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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 3dimensional 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.
For a while, it was thought that some fermented plant
products had small amounts of B12. It turns out, it’s a
similar chemical but is not active in our bodies. I’ll probably
bring all this up again when we get to vitamins.