Hans Adolf
Krebs
By: Victoria Sendanyoye, Carter
Beaupre-Mcphee and Iyoma
Edache
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
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
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)
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)
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)
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)
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
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,")
References
1.
2.
3.
4.
5.
6.
7.
8.
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/krebslecture.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 hans adolf. (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