1 Glutathione as a Prebiotic Answer to α‑Peptide Based Life 2 Be ́ la Fiser, †,‡ Bala ́ zs Jó ja ́ rt, † Mila ́ n Sző ri, † Gyö rgy Lendvay, § Imre G. Csizmadia, †,∥ and Be ́ la Viskolcz* ,† 3 † Department of Chemical Informatics, Faculty of Education, University of Szeged, Boldogasszony sgt. 6, Szeged, Hungary-6725 4 ‡ Department of Organic Chemistry I, University of the Basque Country/UPV-EHU, Manuel de Lardizabal 3, Donostia-San Sebastia ́ n, 5 Spain-20018 6 § Institute for Materials and Environmental Chemistry, Research Center for Natural Sciences, Hungarian Academy of Sciences, 7 Magyar tudó sok krt. 2, Budapest, Hungary-1117 8 ∥ Department of Chemistry, University of Toronto, 80 St. George Str, Toronto, Ontario, Canada, M5S 3H6 9 * S Supporting Information 10 ABSTRACT: The energetics of peptide bond formation is an important factor 11 not only in the design of chemical peptide synthesis, but it also has a role in 12 protein biosynthesis. In this work, quantum chemical calculations at 10 13 different levels of theory including G3MP2B3 were performed on the 14 energetics of glutathione formation. The strength of the peptide bond is found 15 to be closely related to the acid strength of the to-be N-terminal and the 16 basicity of the to-be C-terminal amino acid. It is shown that the formation of 17 the first peptide activates the amino acid for the next condensation step, 18 manifested in bacterial protein synthesis where the first step is the formation of 19 an N-formylmethionine dipeptide. The possible role of glutathione in prebiotic 20 molecular evolution is also analyzed. The implications of the thermodynamics 21 of peptide bond formation in prebiotic peptide formation as well as in the 22 preference of α- instead of β- or γ-amino acids are discussed. An empirical 23 correction is proposed for the compensation of the error due to the incapability of continuum solvation models in describing the 24 change of the first solvation shell when a peptide bond is formed from two zwitterions accompanied by the disappearance of one 25 ion pair. 26 ■ INTRODUCTION 27 In the biosynthesis of bacterial proteins, the amino acid (AA) 28 polymerization always begins with formation of a peptide bond 29 to the carboxyl group of a modi fied methionine, N- 30 formylmethionine (fMet). In the first step of bacterial protein 31 synthesis, the amino group of methionine is protected by 32 enzymatic formylation of the NH 2 group so that the next s1 33 residue can attack only its carboxyl group 1 (Scheme 1). 34 The first amino acid residue that will connect to fMet later 35 will be the N-terminal end of the protein. The peptide chain is 36 then built step by step, each new peptide bond being formed by 37 the carboxyl group of the C-terminus amino acid whose α- 38 amino group is involved in an existing peptide bond. Finally, 39 the methionine is removed from the N-terminus of the protein. 40 In fact, N-formylmethionine acts like a catalyst or an activator: 41 connecting to the amino group, it makes the would-be N- 42 terminal amino acid capable of forming a new peptide bond at 43 the C-terminus. Similar “activation” seems to operate in the 44 biosynthesis of other peptides, too. For example, in the 45 synthesis of glutathione (γ-L-glutamyl-L-cysteinyl-glycine, GSH, f1 46 Figure 1, bottom right), in spite of being performed by 47 completely different enzymes in different organisms, the first 48 step is always formation of the peptide bond involving the γ- 49 carboxyl group of glutamic acid and the amino group of 50 cysteine. The common features of these processes indicate that 51 the chemistry, in particular, the themodynamical characteristics, 52 can be similar. Investigation of the simpler case, the energetics 53 of glutathione formation, can help one to understand how this 54 “activation” works. 55 GSH is accumulated in several cellular compartments such as 56 the cytosol, nucleus, and mitochondria (in as high concen- 57 tration as 1−11, 3−15, and 5−10 mM, respectively). 2 Besides 58 many of its other features, it is one of the most important 59 antioxidants, 3−5 and it contributes to amino acid transport 60 through the cell membrane. 6,7 GSH has an essential role in 61 numerous biochemical processes like cell differentiation, 62 proliferation, apoptosis, signal transduction, and gene ex- 63 pression. 8,9 A large variety of human diseases like cystic 64 fibrosis, cancer, and neurodegenerative diseases are closely 65 related to the irregular GSH homeostasis. 10−13 Its omnipre- 66 sence indicates that it has some structural element that lends it 67 the capability of performing a special function, as well as of 68 surviving and remaining active in drastically different environ- 69 ments. 70 What is unique in GSH is that the energetics of its formation, 71 together with those of its α analogue (L-glutamyl-L-cysteinyl- Received: November 19, 2014 Revised: February 15, 2015 Article pubs.acs.org/JPCB © XXXX American Chemical Society A DOI: 10.1021/jp511582m J. Phys. Chem. B XXXX, XXX, XXX−XXX jem00 | ACSJCA | JCA10.0.1465/W Unicode | research.3f (R3.6.i7:4236 | 2.0 alpha 39) 2014/12/19 13:33:00 | PROD-JCA1 | rq_3326767 | 2/25/2015 15:13:56 | 8 | JCA-DEFAULT