Carsten Krebs Associate Professor of Biochemistry and Molecular Biology Associate Professor of Chemistry 306 South Frear Laboratory, University Park, PA 16802 Phone: (814) 865-6089 Fax: (814) 863-7024 Email: ckrebs@psu.edu B.S. in Chemistry, Ruhr-Universität Bochum, Germany Diplom in Chemistry, Ruhr-Universität Bochum, Germany Ph.D. in Chemistry, Max-Planck Institut für Strahlenchemie, Mülheim, Germany Postdoctoral, Physics, Emory University, Atlanta Georgia Krebs Lab Web Site

Bioinorganic Chemistry - spectroscopic and kinetic studies on the mechanisms of iron-containing enzymes

Enzymes that contain the transition metal iron in their active sites exhibit great structural and functional diversity and play important roles in almost every aspect of life. The goal of our interdisciplinary research program is to combine biochemical, kinetic, and spectroscopic methods to study Fe-containing enzymes. The main technique used in our laboratory is ⁵⁷Fe-Mössbauer spectroscopy. This technique provides information about oxidation state, spin state, coordination environment, and nuclearity of all chemically distinct iron species contained in a sample. In addition, it is possible to quantify all iron species. We combine this method with the rapid freeze quench (RFQ) method, and this allows us to monitor changes occuring at an iron site during a biochemical reaction. These studies (in conjunction with other techniques, such as stopped-flow absorption or RFQ EPR) provide detailed insight into the reaction mechanisms of iron-containing proteins.

Non-heme enzymes:

Our main focus in this area is the oxygen activation reaction of the Fe(II) and a-ketoglutarate(a-KG)-dependent dioxygenase enzyme family. These enzymes play important roles in biochemistry (oxygen sensing and initiation of response to hypoxia, DNA repair, biosynthesis of antibiotics, etc) and they are believed to operate by a common mechanism. In collaboration with the group of J. Martin Bollinger, Jr., we study one member of this class, taurine:a-KG dioxygenase (TauD), and we identified the first reaction intermediate observed in this class of enzymes. This species contains a Fe=O unit, in which the iron is formally in the oxidation state +IV in the high-spin (S= 2) configuration. This species is the key species that abstracts an H-atom from the substrate for subsequent hydroxylation.

Heme enzymes:
We study the electronic structure of several high-valent intermediates using ⁵⁷Fe-Mössbauer spectroscopy in collaboration with the group of Michael T. Green.

Iron-sulfur cluster enzymes:
Our main focus in this area is the study of the ‘Radical-SAM’ enzymes. These enzymes utilize a reduced [4Fe-4S] cluster to cleave S-adenosylmethionine (SAM) to methionine and a 5’-deoxyadenosylradical (5’-dAdo·) intermediate. The 5’-dAdo· is then used for various purposes. For example, we study the enzyme lipoate synthase using ⁵⁷Fe-Mössbauer spectroscopy with the group of Squire J. Booker.

Representative Publications:

• Krebs, C.; Huynh, B. H.  "Intermediates in oxygen activation reactions of diiron enzymes" in "Iron Metabolism." Ferreira, G. C., Moura, J. J. G. and Franco, R., Ed.; Wiley-VCH Verlag GmbH, Weinheim, Germany, 1999, pp 253-273.
• Hwang, J.; Krebs, C.; Huynh, B. H.; Edmondson, D. E.; Theil, E. C.; Penner-Hahn, J. E.   "A short Fe-Fe distance in peroxodiferric ferritin: control of Fe substrate versus cofactor decay?"  Science 2000, 287, 122-125.
• Agar, J. N.; Krebs, C.; Frazzon, J.; Huynh, B. H.; Dean, D. R.; Johnson, M.K. "IscU as a Scaffold for Iron-Sulfur Cluster Biosynthesis: Sequential Assembly of [2Fe-2S] and [4Fe-4S] Clusters in IscU"  Biochemistry 2000, 39, 7856-7862.
• Krebs, C.; Davydov, R.; Baldwin, J.; Hoffman, B. M.; Bollinger, J. M., Jr.; Huynh, B. H.   "Mössbauer and EPR Characterization of the S = 9/2 Mixed-Valence Fe(II)Fe(III) Cluster in the Cryoreduced R2 Subunit of Escherichia coli Ribonucleotide Reductase"  J. Am. Chem. Soc. 2000, 122, 5327-5336.
• Baldwin, J.; Krebs, C..; Ley, B. A.; Edmondson, D. E.; Huynh, B. H.; Bollinger, J. M., Jr.   "Mechanism of Rapid Electron Transfer during Oxygen Activation in the R2 Subunit of Escherichia coli Ribonucleotide Reductase. 1. Evidence for a Transient Tryptophan Radical". J. Am. Chem. Soc. 2000, 122, 12195-12206.
• Krebs, C.; Chen, S.; Baldwin, J.; Ley, B. A.; Patel, U.; Edmondson, D. E.; Huynh, B. H.; Bollinger, J. M., Jr. "Mechanism of Rapid Electron Transfer during Oxygen Activation in the R2 Subunit of Escherichia coli Ribonucleotide Reductase. 2. Evidence for and Consequences of Blocked Electron Transfer in the W48F Variant. J. Am. Chem. Soc. 2000, 122, 12207-12219.
• Krebs, C.; Henshaw, T. F.; Cheek, J.; Huynh, B. H.; Broderick, J. B. " Conversion of 3Fe-4S to 4Fe-4S Clusters in Native Pyruvate Formate-Lyase Activating Enzyme: Mössbauer Characterization and Implications for Mechanism"  J. Am. Chem. Soc. 2000, 122, 12497-12506.
• Krebs, C..; Agar, J. N.; Smith, A. D.; Frazzon, J.; Dean, D. R.; Huynh, B. H.; Johnson, M. K.   "IscA, an Alternate Scaffold for Fe-S Cluster Biosynthesis".  Biochemistry 2001, 40, 14069-14080.
• Baldwin, J.; Voegtli, W. C.; Khidekel, N.; Moënne-Loccoz, P.; Krebs, C.; Pereira, A. S.; Ley, B. A.; Huynh, B. H.; Loehr, T. M.; Riggs-Gelasco, P. J.; Rosenzweig, A. C.; Bollinger, J. M., Jr.     "Rational Reprogramming of the R2 Subunit of Escherichia coli Ribonucleotide Reductase into a Self-Hydroxylating Monooxygenase"   J. Am. Chem. Soc. 2001, 123, 7017-7030.
• Krebs, C.; Broderick, W. E.; Henshaw, T. F.; Broderick, J. B.; Huynh, B. H.   "Coordination of Adenosylmethionine to a Unique Iron Site of the [4Fe-4S] of Pyruvate Formate-Lyase Activating Enzyme: A Mössbauer Spectroscopic Study".   J. Am. Chem. Soc. 2002, 124, 912-913.
• Lee, D.; Pierce, B.; Krebs, C..; Hendrich, M. P.; Huynh, B. H.; Lippard, S. J.   "Functional Mimic of Dioxygen-Activating Centers in Non-Heme Diiron Enzymes: Mechanistic Implications of Paramagnetic Intermediates in the Reactions between Diiron(II) Complexes and Dioxygen"  J. Am. Chem. Soc. 2002, 124, 3993-4007.
• Krebs, C..; Bollinger, J. M., Jr.; Theil, E. C.; Huynh, B. H. "Exchange coupling constant J of peroxodiferric reaction intermediates determined by Mössbauer spectroscopy". J. Biol. Inorg. Chem. 2002, 7, 863-869.
• Price, J. C.; Barr, E. W.; Tirupati, B.; Bollinger, J. M., Jr.; Krebs, C. "The First Direct Characterization of a High-Valent Iron Intermediate in the Reaction of an a-Ketoglutarate-Dependent Dioxygenase: A High-Spin Fe(IV) Complex in Taurine/a-Ketoglutarate Dioxygenase (TauD) from Escherichia coli" Biochemistry 2003, 42, 7497-7508.
• Price, J. C.; Barr, E. W.; Glass, T. E.; Krebs, C.; Bollinger, J. M., Jr. Evidence for Hydrogen Abstraction from C1 of Taurine by the High-Spin Fe(IV) Intermediate Detected during Oxygen Activation by Taurine:a-Ketoglutarate Dioxygenase (TauD)" J. Am. Chem. Soc. 2003, 125, 13008-13009.
• Tripp, B. C.; Bell, C. B., III; Cruz, F.; Krebs, C.; Ferry, J. G. "A Role for Iron in an Ancient Carbonic Anhydrase". J. Biol. Chem. 2004, 279, 6683-6687.
• Riggs-Gelasco, P. J.; Price, J. C.; Guyer, R. B.; Brehm, J. H.; Barr, E. W.; Bollinger, J. M., Jr.; Krebs, C. EXAFS Spectroscopic Evidence for an Fe=O Unit in the Fe(IV) Intermediate Observed during Oxygen Activation by Taurine:a-Ketoglutarate Dioxygenase" J. Am. Chem. Soc. 2004, 126, 8108-8109.
• Cicchillo, R. M.; Baker, M. A.; Schnitzer, E. J.; Newman, E. B.; Krebs, C.; Booker, S. J. "Characterization of Escherichia coli L-Serine Dehydratase as an Iron-Sulfur Containing Enzyme" J. Biol. Chem. 2004, 279, 32418-32425.
• Cicchillo, R. M.; Lee, K.-H.; Baleanu-Gogonea, C.; Nesbitt, N. M.; Krebs, C..; Booker, S. J.  "Escherichia coli Lipoyl Synthase Binds Two Distinct [4Fe-4S] Cluster per Polypeptide" Biochemistry 2004, 43, 11770-11781.
• Krebs, C..; Price, J. C.; Baldwin, J.; Saleh, L.; Green, M. T.; Bollinger, J. M., Jr.  "Rapid Freeze-Quench 57Fe Mössbauer Spectroscopy: Monitoring Changes of an Iron-Containing Active Site during a Biochemical Reaction". Inorganic Chemistry 2005, 44, 742-757. 

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