Thrice round the cycle

Thrice round the cycle

169 Teaching the TCA Cycle At a recent meeting of the Education Group of The Biochemical Society (Belfast, September 1986) the topic for the day was ...

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169 Teaching the TCA Cycle

At a recent meeting of the Education Group of The Biochemical Society (Belfast, September 1986) the topic for the day was 'What to Leave Out: Curriculum Development in Biochemistry' and the main talks were reported in Biochemical Society Transactions 14, 415-431 (1986). (Offprints of these proceedings are available from The Editorial Office.) In the afternoon a workshop was held in which four speakers (teachers of biochemistry) were each given 10-15 minutes in which to describe how they would teach the TCA Cycle. They were asked not to confer with each other in advance. Their remit was how to teach the cycle in an interesting way to first-year science students who thought they had already 'done' the Krebs Cycle in highschool biology. They were also asked to say how they would bring out the central importance of metabolism and to cope with the feeling held by students and some molecular biologists that metabolism is 'ancient history'. Finally they were encouraged to use and demonstrate audiovisual aids. Margot Kogut and Bernar.d Brown teach in university departments and Terry Vickers and David Fell teach in polytechnic institutions. Only a small fraction of their students will be taking biochemistry as a single subject: most will be doing it as a subsidiary to a life science B.Sc. course (pharmacology, physiology, genetics, agriculture) or professional course (nursing, food science, technical education certificate). These speakers were invited to write down what they thought they said at the Workshop and their offerings are published below. Almost all of them used a scheme of the TCA cycle, but it was decided that in order to save space it was unnecessary to reprint these here. Thrice R o u n d the C y c l e BERNARD S BROWN

Biochemistry Department Medical School University of Manchester Manchester M13 9PT, UK Introduction

The citric acid cycle is not difficult. In fact it's fun! This is the message I would try to convey if I were teaching the cycle to a group of first-year students who had already 'done' it at school. To this end I would take them on three trips round the cycle. First trip m a guided tour

I would begin by presenting the whole cycle, names and structures, on an O H P slide, and also on a hand-out. Using this as a map I would take the class on a guided tour of the cycle by asking them questions designed to highlight its main features. I would ask them how many carbons there were in oxaloacetate; in acetyl; in citrate; in 2-oxoglutarate; in succinate. Reference to the hand-outs would ensure that there were no wrong answers! To professional biochemists these questions may seem obvious, even childish. But the succession of correct answers so early in the session would encourage the students. And some, I suspect, would gain an insight into the working of the cycle that they had previously not possessed. BIOCHEMICAL EDUCATION

14(4) 1986

Having let them in gently I would then ask a more difficult question: What does it do? This would probably produce an immediate silence, which might eventually be broken by the utterance of a number of 'standard' answers (Fig 1). Please don't misunderstand me: all these answers are correct! But do they indicate an understanding of the simplicity and design of the cycle? I doubt it!

Figure 1 So, to nudge the students towards the answer I want I'd overlay my first O H P slide to draw their attention to what goes into the cycle (acetyl) and what comes out (CO2 and N A D H / F A D H a ) , and hopefully, after such gentle persuasion, someone would come up with the answer: "It oxidises acetyl"! Yes! Basically the cycle oxidises acetyl groups. To emphasise this point I would use slides and overlays (Fig 2), and would probably say something like this: "The citric acid cycle starts with acetyl (Fig 2a) and rips it apart into carbon dioxide and water (Fig 2b) with the release of energy. "Now if you count the atoms you'll see that acetyl contains two C A R B O N S which end up as CO2; it contains four H Y D R O G E N S which end up as H 2 0 ; but although it contains only two O X Y G E N S , six have emerged from the reaction in 2CO2 and 2H20~ Four extra oxygens have to be supplied. "These extra oxygens come from two sources (Fig 2c); Two are derived from components of the cycle (and end up in CO2); and two are encountered at the end of the electron transport chain (and end up in HzO ). "The lines radiating from the diagram (Fig 2c) represent the energy that is released, some of which is trapped as ATP, and some of which 'escapes' as heat." Second trip m as easy as A - B - C

The above bird's-eye-view completes the first trip round the cycle. I would then take a second trip to examine the cycle's workings in more detail. Referring to the handout, and with the corresponding slide on show, I would point out the three phases that comprise the cycle. Indeed, the cycle is "as easy as A - B - C , " I would say, as with successive overlays I revealed (Fig 3) Attachment to carrier, Breaking up of carrier,~ and Carrier regeneration. "One day I'll publish this and make a fortune, ''2 I'd say, "meanwhile it's yours for free!"


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Figure 2 Here, then, are three pegs onto which to hang the cycle. Other facts, and even the individual reactions and structures of the cycle, can be considered in the context of these pegs. So we've been round the cycle twice; once for a fairly superficial view to give the overall picture, and once for a slightly more detailed view to give a glimpse of its inner workings. What more is there to do?

Third trip - - mini-sagas and mnemonics One more thing I would do, in an attempt to sustain interest, would be to present the students with some m e m o r y aids. And I would encourage them to devise their own summaries and mnemonics. My attempts are shown in Figures 4 and 5. My fourteen-word-summary (Fig 4) - a mini-saga, if you like - - attempts to capture the essence of the cycle with the smallest number of words. Could it be improved, or even shortened? I would encourage my ~tudents to do better, asking them to imagine themselves having to telegraph a summary to s o m e o n e on the other side of the world at, say, £5 a word! BIOCHEMICAL


14(4) 1986

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My two mnemonics (Fig 5) are acrostics using the initial letters of words as reminders of the cycle. The first (Fig 5a) extends the A - B - C of the cycle into further regions of the alphabet, and reveals more of the cycle's inner workings than does the simple A - B - C scheme. My second mnemonic (Fig 5b) exposes the cycle by means of the initial letters of the names of its intermediates. ~CETYL C~A

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171 The purpose of these aids is not to encourage parrotlearning. Rather, their aim is to stimulate interest, and to make an unfriendly topic look a little less unfriendly. If we can achieve this, then we've won half the battle.

But what about afterwards? The lecture theatre is empty, the students have gone. In a few hours they will discover that they have been mildly conned: that, despite the mnemonics and summaries, they will still have to do some work on the cycle in order to understand and learn it. Can they be helped at this stage? Of course they can! We all know what we'd do, don't we? We'd give them a sheet of questions to work through at home, wouldn't we? And so would I . . . . But here again I would try to introduce some fun into the work. Have you ever wondered why some students will happily spend hours working on, say, a crossword puzzle, but be completely turned off by something like: Outline the ways in which the citric acid cycle contributes to catabolic and anabolic pathways? Could it be that, whereas they are curious about (hence interested in) the answer to a crossword clue, they couldn't care less about the answer to the above cunningly devised question? Unbelievable isn't it! But probably true nonetheless. So why not keep our cunningly devised question for the exam paper and, in the meantime, give the students questions that at least appear interesting. We can try. Table 1 gives four questions which transport the citric acid cycle from the dusty pages of the textbook to the everyday world of food and drink. They are certainly more intriguing than the bald request to lay bare the cycle's metabolic interrelationships. Question 1 covers a lot of biochemistry, and could form the basis of a tutorial discussion. It also embraces a fact which puzzles some students: that we cannot convert fatty Table 1 Four questions to lay bare the cycle's metabolic relationships QI Consider a ham sandwich. (a) Outline how its components can enter the citric acid cycle. (b) Show whether your biochemistry will permit you to convert (i) excess bread to body fat; (ii) excess butter to body glycogen. Q2 Suggest why it is that people who have dedicated their lives to the drinking of alcohol often accumulate excessive amounts of fat. Q3 Suggest why eating grapefruit or drinking lemon juice might be helpful to a person wishing to slim. Q4 Consider the teaspoon of vinegar you put on your chips. (a) How much acetic acid does it contain? (b) How much energy does this represent? (c) How much ATP would be obtained if all this acetic acid was oxidised in the citric acid cycle? (d) How much energy does this ATP represent? (e) How efficient is the cycle at extracting useful energy from vinegar?

B I O C H E M I C A L E D U C A T I O N 14(4) 1986

acids to glucose. Questions 2 and 3 consider two common 'abnormalities' involving the citric acid cycle in which metabolism is disturbed. 3'4 Question 4 invites energy calculations to be made. It could be turned into a miniresearch project by requiring the students to find all the information for themselves. The efficiency obtained from this calculation5 agrees with that quoted in the textbooks.

Table 2 Three activities to stimulate creativity concerning the cycle Q5 Summarise the essential features and relationships of the citric acid cycle on one side of a 3 x 5 inch card. Q6 Devise aids to help you to understand and remember the citric acid cycle. Q7 Be ready at the next class to stand up and explain the citric acid cycle in no more than ten minutes. My final three questions (Table 2) allow the exercise of the creative and artistic talents that lie dormant in everyone, even biochemistry students! Answering these questions would stimulate subconscious, hence painless, learning. Perhaps a small prize (a copy of 'Waltz round the cycle')6 might be offered for the best attempt at question 7. Perhaps after all this, the students themselves would agree with my opening statement: "The citric acid cycle is not difficult. In fact it's fun!" And this, like the cycle itself, takes us back to where we came in!

References t Some might argue with 'Breaking up of the carrier' as an adequate term for the oxidative decarboxylations that comprise this phase, but the phrase does begin with B, and (more important) it does serve as a reminder that the two CO2 molecules that emerge come from the carrier rather than the acetyl! 2Not yet, though: Biochemical Education doesn't pay contributors! Still, its amazing how statements like the one I have included adds a bit of liveliness to the proceedings: on a good day its use would provoke several restrained titters!

3Question 2 Alcohol is oxidised by liver enzymes to acetaldehyde and acetate, NAD being reduced to NADH. The depletion of NAD slows down NAD-requiring reactions, of which there are three in the citric acid cycle. Acetyl CoA accumulates and is diverted into (among other pathways) fatty acid synthesis. Hence fat accumulates.

4Question 3 Grapefruit and lemon juice contain citric acid (the name citric is derived from the Greek citron, a lemon). The presence of citric acid in body cells might be expected to speed up the cycle and therefore increase the flow of intermediates (eg acet~,l CoA from fat breakdown) through it. However, for this to work, the ingested citric acid has to get into the mitochondria. 5Question 4 Vinegar contains about 5% v/v acetic acid (mol wt 60; density 1.05; heat of combustion approx. 900 kJ/mol). Hence (a) one teaspoonful (5 ml) contains 0.25 ml, 0.26 g, 4.37 mmoi acetic acid. (b) This represents 4.37 × 0.9 = 3.93 kJ. (c) Each acetyl yields 12 ATP via the citric acid cycle and electron transport chain, hence 4.37 mmol will yield 4.37 x 12 = 52.4 mmol ATP. (d) If hydrolysis of the terminal phosphate of ATP yields 31 kJ/mol, 31J/mmol, then 52.4 mmol ATP will yield 52.4 x 31 = 1626J or 1.63 kJ. (e) Efficiency of energy trapping is 100(1.63/3.93) = 41.5%. 6Baum, H (1982) The Biochemist's Songbook, Pergamon Press, Oxford, New York, Toronto, 7-11