C, the vitamin K-dependent protease that hydrolyzes activated Factor V in the blood clotting cascade (Section 5.3.2). Trimethyllysine and y-butyrobetaine hydroxylases are required for the synthesis of carnitine (Section 14.1.1). 

In general, the effects on collagen synthesis are more marked and more important than those of decreased formation of carnitine (as a result of impaired activity of trimethyllysine and y-butyrobetaine hydroxylases Section 14.1.1), impaired xenobiotic metabolism, or hypercholesterolemia (Section 13.3.8). However, depletion of muscle carititine may account for the lassitude and fatigue that precede clinical signs of scurvy. 

Carnitine is synthesized from lysine and methionine by the pathway shown in Figure 14.2 (Vaz and Wanders, 2002). The synthesis of carnitine involves the stepwise methylation of a protein-incorporated lysine residue at the expense of methionine to yield a trimethyllysine residue. Free trimethyllysine is then released by proteolysis. It is not clear whether there is a specific precursor protein for carnitine synthesis, because trimethyllysine occurs in a number of proteins, including actin, calmodulin, cytochrome c, histones, and myosin. 

Figure 14.2. Biosynthesis of carnitine. Trimethyllysine hydroxylase, EC 1.14.11.8 aldolase, EC 4.1.2. x aldehyde dehydrogenase, EC 1.2.1.47 -butyrohetaine hydroxylase, EC 1.14.11.1. Relative molecnlar mass (Mr) carnitine, 161.2.

Both hydroxylation reactions in the synthesis of carnitine from trimethyllysine are ascorbic acid-dependent, 2-oxoglutarate-linked, reactions (Section 13.3.3), and impaired synthesis of carnitine probably accounts for the muscle fatigue associated with vitamin C deficiency. 

In general, the effects on collagen synthesis are more marked and more important than those of decreased formation of carnitine (as a result of impaired activity of trimethyllysine and y-butyrobetaine hydroxylases Section... 

Y. Kakimoto and S. Akazawa, Isolation and Identification ofNG, NG-Dimethylarginine, N-Epsilon-Mono-, Di-, and Trimethyllysine, and Glucosylgalactosyl- and Galactosyl-Delta-Hydroxylysine From Human Urine, Journal of Biological Chemistry 245 (1970) 5751 -5758. 

The typical Western diet supplies about 100 mg of carnitine per day, Meat and milk are good sources beef contains about 500 mg/kg, and cow milk contains 5 to 40 mg/kg of fluid. Hfen eggs and plant food contain little or no carnitine. The trimethyllysine present in food proteins contributes, to a very small extent, to Ihe carnitine made in the body. There is, however some concern about a transient deficiency of carnitine in newhom infants fed carnitine-free diets. Soy-based infant formula may lack carnitine, and the neonate seems to have a lesser capacity to synthesize the cofactor than adults. Breast milk contaiirs high levels of carnitine after about a month of lactation, the content declines to about half of its initial value. There is little evidence that the lowered levels of plasma carnitine found in infants fed soy formulas are associated with an impairment of the oxidation of long-[Pg.225]

Carnitine acts catalytically and might be considered to be a cofactor of carnitine fatty acyltransferase. Unlike most cofactors, however, carnitine is not a vitamin and is not derived from a vitamin. Generally, all of the carnitine needed by the body can be S5mthesized by the body. The biosynthesis of carnitine begins in an unusual manner. The starting material, the lysine residues of a variety of proteins, undergo post-translational methylations, as shown in Figure 4.55. The methyl donor is S-adenosylmethionine. Trimethyllysine, liberated from the protein by intracellular hydrolysis, is hydroxylated and then converted to carnitine in three... 

---