LeRoy Kuehl Depariment of Biochemistry University of Utah College of Medicine Salt Lake City, Utah 841 12
A Novel Method for Presenting the Amino Acids in an Introductory Biochemistry Course
At the University of Utah, freshmen medical students are required to take a three-quarter course in biochemistry. The organization of this course follows traditional lines: the structure and properties of biomolecules are covered first; metabolic pathways are dealt with second; a section on molecular biology is presented next; and finally, various special topics such as nutrition and endocrinology are discussed. Maintaining the students' interest during the first part of the course has proven particularlv difficult since the initial lecrures are, fnr the most part, little more than a tabulation of the structuresand properties of various biologically imponant molecules. This year a novel teaching was employed during the presentation ofone portion of thc introductory material: namely, that dealing with the amino acids. This~approach, which will he described below, appeared quite effective in stimulating the students' interest in a topic which they usually find quite tedious. Two 50-minute periods were devoted to the amino acids. During the first 25 minutes, general information about the . amino acids and their role in protein structure was presented usingstandard lecture format. The last half of the first period was devoted to a readina of the voem, "Amino Acid Tales." which is reproduced helow. ~ h i s i o e mwritten ; in the style of Chaucer's "Canterhurv Tales." includes a descrivtion ot'the structure and principal physi'cal and chemical Goperties of each amino acid alone with additional information for those amino acids which a particularly important or unique role in prutein structure or in metabolism. Slides illustrating the material described in the poem were shown a t approGiate points during the reading. During the second period, the verses were reread. This time the reading was interrupted at intervals to discuss in greater depth important concepts which are vresented onlv, sunerficiallv in the noem. . The initial reading ot'the poem h a s made as dramatic as vossihle. The students were not informed in advance that the iectures on amino acids would be different from any others. Just before the verses were read, the class was told that they could dispense with note taking since they would be supplied with a complete copy of the material which was to follow. Advanced planning assured that the slide changing would occur a t the proper times and with no interruption of the presentation. The poem was read slowly and with considerably
more dramatic emphasis than is customary in a biochemistry lecture. During the reading of the prologue the mood of the class was jocular and somewhat boisterous. When the verses dealing with the individual amino acids were read, however, the students became very quiet, giving the impression that they were concentrating intently on the material being presented. At the couclusion of the poem, the class responded with a round of sustained and viaorous avplause and afterwards a number of .. students made i;;,sitive commcntsahout the use of the poem for teaching the amino acids. Thus, one obtained the i p pression that the presentation as described above had succeeded in stimulating; the students' interest and holding their attention. This impr&sion was reinforced hy the r e d & of an anonymous questionnaire given about a week afterwards in which the students were asked: "As compared with a wellprepared traditional lecture, how stimulating did you find the presentation based on the poem?" Of 91 students who responded, 79 found the presentation "much more stimulating than a traditional lecture:" 11 found it "somewhat more stimulating," and one found it "about the same." No one found it somewhat or much less stimulatina. The same auest~onnoirt:indicated that mcgst s t u d m t i (64%) spent 50% or mure of their vrevaration time studvine the ooem and that they considered it a more effective learning device than standard lecture notes. 'l'he poem, "Amino Acid 'l'ales," is reproduced below together with a few revresentativr examrdes of the illustrations which accompanied the text. Amino Acid Tales to G.Choucer
with apologies Prologue
When fall hnth come, and days grow shwr and cool, Then eager students hasten hack to school; And freshmen who would gladly doctors be Begin to study biochemistry, And memorize a host of useless structures Because they know that pleases their instructors, But also so that they their boards might pass, And go to practice medicine at last; For they would fain restore the sick to health,
Volume 55, Number 11, November 1978 / 711
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A second center of asymmetry. Now serine oft is cleaved within the cell To glycine and a smaller piece as well. The latter's then to synthesis remanded When a one-carbon fragment is demanded. Melhionlna
To alanine an extra carbon lend, And next attach a sulfur t o the end, Then finally if you methylate the S Mcthwnine is u hat you uill pusses% Examine now the R-group carefully, It's t r u h h v d m u h h r , as you'll see. ~ e a c t i o k khich s in livine Ee~lstransoire Quite often d o n methyl group IP~IIIIP: And usually does the cell s w h unlts d ~ n n Frum thc S-methyl ofmcthiunine ~
Figure l.Valine.
Figure 2. Cysteine
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Cyst*tn*
And also would win fame, respect, and wealth. As first to teach in biochemistry. The section treating structures falls t o me. With the amino acids we begin, The building blocks of muscle, enzymes, skin. Glycine
For R-group glycine has an H, that's all. I t boasts no isomers and is so small; But when in protein structure space is tight, Then glycine's chosen because it is slight; And this, dear students, is the reason why In collagen the glycine content's high. Alanlne
Draw glycine, then with pen a methyl add, And alanine will he there on your pad. The methyl group, apolar as you know, Gives alanine a hydruphobic glow. If alanine you now should modify And to its methyl various groups apply, All the amino acids we will learn Can quickly be produced, each in its turn. Valine
To d i n e learn, imagine, if you can, A structure with the outline of a man. He's hydrophobic from the waist on down, And hydrophilic is from waist to crown. Leuclne and lsoleucine
T o valine's leg affix one carbon more, And isoleucine joins the growing corps. In valine's trunk instead a C insert, And d i n e then to leucine does convert. Their R-groups are like little drops of oil; From water they with loathing do recoil. At isoleucine look now carefully; Two asymmetric carbons you will see. Proline
Five carbon atoms fastened end t o end, Just look, my students, notice how they bend Until, in sooth, the circle is perfected, And the last C is t o the N connected To form a hydrophobic little ring, And the amino a substituent bring. Amino acid proline's truly not, For an imino group instead it's got. Now polypeptide chains coil often round. In many proteins are such spirals found. As alpha helices by scientists known, These coils are by H-bonds together sewn. But should thechain with proline he corrupted, Then is the alpha helix interrupted. Serlne and Threonlne
T o alanine an OH group append, And serine's what you're left with in the end; And if you add a methyl group as well Then you have threonine, so chemists tellIndeed, a very bydrophilic pair, Because of the hydroxyls that they bear. Check threonine most carefully and you'll see 712 / Journal of Chemical Education
Just add a n S H group t o alanine; The compound that is formed is cysteine. Its S H can a proton liberate, The pK of this group being close to eight. But more important, you should realize, That the sulfhydryl group can oxidize, And that, thereby, two eysteines are joined (For such a pair the name eystine is coined) If cyitrinesaw lmked, it'ssurely true. The pepr~dechams they're part uf ore jorned, too Thus urotein structures. full of folds and kinks. Are hlld together by &tine cross-links. Phanylalanine and Tyroalns
We now consider phenylalanine, Whose name alone the structure does convene. And tryosine, in structure close related, Just phenylalanine hydroxylated. When phenyl group has a hydroxyl gained Then are its orooerties suhstantiallv changed: Decreased is.its hydrophobicity; More strongly it absorbs in the uv. And should the pH over ten arise, Then does this new hydroxyl ionize. In proteins this OH is wont to form H-bonds, and these, and others, do transform A random polypeptide, as a rule, T o a precisely folded molecule. An enzyme found within each living cell Performs this same hydroxylation well; But should there in this enzyme lie a fault, Phenylketonuria is the result. Tryptophan
Let alanine an indole function gain, And from the two arises tryptophan. (The indole group, in case you don't remember, Has benzene ring and pyrrole fused together. And pyrrole-is i t hard remembering?Has four carbons and an N joined in a ring.) Now indole is a planar residue; Aside from this, it's hydrophobic, t w . The indole group so strongly resonates That i t imoineine-. ohotons caotivatesTo an absorption spectrum this gives rise Which is presented for you to apprise.
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tH2 Cysteine SH
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+H3N-C-H +H#-C-H I I Cystine CH2- S-S-CHZ Figure 3. Oxidation of cystsine.
+HzO
Arpartb and Glularmc Adds
Now aspartate has carbon atoms four; And glutamate has these and then one more Carboxyl groups a t each extremity Make these compounds acidic, you'll agree. Alpha carhoxyls have pK's near two, So it may come as a surprise to you, That p K values close to four attend Carboxyl groups placed a t the other end. And now about an enzyme I'll relate Which the amino cleaves from glutamate Tu vield ammonia. there inside the cell. And alpha ket&;nmte as wcll. A rermd enzyme then the latter takes. And l r w l it glutantare regenerates. For this amino groups are now required, And from amino acids they're acquired. Thus using glutamate, as you can see, The cell has this broad capahility: Diverse amino acids can it take. And wcry me of them denmm&. And r r s d u t s which then are left hehind. To mrrnln,l~cpnrhways are conrigned. Asparaghe and GM& Aspartate's amide is asparagine, And glutamate's is known as glutamine. The two are neutral-amides have no charge, But polar still with dipole moments large.
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Arglnlne
If alanine's two carbons more extended, And a guanido's t o the end appended, A compound's formed which arginine we callMost base amino acid of them all; For the guanida group has pK high; At nearly 12.5 it's known to lie. (Now the guanido group, my freshmen friends, Is but a C surrounded by three N's.) A liver enzyme, arginase by name, Does act on arginine and cleaves the same By hydrolysis, for water comes between, To yield urea and also ornithine. The latter converts back to arginine By a complex, but key, reaction scheme In which excess ammonia is consumed. Except for t h ~the i cell were F I I T P Idoomed. ~ Thuzarg~nine-you should remcmbcr thrsIs source of the urea in your piss. ~~
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Figure 4. Disulfide cross-links in a protein Lyslne
This unbranched basic molecule is lysine. I t has four carbons more than are in glycine, And an amino group on its tail end Which has a pK value over ten. Hlstidlne
The residue which histidine we call Is analine with an imidazole. The latter is-now listen closely, pleaseA pentagon with two N's and three C's. To histidine s proton can affix; Its R-group has a p K close to six. Dear students, this is, as you know full well, Not far from the pH within the cell. And since the pK of a group may change Influenced by other groups lying at close range So histidine within a protein structure Shows sometimes one ion form, sometimes the other. EpllDgue
With the amino acids we are through. The learnine vou. " of them now is un . to . Do not desoair. hut work industriouslv: ,. And you wtll hare them mastered prtaenrly: And thmk, when i t is late and !ou gruw bured. Of the M.D. that will be your reward. ~
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Volume 55. Number 11. November 1978 / 713
