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Lecture 10

NUTR 3210 Lecture Notes - Lecture 10: Proteinogenic Amino Acid, Pyrrolysine, SelenocysteinePremium

25 pages81 viewsWinter 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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