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Lecture 7

NUTR 3210 Lecture Notes - Lecture 7: Branched-Chain Amino Acid, Protein Quaternary Structure, Aminopeptidase

8 pages98 viewsFall 2016

Department
Nutrition
Course Code
NUTR 3210
Professor
Jennifer Monk
Lecture
7

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NUTR3210 Proteins
What is Protein?
Macronutrient
**Health Canada recommends that protein intake comprises 10-30% of total energy
(ME)
Main Functions:
o Protein synthesis
o Support of blood glucose
o Energy, ME=4 kcal/g
**Requirements are for amino acids rather than whole proteins
Very important: National and international recommendations for protein intake are based
on animal sources of protein (such as meat, milk and eggs)
o Plant proteins may be less digestible due to intrinsic differences in the nature of
the protein and the presence of other factors (like fibre which reduces protein
digestibility by <10%)
Protein requirements vary with life stage (e.g. infancy, childhood, adolescence,
pregnancy)
Amino Acid Structure:
*see slide
connected by peptide bonds:
o amino acids join through condensation reactions, in which water is lost by a
reaction between the carboxyl group of one amino acid + amino group of another
o peptide bonds eliminate charge and decrease water solubility zwitterionic free
amino acids have a high water solubility
Protein Synthesis:
peptide cannot be used interchangeable with protein quaternary structure
o 2 aa = dipeptide
o 3 aa = tripeptide
o 4-50 aa = olgiopeptide
o >50 aa = polypeptide
o *aa are added one at a time in the correct sequence
o 1 or more polypeptides biologically active protein
linear polypeptide chain 3D folded protein (tertiary/quaternary structure)
Native vs Denatured
native protein normal 3D configuration
proteins are denatured by:
o heat
o salt treatment
o detergents
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o pH (acids; stomach acid with HCl)
When denatured, protein loses biological activity
Denaturation affects secondary-quaternary structure (peptide bonds between aa are intact)
Result: open up the protein so enzymes break peptide bonds
E.g. Albumin (egg protein)
o Albumin in native form is transparent and liquid
o When cooked, the albumin becomes opaque and hard denatured
Amino Acid Classification
Essential 9 in adults; 10 in infants *cannot be made by the body or made quickly
o Lysine
o Threonine
o Isoleucine*
o Leucine*
o Methionine
o Phenylalanine
o Tryptophan
o Valine*
o Histidine
o Arginine (in infants)
o * = branched chain amino acids
Non-essential
o Can be synthesized in the body and are not essential components of the diet
Protein Digestion:
Dietary protein homogenization (mouth: mechanical, lubrication)
protein denaturation in the stomach (HCl)
denatured protein olgiopeptides (by zymogens activating enzymes; activator=HCl)
o pepsinogen pepsin
o parapepsinogen 1&2 parapepsin 1&2
o **process occurs by proteolytic cleavage or HCl activation*
olgiopeptide (from stomach) small intestine
o s. intestine secretes aminopeptidase (already active; not a zymgogen) and
enterokinase (activates trypsin)
o pancreas secretes trypsinogen, chymotyrpsinogen, proelastase,
procarboxypeptidase A&B
*trypsin is activated first then trypsin activates other proteolytic enzymes
free amino acids and small peptides
Enzymes:
exopeptidases:
o amino peptidase cleaves off NH2 from amino acids
o carboxypeptidases cleaves COOH from end
endopeptidases:
o trypsin cleaves basic aa peptide bonds inside amino acid chain
o elastase cleaves neutral aa peptide bonds
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o pepsin, parapepsins, chymotrypsin cleaves large neutral aa peptide bonds (with
complex side chains)
*See Secretion Summary:
enterokinase (enteropeptidase) and aminopeptidase are secreted by the intestinal mucosa
in active forms NOT a zymogen
Zymogen (inactive)
Source of
Zymogen
Secretion
Enzyme/Activa
tor
Enzyme (active)
Site of
Activity
Substrate
Pepsinogen
Stomach
HCl or pepsin
Pepsin
Stomach
Most amino
acids (no
terminal aa)
Parapepsinogen 1&2
Stomach
HCl
Parapepsin 1&2
Stomach
Most amino
acids (no
terminal aa)
Trypsinogen
Pancreas
Enteropeptidase
Trypsin
S.
Intestine
Basic aa
Chymotrypsinogen
Pancreas
Trypsin
Chymotrypsin
S.
Intestine
Aromatic
aa, Met,
Asn, His
Proelastase
Pancreas
Trypsin
Elastase
S.
Intestine
Neutral aa
Procarboxypeptidase
A&B
Pancreas
Trypsin
Carboxypeptidase
A
Small
Intestine
C-terminal
neutral aa
Carboxypeptidase
B
C-terminal
basic aa
Aminopeptidases
N-terminal
aa
Amino Acid Absorption into Enterocytes (through Apical Membrane)
1. Facilitated Diffusion
Depends on concentration gradient; high [aa] in the lumen and low [aa] inside
Diffusion cannot occur if intracellular concentration of aa rises above
concentration in the lumen
Does not require Na+ or ATP
2. Sodium-dependent amino acid transporters (active transport)
Aa and Na+ are cotransported (Na+ moves down concentration gradient)
Requires ATP to maintain Na concentration gradient
Di/Tri-peptides (small) are cotransported by PEPT1
Once inside the ell, they are further broken down by cytosolic peptidase
into individual amino acid
Protein Absorption: Enterocyte Circulation
Amino acids cross the enterocyte basolateral membrane by facilitated diffusion
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