Chapter 6 Reading Guide
1. Describe the structure of an amino acid (NOT a protein…yet)
2. What’s the difference between an amino acid and a protein? Where do dipeptides, tripeptides and polypeptides fit in?
3. How many amino acids are there, and what’s different about them?
4. How many of the amino acids are essential? What’s the difference between essential and nonessential amino acids?
5. Are proteins generally straight-chain structures or highly folded?
6. What is protein denaturation? Provide a food example. How do you think that relates to digestion; for example, pepsin is active in acid solutions. The small intestine is slightly basic. What happens to the activity of pepsin in the small intestine, and why?
7. Generally describe the structure of hemoglobin and insulin. What is the function of each? Are these proteins or amino acids?
8. Describe several other functions of proteins in the body… don’t forget enzymes!!
9. Where in the GI tract (digestive tract) does enzymatic digestion of protein begin?
10. What is the active protease in the stomach? What is its inactive form? How is the inactive form converted to the active form?
11. Where are intestinal proteases produced?
12. What’s the difference between proteases and peptidases? Where are peptidases located?
13. Describe how amino acids are absorbed. Do they enter the blood capillary or the lacteal of a villus?
14. There is a specific type of honey produced in New Zealand which contains small amounts of a unique enzyme produced by bees. Some people believe this enzyme has health benefits. What is a potential flaw in this belief? (Not that I’m bashing honey, it is definitely a preferred source of sugar!)
15. Name the proteases that are produced by the pancreas and are sent to the small intestine. *Note: I recommend you check out my supplemental lectures in addition to the book before attempting to answer this one.
16. Where are the instructions for making proteins stored in a cell?
17. What ARE the instructions; ie, how would you make a protein, if you could?
18. What molecule is a “copy” of a gene, carrying the instructions for making one specific polypeptide?
19. What structures in a cell “read” the instructions from the aforementioned “copy?”
20. What structures deliver amino acids to ribosomes?
21. What happens to the functionality of a protein if the amino acid sequence is altered, for example, by a mutation to a gene?
22. Explain several roles of proteins
23. Why can a protein deficiency cause edema?
24. What types of people are (at least should be) in positive nitrogen balance? Why is that?
25. What does “protein turnover” mean? Talk about this in terms of insulin’s effect on most cells’ uptake of glucose (and the subsequent lack of insulin). In other words, what do cells do to allow glucose in when insulin is present; and, when insulin is absent, why is glucose unable to get in?
26. What compounds can be made from the amino acid tyrosine? From the amino acid tryptophan?
27. Of the following: when is the most likely time urea would be produced: a) fatty acids are used for energy, b) amino acids are converted to glucose or fatty acids, c) proteins are being built
28. Describe how urea is produced.
29. Between plant and animal proteins in general, which are higher quality? What is an example of a low-quality animal protein? A high-quality plant protein?
30. To what does the term “complementary proteins” refer? Provide some examples of food combinations that have complementary proteins (fyi: peanuts are legumes).
31. Why do cells need access to all 20 amino acids?
32. How many of the amino acids are essential?
33. What’s the difference between acute PEM and chronic PEM. Describe kwashiorkor and marasmus.
34. What are some examples of people most commonly diagnosed with PEM in the US?
35. What are some diseases that some researchers have linked with high protein diets? Is evidence of this link conclusive?
36. Is protein primarily used for energy or for functional purposes by the body?
37. What are some problems in trying to evaluate health effects of a vegetarian diet? FYI- these are some of the same problems in trying to evaluate high-meat diets: most folks who eat a lot of meat don’t eat lots of fruits and veggies; so, is it the meat or the lack of fruits and veggies? (This last question is not for you to answer: no one knows absolutely)
38. What’s the difference between a lacto-ovo-vegetarian and a vegan?
39. What are some health benefits that are CORRELATED with a vegetarian (or, low animal fat, high whole plant food) diet?
40. Which vitamin can only be obtained (naturally) from animal sources? What are some other vitamins and minerals that vegetarians may need to supplement?
41. Discuss some nutritional concerns for pregnant and lactating vegan women.
42. What is “gluconeogenesis?”
Supplemental Lectures
I. Protein structure- I just want to point out emphatically that the difference between the tens of thousands of different proteins in your body is this: each protein is made of amino acids that are linked together in a unique sequence. The sequence of amino acids is exactly the same in every molecule of keratin in your body. But, the sequence of amino acids in keratin is different than that of hemoglobin. Each protein is folded into a complex and SPECIFIC 3-dimensional shape. The shape of each protein determines its function. What dictates the shape of each protein? The sequence of amino acids. So, a protein’s function is determined by its shape, which is determined by the sequence of amino acids.
Now, the shape that each protein takes also depends on the surrounding environment: pH, temperature, etc. If a protein is exposed to conditions outside of its normal range, the amino acids re-align themselves, the folding pattern (therefore shape) changes, and the protein loses its function. This is protein denaturation.
II. Cells make proteins! If you had to narrow the “job” of cells down to one simple statement, that would be a reasonable one. Remember, the DNA is mostly about storing the instructions for making proteins! So, you don’t need whole proteins from your diet in your blood… all you need are the amino acids. When you ingest a protein, like myosin from animal meat, you do not absorb the protein whole. Proteins are WAY to big to be able to pass through tunnels in the intestinal cells and get to the blood. Digestion of proteins involves clipping the protein chains into single amino acids, dipeptides and tripeptides which are small enough to be absorbed. Then, they travel through the blood and cells can take up the amino acids to make the proteins they need.
A side note for your interest (I won’t test you on this): there are a few proteins which can be taken up whole; the mechanism is different than normal absorption, and they are somehow able to escape being digested by proteases. Two notable examples are: 1)nursing infants are able to absorb whole antibodies (these are proteins) from mothers milk, and 2)prions (proteins that probably cause mad cow )are taken up whole… which is a shame, because if they were just digested like normal proteins, they wouldn’t cause a disease!
III. The protease and peptidase populations
A. All proteases (not peptidases) are released in an INACTIVE form and must be activated in order to be functional. In the stomach, pepsinogen is the inactive protease. Once in the lumen of the stomach, HCl activates it to pepsin.
Several inactive proteases are released into the intestine. They are all made by the pancreas (how do they get to the small intestine again?). Cells of the small intestine make an enzyme that activates ONE of the inactive proteases from the pancreas. Once that one is activated, it will activate the rest. Here are the details:
The enzyme already in the small intestine is called enteropeptidase. It converts (activates) the inactive trypsinogen to the active trypsin (recall, trypsinogen is one of the pancreatic proteases). Trypsin is an active protease. In addition to digesting proteins, trypsin will also activate the rest of the pancreatic proteases: chymotrypsin, carboxypeptidase, elastase, and collagenase.
B. The peptidases are produced by the cells lining the villi of the small intestine. The peptidases are actually attached to the microvilli. They finish off the job started by the proteases… proteases produced small peptides, and peptidases will chop the small peptides into amino acids, dipeptides and tripeptides, which can be absorbed.
IV. Protein deficiency and edema: water is attracted to proteins. Blood plasma has tons of proteins in it, for example albumins and antibodies. There are very few proteins in the interstitial fluid (fluid surrounding cells). So, when arteries branch into the tiny capillaries that service cells with nutrients, not much water leaches out of the capillaries, even though there are tiny little holes in the capillaries. That’s because all the proteins in the blood, which are too big to leave capillaries, hold on to the water.
When excess water leaves the blood and fills up the space between cells, this is swelling (edema).
A protein deficiency can lead to edema, because not enough proteins will be in the blood to keep the water from leaking out.
I want to expand on edema a bit to clarify the text’s explanation. There are different causes of edema; protein deficiency is not the only cause. The author explains that edema can occur when excess proteins accumulate in the interstitial space. This is true, but NOT because of protein deficiency. Two examples of why this might
happen are: 1) chronic hypertension, in which high blood pressure damages capillaries and pushes out excess proteins and fluid; 2) blockage of the lymphatic system, in which fluids/proteins cannot be adequately cleared from the interstitial space.
On the other hand, with a protein deficiency, there are not enough plasma proteins to hold plasma in the capillaries. It's not so much that excess proteins are leaving the blood. Instead, it is that the "water-holding" ability of the blood is diminished, because of a lack of proteins in the blood. When blood gets to capillaries, which are "leaky" by nature, more fluid than normal leaves the capillaries simply because proteins aren't in the blood to hold the fluid in.
There may also be extra loss of proteins into the interstitial space, as the proteins that hold the vessel walls in place may also be compromised. But the loss of osmotic pressure (which is what I described above) is the big issue.
V. Proteins are used primarily for structure and function. There are 3 major reasons that proteins (amino acids, really) would be used for energy by the body rather than structure/function:
a. The body NEEDS another source of energy, for example if you are fasting or starving. In this case, structural and functional proteins- like the contractile proteins in your muscles- will be sacrificed, digested, and their amino acids used for energy.
b. The body needs glucose specifically. Remember, even if you have plenty of fat stores, or fat intake, fatty acids cannot be converted to glucose. With a low-carbohydrate intake (less than 50-100 g/day), the amino acids pool, and then structural proteins, become a very important resource for making glucose for the brain.
After a few days of low carbohydrate intake (or fasting), the metabolism of fatty acids by most cells increases dramatically. At this point, ketone bodies will be produced more and more… as you know, this will provide another source of energy to the brain and will spare some structural proteins.
The health effects of low-carb diets, by the way, have not been investigated long-term… but there is not much evidence that they work much better than other restrictive diets. They may have some specific negative effects, such as leaching calcium from bones.
c. You take in more protein than you need for growth/maintenance. Then, excess protein (really, amino acids) will simply be converted to fat for storage.
VI. Some random thoughts
a. Mutations: just for your own information- not all mutations create negative effects. For example, different hair colors reflect different versions of the pigment melanin. All of the different versions originally came from mutations. Mutations are the reason we have genetic variability. However, most mutations are negative, especially when they affect really important functional proteins.
b. The “transporters” listed by the book are the “tunnels” I’ve been talking about, that let water-soluble substances pass in and out of cells. For example, remember insulin tells cells of the body to build tunnels to let glucose in? Well, these tunnels are made of protein. By the way, when insulin is absent, the cell digests the protein and recycles the amino acids. (This is an example of protein turnover!)
c. When an amino acid-or any other non-carb substance- is used to make glucose, the process is called “gluconeogenesis” (New Glucose Rising)
d. Vitamin B12 is the most chemically complex of all the vitamins. It is ONLY produced by bacteria. These bacteria happen to live in the intestines of animals (E. coli). Certain animals, like cows, house HUGE numbers of these bacteria in their upper digestive tracts (stomachs). So, they get to absorb the B12 produced by their symbionts. Unfortunately for us, our bacterial populations are way down in the large intestine, and we get VERY LITTLE (if any) B12 from them. Want to get some B12 from your own symbionts? Easy! Become a coprophage (animal that eats feces)! Okay, I am joking, but in all seriousness, there are animals that are coprophagous, partially for the B12. Rabbits are an example. Their intestinal bacteria are also far down in their digestive tracts. WITHOUT B12, YOU DO NOT HAVE A NERVOUS SYSTEM AS YOU KNOW IT! And, B12 can ONLY be gotten naturally from animal sources. This includes: meat and all other parts of the actual animal, milk, eggs, insects (yep, they’re animals with bacterial symbionts too), and feces.