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Lecture

Chapter 7 - Proteins
(On the syllabus this is listed for Week 9)

Proteins are made of chainlike strands of amino acids linked together by peptide bonds. These strands fold over on themselves to create proteins with unique three-dimensional structures.  The shape of a protein determines its function.

Amino acids are the building blocks of protein.

Like carbohydrates and fats, proteins contain the elements carbon, hydrogen, and oxygen, but proteins also contain nitrogen.

Approximately 20 different amino acids exist in nature. Different combinations of these 20 different amino acids is what makes up all the proteins that exist in nature.

Of the 20 amino acids found in protein, 9 cannot be made by the adult human body. These amino acids, called essential amino acids, must be consumed in the diet. If the diet is deficient in one or more of these essential amino acids, new proteins containing them cannot be made without the breaking down of other body proteins to provide them.

The 11 nonessential amino acids can be made by the human body and are not required in the diet. Most of the nonessential amino acids can be made by the process of transamination, in which an amino group from one amino acid is transferred to a carbon-containing molecule to form a different amino acid.
 
Amino acids consist of a central carbon atom (C) with a hydrogen atom (H), an amino group (NH2), an acid group (COOH), and a side chain that varies in length and structure.

To form proteins, amino acids are linked together by a type of chemical bond called a peptide bond. Two amino acids linked with a peptide bond are called a dipeptide; three linked amino acids form a tripeptide. Many amino acids bonded together constitute a polypeptide. 

A protein is made of one or more polypeptide chains folded into a complex three-dimensional shape.

The sequence of amino acids in each protein determines its unique shape and function.

If the shape of a protein is altered, its function may be disrupted. In the genetic disease, sickle-cell anemia, a single amino acid in the hemoglobin molecule is altered. This makes the red blood cells have a distorted shape that resembles a crescent or sickle. Sickle-shaped red blood cells can block capillaries, causing inflammation and pain. They also rupture easily, leading to anemia from a shortage of red blood cells.

Changes in protein shape can also be caused by changes in temperature and acidity. This change is called denaturation. During digestion, stomach acid denatures proteins, opening up their structure to allow digestive enzymes better access to the peptide bonds.  In food, cooking denatures protein, changing its shape and physical properties. For example, a raw egg white is clear and liquid, but once it has been denatured by cooking, it becomes white and firm.

Protein in the Diet
Protein eaten in the diet provides the raw materials to make all the various types of proteins that the body needs.

Approximate Amounts of Protein in Foods:

Proteins requirements are determined by studying nitrogen balance.  For healthy adults, the RDA for protein is 0.8 g per kilogram of body weight.  Growth, pregnancy, lactation, and physical stress can increase protein requirements.

Dietary protein comes from both animal and plant sources.  Animal proteins contain a pattern of amino acids that matches the needs of the human body more closely than the pattern of amino acids in plant protein.  Therefore, they are said to be higher quality or more complete proteins than most plant proteins.  Plant proteins are said to be incomplete proteins or lower quality proteins.

The quality of protein in the diet is as important as the amount of total protein.  There are many ways of assessing protein quality in the laboratory.  In diet planning, animal proteins are generally considered to be complete proteins-that is, they provide the right combination of amino acids to meet needs.  Diets that include little or no animal protein can provide adequate protein if the sources of protein are complemented to supply enough of all the essential amino acids.

If a person is not eating animal products, the complementary relationship between plants works like this:

                          Grains                          Seeds/nuts




                                                             Legumes

On your printed version, draw a line with arrowheads at both ends going from Grains to Legumes; then draw a line with arrowheads at both ends going from Legumes to Seeds/nuts.

The essential amino acids missing in Grains are present in Legumes.
The essential amino acids missing in Legumes are present in Grains.
By eating Grains and legumes within an ~ 24 hour period, you would be getting the essential amino acids needed by the body. This is an example of complementary protein. The protein in this complementary relationship is as good as animal protein/complete protein.

The essential amino acids missing in Legumes are present in Seeds/nuts.
The essential amino acids missing in Seeds/nuts are present in Legumes.
By eating Legumes and Seeds/nuts within an ~ 24 hour period, you would be getting the essential amino acids needed by the body.  This too is an example of complementary protein. Again, the protein in this complementary relationship is as good as animal protein/complete protein.

This same relationship does not exist between Grains and Seeds/nuts. Therefore, no arrow is drawn between them.

Vegetarian diets can meet all amino acid needs if carefully planned.  Lacto and lacto-ovo vegetarian diets provide high-quality animal proteins. Vegan diets must be carefully planned and care must be taken to make sure vitamin B-12 needs are met.

Digestion breaks protein into small peptides and amino acids. It's the amino acids which can be absorbed.  Amino acids can be used for the synthesis of protein and other nitrogen-containing molecules and can be deaminated and used for energy or to synthesize glucose or fatty acids.

Protein turnover refers to the continuous synthesis and breakdown of body proteins.  Our cells have a collection of amino acids called the amino acid pool. These amino acids come from both the breakdown of body proteins and from protein that's eaten.  Protein is made by the body in amounts necessary to maintain homeostasis.

The specific body proteins that are made are determined by which genes are expressed.  Genes are segments of DNA that code for the synthesis of proteins.  The information in a gene is transferred to mRNA.  Messenger RNA (mRNA) carries the coded message from the nucleus to ribosomes in the cytoplasm where the protein is made.

Functions and types of proteins in the body:
Body proteins provide structure and regulate body functions.  Enzymes and some hormones are proteins.  Proteins help transport molecules in the blood and into and out of cells.  Antibody proteins are essential for immune system function and contractile proteins are needed for muscle contraction. Proteins help regulate fluid balance and acid balance.  Proteins can also be used to generate ATP or to synthesize glucose or fat.

Protein-energy malnutrition (PEM) is a public health concern, primarily in developing countries.  Kwashiorkor occurs when the protein content of the diet is insufficient to meet needs.  It is most common in children. Marasmus occurs when total energy intake is deficient.  Most people in developed countries, like the USA, consume more than enough protein; although unnecessary, protein and amino acid supplements remain popular.  Excess amounts of individual amino acids can interfere with the absorption of other amino acids that share the same transport systems.

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Study Guide

1. What chemical element distinguishes a Protein from a carbohydrate and fat?
2. Approximately how many amino acids exist in nature? What makes an amino acid
   "essential"? 
3. What is the name of the bond that links amino acids together? Define the following: dipeptide, tripeptide, and polypeptide. To what does "amino acid sequence" refer? What is responsible for the twisted, tangled shapes of proteins? What enable proteins to perform different tasks in the body? What is denaturation?
4. Where does digestion of protein begin? What happens to protein when it enters the stomach? What is the function of hydrochloric acid? What is pepsin? What happens to polypeptides when they enter the small intestine? Where does absorption of amino acids take place? What is the function of a peptidase? What happens to orally ingested amino acids (predigested protein)?
5. What informs a cell of the amino sequence needed for a specific protein? What is the function of transfer RNA? What can happen if a protein sequence is altered? What is the disease associated with a sequencing error in hemoglobin?
6. Define the following terms: collagen, enzymes, and hormones. Provide 2 examples of hormones and their functions. How does protein help maintain fluid balance? How do proteins respond when the body becomes too acid? What is acidosis? What is alkalosis? What is the function of transport proteins? What is the transport protein that carries oxygen from the lungs to the cells? What transport proteins carry lipids throughout the body? What is the transport protein for iron? What are antibodies? What is an antigen? What is fibrin? Summarize the various roles of proteins in the body.
7. What is protein turnover? To what does "amino acid pool" refer? What is nitrogen balance? What is nitrogen equilibrium? Provide examples of people in positive nitrogen balance. Provide examples of people in negative nitrogen balance. What compounds can be made from the amino acid tyrosine? What compounds can be made from tryptophan? Under what conditions will amino acids be used for energy? What is deamination? What is the by-product of deamination? What is the fate of ammonia in the body? What happens if a person eats more protein than the body needs?
8.What is protein-energy malnutrition (PEM)? Where is PEM most prevalent in the world? How does PEM affect the United States? What is marasmus? What population is most affected by marasmus? What are the symptoms of marasmus? What is kwashiorkor? What population is most affected by kwashiorkor? What are the symptoms associated with kwashiorkor? How should nutrition therapy be applied in children with PEM?

Fill in the blanks below to follow proteins through the GI tract, through absorption, and through metabolism.
1. Hydroysis of protein begins in the ___________.
2. The loss of a protein's shape and function is known as _____________.
3. The active form of the enzyme pepsinogen is ______.
4. Polypeptides in the small intestine are hydrolyzed by ______.
5. Peptides in the small intestine are hydrolyzed by specific ______.
6. Free amino acids, dipeptides, and tripeptides are absorbed into the _____after digestion.
7. Amino acids that can be produced by the body are referred to as _____amino acids.
8. The removal of the nitrogen group from an amino acid is known as ______.
9. _____ is the toxic by-product of deamination.
10. Ammonia is converted to ______ in the liver.
11. Urea is excreted as urine from the ______.

Answer Key to Fill in the Blanks:
1. stomach
2. denaturation
3. pepsin
4. proteases
5. peptidases
6. small intestine
7. nonessential
8. deamination
9. ammonia
10. urea
11. kidneys

1. What is a limiting amino acid? What is a high quality dietary protein? What food sources generally provide high quality proteins? To what does "complementary protein" refer? 
2. What types of cancers may be associated with high-meat diets? How might a high protein intake relate to the development of osteoporosis, obesity, and kidney disease?
3. What are the two reasons the body needs dietary protein? What is the recommended intake of protein as a percent of total kcals? What is the advice from health experts regarding protein intakes? What are the steps necessary to determine individual protein needs (RDA)?
4. How digestible are protein supplements compared to protein foods? What can happen when an excess of single amino acids is eaten? 
 

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