Hans Adolf Krebs

1. Hans Adolf Krebs By: Victoria Sendanyoye, Carter Beaupre-Mcphee and IyomaEdache

2. Overview • About Hans Adolf Krebs • Prior knowledge of metabolic reactions • Krebs’ experimental design • Observations and Results • Interpretation of the results • Conclusion and major discovery • Efficiency of Krebs’ Cycle

3. History: Hans Krebs (1900-1981) • German physician and biochemist • Identified two important metabolic processes: the urea cycle (ornithine cycle) and the citric acid cycle, which was discovered in 1937 (Ref. 2 Helmenstine) • Earned a Noble Prize for the citric acid cycle (Krebs’ cycle) in 1953 • He taught at Cambridge and at the University of Sheffield and after 1954 was a professor of biochemistry at Oxford(Ref. 6 "Krebs, sir hans," 2007) • Krebs Cycle was originally known as the tricarborxylic acid cycle

4. History: Knowledge of Metabolic Processes (Early 1900’s) • Progress was made in the study of fermentation • Very little was known about the oxidization of sugar in living cells • In 1935, biochemist Albert Szent-Gyorgyi was able to describe the sequence of reactions of succinate oxidation, specifically succinate to fumarate to malate to oxaloacetate (dicarborxylic acids) • It was confirmed that the organic acids act as cataylsts • Martius and Knoop discovered another part of the sequence. That is, citrate to isocitrate to alpha-ketoglutarate to succinate (tricarboxylic acids) (Ref. 1 Caprette)

5. Krebs’ Experimental Design • Independent Variable: Malonate (competitive inhibitor of the enzyme succinate dehydrogenase) • Dependent Variable: Accumulation of succinate • Controlled variables: type and amount of muscle tissue (minced or grinded pigeon breast muscle), type and amount of competitive inhibitor, and amount of the organic acids. • Malonate was added to muscle suspensions in aqueous solutions in the presence of the dicarboxylic and tricarboxylic acids (Ref. 3 Krebs, 1953)

6. Observations and Results • When the malonate was added to the muscle suspension in the presence of these organic acids, there was major accumulation of succinate (Ref. 1 Caprette) • This also inhibited the oxidation of the pyruvate since there was a limited amount of oxaloacetate • In the uninhibited system, one oxaloacetate molecule could oxidize many pyruvates • In the poisoned system, only one pyruvate could be oxidized per one oxaloacetate molecule • Surprise breakthrough! Citrate was readily formed in muscle provided that oxaloacetate was present (Ref. 3 Krebs, 1953)

7. Interpretation of the Results • The fact that malonate inhibited the entire sequence of the reactions when it was added to each of the organic acids indicated that the sequence was cyclic (Ref. 1 Caprette) • It also indicated that succinate and succinate dehydrogenase are essential components in the enzymatic reactions • Some citrate and alpha-ketoglutarate accumulated as well which suggested that they are produced before succinate • It was assumed that the formation of citrate from oxaloacetate occurred as a result of the oxaloacetate condensing with a substance derived from a carbohydrate, such as pyruvate or acetate (Ref. 3 Krebs, 1953)

8. Conclusion and Major discovery (1937) • The discovery of the synthesis of citrate from oxaloacetate completed the scheme of carbohydrate oxidation (Ref. 1 Caprette) • This concept explained the catalytic nature of the di- and tricarboxylic acids and their ability to oxidize in tissues that oxidize carbohydrates, as well as fatty acids, and amino acids • Years later, the citric acid cycle was found to function in the tissues of aerobic plants and micoorganisms ( Ref. 7 "The citric acid,"). Original citric acid cycle

9. The Efficiency of the Citric Acid Cycle (1940’s) • The details of the citric acid cycle were worked out by the study of highly purified enzymes of the cycle • Some questioned whether these enzymes really did function in a cycle in living cells and whether the rate was high enough to account for all the glucose oxidation in animals • Metabolites such as pyruvate and acetate were isotopically labeled with 13C or 14C and were traced throughout the pathway of the citric acid cycle (isotope tracer technique) • It was confirmed that it does take place in living cells and that it does occur at a high rate ( Ref. 7 "The citric acid,")

10. References • Caprette, D. (n.d.). Hans krebs (1900-1981). Retrieved from http://www.ruf.rice.edu/~bioslabs/studies/mitochondria/krebs.html • Helmenstine, A. (n.d.). Overview of the citric acid cycle. Retrieved from http://chemistry.about.com/od/biochemistry/ss/citricacidcycle.htm • Krebs, H. (1953, December 11). The citric acid cycle. Retrieved from http://www.nobelprize.org/nobel_prizes/medicine/laureates/1953/krebs-lecture.pdf • Wilson, B. A., Schisler, J. C., & Willis, M. S. (2010). Sir Hans Adolf Krebs: Architect of Metabolic Cycles. Lab Medicine, 41, 377-380. doi:10.1309/LMZ5ZLAC85GFMGHU. • No author (n.d.). Hans_Krebs_Citric_Acid. Oxford University Press - homepage. Retrieved October 17, 2012, from http://www.oup.com/us/companion.websites/9780195305753/pdf/Hans_Krebs_Citric_Acid.pdf • Krebs, sir hansadolf. (2007). Retrieved from http://www.factmonster.com/ce6/people/A0828224.html • The citric acid cycle. (n.d.). Retrieved from http://www.bioinfo.org.cn/book/biochemistry/chapt15/sim3.htm • The citric acid cycle. (n.d.). Retrieved from http://www.bmb.leeds.ac.uk/illingworth/metabol/krebs.htm