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Lecture 10

NUTR 3210 Lecture 10: Protein Metabolism
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25 Pages
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Winter 2017

Department
Nutrition
Course Code
NUTR 3210
Professor
David Mutch
Lecture
10

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Protein Metabolism
Macronutrients so Far:
1. Carbohydrates
- Energy Source
- No specific “essential” CHO
- Includes Dietary Fibre
o Dietary goal increase intake of non-digestible CHO
- Recommended intake 45-65%
2. Lipids
- Energy source
- Two essential fatty acids: alpha-linolenic (n-3) and linoleic (n-6)
- Key roles as precursors for signaling molecules, structural role in membranes, etc.
- Goal is to decrease intake of total fat (specifically SFA and trans fats) and increase of
MUFA and n-3 fats
- Recommended intake 25-35%
3. Protein
- Energy source
- Substrate for glucose synthesis
- Provides amino acids (AAs) for protein synthesis and other areas of metabolism
- 21 proteinogenic AAs (i.e used to make protein)
o Includes Selenocysteine
o 22 AAs in reality BUT the 22nd amino acid, pyrrolysine is used by bacteria,
not humans
- 9 AAs considered “essential” or “indispensable” for humans
o Histidine was the last AA to be added to the essential list
- Health Canada recommends a daily protein intake between 10-30%
o Average consumption in North America about 16% of daily calories
o We don’t need protein per se, but rather the AAs in protein
Protein Overview
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Where do we find protein in our bodies?
- Whole Body Average (overall):
o Carbs 0.2%
o Minerals 2%
o Protein 15%
o Water 60%
o Fat (lipid) 20-25%
- Blood (RBC)
- Connective Tissue
- Eye Lens
- These 2 tissues have the most protein % - 34-37%
- Adipose tissue has the least protein %
- Skeletal Muscle, Cortical Bone, Skin In the middle 20-25%
Where do we find protein in our diets?
- % protein content of animal products is generally higher than plants
- Ex: Lentils 9 g vs. meats have 25-30 g
- Plants don’t have the same amino acids needs like us. Ex. Lysine not present in
wheat. Therefore, must couple food if vegetarian
Amino Acid Structure
- Amino acid = building block proteins
- Can be considered a monomer
Standard vs. Non-Standard AAs
- 2 types of amino acids in the body
1. Standard Amino Acids
- 21 proteinogenic AAs (involved in
protein synthesis)
- 20 classified as “standard”
- B/c 20 are encoded in eukaryote
genome, except selenocysteine
2. Non-standard Amino Acids
- Formed by post-translational modification
of other AAs OR as intermediates in the metabolic pathways of standard AAs.
o Ex: GABA neurotransmitter is a metabolite of glutamate
- Many exist, but they are rarely used to make proteins
D vs. L amino acids
- D vs. L enantiomers
- All standard AA exist as enantiomers
o Except glycine
- L form of AA are naturally occurring
- D form made by post-translational modifications
Zwitterions
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- At physiological pH, AAs are ionized
o Protonated amino group
o Deprotonated carboxyl group
- No overall charge except on the R group
- Increased polarity AA are water soluble
AAs connected by Peptide Bonds
- Peptide bonds (also known as amide bonds) are covalent chemical bonds
- The carboxyl group of one AA reacts with the amino group of another AA, releasing
water
o Condensation reaction
How to break a peptide bond? Hydrolysis add H2O
Protein Synthesis: From AA to Protein
- The terms peptide and protein CANNOT be used interchangeably
- AAs are joined by condensation reactions
- 2 AAs = Dipeptide
- 3 AAs = Tripeptide
- 50 AAs = Oligopeptide
- >50 AAs = Polypeptide
- So, we form a dipeptide and add n number of amino acids, one at a time, in the
correct sequence = linear polypeptide chain
- Polypeptide chain folds = 3-D folded protein (biologically active protein)
- 1 or more polypeptides can make up a protein
- Proteins begin to fold and take shape as they are synthesized by ribosomes
- Protein folding is assisted by chaperons (HSP70)
1. Transcription
2. Translation
Primary Structure of Proteins
- Determined by the DNA sequence
- Primary structure = a linear sequence of AAs
- Amino acids are held together by peptide bonds, which are made during translation
o Chaperones
- Carboxyl and amino terminus
- Counting of amino acids always starts at amino ends
Secondary Structure of Proteins
- Coiling, folding, and/or bending of the protein
- Various interactions between and among amino acids within the proteins contribute to
the overall levels of organization
o So, hydrogen bonds contribute to the secondary structures
The hydrogen bonds form a stabilized structure
o Electrostatic interactions (ion interactions) occur between oppositely charged
side chains of amino acids to impact the secondary structure too
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Description
Protein Metabolism Macronutrients so Far: 1. Carbohydrates Energy Source No specific essential CHO Includes Dietary Fibre o Dietary goal increase intake of nondigestible CHO Recommended intake 4565 2. Lipids Energy source Two essential fatty acids: alphalinolenic (n3) and linoleic (n6) Key roles as precursors for signaling molecules, structural role in membranes, etc. Goal is to decrease intake of total fat (specifically SFA and trans fats) and increase of MUFA and n3 fats Recommended intake 2535 3. Protein Energy source Substrate for glucose synthesis Provides amino acids (AAs) for protein synthesis and other areas of metabolism 21 proteinogenic AAs (i.e used to make protein) o Includes Selenocysteine nd o 22 AAs in reality BUT the 22 amino acid, pyrrolysine is used by bacteria, not humans 9 AAs considered essential or indispensable for humans o Histidine was the last AA to be added to the essential list Health Canada recommends a daily protein intake between 1030 o Average consumption in North America about 16 of daily calories o We dont need protein per se, but rather the AAs in protein Protein Overview
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