Cysteine Cystine Methionine and Taurine Figure

Methionine is converted to cysteine and its dipeptide cystine. In addition methionine is a precursor for creatine (see arginine). The potential for formation of disulfide bonds between its thiol (-SH) groups makes protein-bound cysteine important in the folding and structural assembly of proteins. Reduced cysteine thiol groups are found in protein (albumin), free cysteine, and in the principal intracellular anti-oxidant tripeptide glutathione (see glycine, glutamic acid) for which free cysteine is the synthesis rate-limiting constituent. Through the formation of disulfides (e.g., cystine, cysteinyl-glutathione, glu-tathione disulfide, mercaptalbumin) thiol-containing molecules can scavenge oxygen-derived free radicals. The ratio between oxidized and reduced thiol groups reflects the cellular redox state. Owing to its small pool size cysteine deficiencies rapidly occur during malnutrition.

Cysteine is also the precursor for taurine, which is abundant in all mammalian cells, particularly in neuronal cells and lymphocytes, but is not a true amino acid and is not incorporated in proteins. Taurine is involved in the conjugation of bile acids

Arginine Creatine ^^^

(constituent/energy -Methionine source for skeletal muscle) \



Glutathione 4___ Cysteine _ Cystine

(an«oxiclant) (protein folding) (sulfur storage)


(bile acid conjugation, neuronal cell development, regulation membrane potential, calcium transport, antioxidant)

Figure 2 Specific functions of sulfur-containing amino acids.

and may act as an antioxidant. Moreover, taurine is an osmolyte by virtue of the fact that through its transporter its intracellular concentrations are between 50 and100-fold higher than in the extracellular compartment. This gradient contributes to the maintenance of the cellular hydration state. Similarly, it has been proposed that taurine is involved in stabilization of cell membrane potential and regulation of Ca2+ transport through several calcium-ion channels. Based upon these characteristics it has been suggested that taurine is involved in the control of cardiac muscle cell contraction, which has led to the addition of taurine to commercially available energy drinks. Its high level in lymphocytes suggests an important role in immunological resistance to infections. Taurine plays an important part in the development and maintenance of neuronal and especially retinal cells.

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