148 PROTEINS 



2. Give the graphic formula for (1) cystine, (2) tyrosine, (3) tryptophan, (4) 

 lysine, (5) uric acid. Write out the graphic formula for (a) a dipeptide, (b) a 

 tripeptide. Give the elements contained in hematin. 



3. Name eight chemical groups contained in amino acid molecules. Which of 

 the above are responsible for color tests, and which reagent is used to show the 

 presence of the particular group? 



4. What is meant by the term "incomplete protein"? Name two proteins that 

 are incomplete, and show in what respect they are incomplete. 



5. Explain the use of the following reagents ia the Kjeldahl determination of 

 crude protein, (1) concentrated H2SO4, (2) Na2S04, (3) concentrated NaOH. What 

 is meant by the term "crude protein"? As usually determined, what assumptions are 

 involved? 



6. Which three proteins are the most abundant in the food that you have eaten 

 in the last three meals? Give data on which you base your conclusions. 



7. Approximately how many grams of tryptophan are there in a glass of milk? 



8. Which two amino acids are found most abundantly in proteins? 



9. Name two kinds of reagents that may be used to precipitate proteins. In each 

 case explain how the reagent brings about the precipitation, and name the product 

 formed. 



10. Give the name and source of (1) three proteins that are enzyines, (2) three 

 proteins contained in industrial nonfood products, (3) one protein that is a hormone, 

 (4) two proteins that are respiratory pigments, (5) two proteins other than those 

 named in (1) to (4) that have been crystallized. 



11. Which protein color tests will be positive with the tripeptide, tyrosyltryptophyl- 

 cystine? Give reasons for conclusion in case of each test. 



12. Correct the following statements if incorrect: 



(1) Milk and eggs are proteins that belong in every diet. 



(2) In calculating crude protein it is assumed that all the protein of the sample 

 is in the form of nitrogen. 



(3) In practical nutrition the deficiencies of incomplete proteins are remedied 

 by the addition of individual amino acids. 



13. If the tyrosine content of a protein is 3.78 per cent and the molecular weight is 

 42,000, calculate the number of moles of tyrosine per mole of protein. 



14. From the data given in the text calculate the number of residues of glycine, 

 alanine, and valine in (3-lactoglobulin. Do 3'our figures in round numbers check with 

 those of the text? 



15. In which two ways does cystathionine appear to be split in S-transfer? 



16. What is the least number of dipeptides that would have to be isolated and 

 identified to prove the sequence of anjino acids in p-amicidin S? 



17. For outside reading: How would you determine whether a dipeptide contain- 

 ing alanine and leucine is alanyl-leucine or leucyl-alanine? 



18. For outside reading: How many isomers of pteroyltriglutamic acid are due to 

 the arrangement of the glutamic acid residues. 



REFERENCES AND SUGGESTED READINGS 



Anson, M. L. and Edsall, J. T., Advances in Protein Chemistry, Vols. I-VII. Aca- 

 demic Press Inc., New York, 1945-51. 



Baldwin, E., Dynamic Aspects of Biochemistry, 2nd ed.. The University Press, Cam- 

 bridge, England, 1952. 



Bemal, J. D., "Structure of Proteins," Proc. Roy. Inst. Gt. Brit., 30, 541 (1939). 



