Protein Flashcards
- basic units of ptn are
aa - each ptn is unique due to the sequencing of the
AAs that comprise its structure
- nutr requirement is not for ptn but some AAs & N
AMINO ACID CLASSIFICATION
Table 6.1
Nutritionally active and natural form: L-isomer
• Each AA side chain (R) distinctive
– neutral, acidic, basic, aromatic etc.
• Over 300 AAs found in animal tissue:
– 20 used for ptn synthesis (tRNA)
– all 20 AAs are essential (some have to be in the diet)
Common Post-Translational Amino Acids
Bone
OH-Lysine
• OH-Proline
Common Post-Translational Amino Acids Muscle
• 3-Methyl histidine
– released during muscle breakdown via urine
INDISPENSABLE (ESSENTIAL) AMINO ACIDS
IAA is one which cannot be made in the body or cannot be
made sufficiently to meet physiological needs, hence must
be supplied in the diet.
e.g., inability to make C-skeleton of AA de novo
• 10 AAs most commonly essential:
PHE HIS ILE LEU LYS MET TRP VAL THR (ARG ?)
– Dependent on species and/or stage of growth:
GLY PRO ARG
• IAA as a misnomer: supply keto analogue (requires a
transaminase) or hydroxy analogue (requires a
dehydrogenase and a transaminase) and animal can
make the AA – used in renal failure patients
– exceptions THR, HIS and LYS
DISPENSABLE (NONESSENTIAL) AMINO ACIDS
DAA can be made in the body from:
a) keto acid & transamination with another AA
e.g., ALA from pyruvate
b) another amino acid by conversion other than by
transamination
e.g., MET
→ CYS, PHE
→ TYR
• IAA and DAA inflexible classifications - too strict?
e.g., TYR essential in PKU, TAURINE [MET] essential in TPN
Conditionally Indispensable Amino Acids
Conditionally Indispensable Amino
Acids and Their Precursors
Amino Acid- Precursors Tyrosine-Phenylalanine Cysteine-Methionine, Serine-Glutamine Proline Arginine- Glutamine or Glutamate, Aspartate Glutamine- Glutamate, Ammonia
SIMPLE PROTEINS:
AAs only
CONJUGATED PROTEINS:
AAs
combined with other non-ptn moiety [e.g.,
lipoprotein]
PRIMARY STRUCTURE
Sequence of strong covalently bonded AAs as dictated by the genetic code which determines the final form of ptn • The side chain of one amino acid differs from that of another amino acid making each amino acid different; however the polypeptide backbones do no differ between polypeptide chains
SECONDARY STRUCTURE:
Determined by attracting forces between nearby groups (R) in the peptide chain. Gives shape to the ptn. α-helix β-pleated sheet random coil
TERTIARY STRUCTURE:
The way a protein folds in a threedimensional space • Due to interactions occurring among R groups that are located at considerable distances from each other on the polypeptide chain e.g., cystine -S-S- • Produces binding and looping of the ptn molecule e.g., enzyme pocket - site of action
QUATERNARY STRUCTURE:
Aggregate of two or more polypeptide chains that form oligomers
held together by H bonds and electrostatic salt bridges (e.g.,
haemoglobin, regulatory enzymes).
Fig. 6-4, p. 186
Quaternary
structure of the
hemoglobin protein
FUNCTIONS OF PTN & AAs
1 Immediate energy
• amino group removed; C-skeleton enters metabolic
pathways
2. Enzymes (-ase)
3. Hormones
• e.g., peptide hormones – insulin, glucagon etc.
4. Structural Proteins (building - growth - and
maintenance)
• Contractile proteins – e.g., actin & myosin in muscle
• Fibrous proteins – e.g., collagen, elastin, and keratin
in bone; teeth; skin; tendons; cartilage; blood
vessels; hair; nails etc.)
5. Immunoproteins
• Immunoglobulins (Ig) or antibodies (Ab)
6. Transport Proteins
• Blood transport ptn - e.g., albumin, transthyretin or
prealbumin, retinol-binding ptn, haemoglobin,
transferrin, ceruloplasmin, lipoproteins)
7. Regulate water balance in blood
• oncotic pressure - e.g., albumin, globulins attract
water to blood compartment)
8. Buffers in blood to maintain pH at 7.4
• e.g., albumin ‘mops up’ H+ ions
9) Conjugated proteins
• e.g., glycoproteins, proteoglycans, lipoproteins,
flavoproteins, metalloproteins
10) Synthesis of other essential body compounds:
a) taurine (bile salts) - CYS
b) melanin pigments - PHE TYR
c) thyroid hormones - PHE TYR
d) carnitine (FA oxidation) - LYS
e) niacin - TRP
f) neurotransmitters: serotonin - TRP; norepinephrine - PHE TYR
g) nucleic acids: pyrimidine - ASP; purine - GLY GLU ASP
h) creatinine - ARG GLY MET
i) choline - MET SER
j) ammonia [NH
3] (conserves K+ and Na+ ions in acidosis) - GLN
Use of Amino Acids for Anabolism
biogenic amines, peptide hormones, plasma pro, structural pro, enzymes, immunoproteins, transport pro
Approximate Amount of Protein in
Body Tissue
70 kg adult male – 10 kg (~15%) protein • 50% muscle • 23% bone • 10% skin • 8% blood • 4% liver • 3% GI tract • 1.5% brain
PROTEIN DIGESTION AND
ABSORPTION
Exogenous ptns are the source of IAAs and N
– normal digestion and absorption is critical
• Endogenous ptns
– e.g., enzymes, mucosal cells also present in digestive
tract
• DIGESTION: Proteolytic enzymes hydrolyse
peptide bonds (add H
2O)
DIGESTION IN STOMACH
Acid environment (excretion of HCl) denatures ptns which ↑ exposure of peptide bond. DENATURE: 1) Disrupt attracting forces in ptn 2) Loss of biological activity. 3) AA sequence retained e.g., heat, detergents, organics, metals & mineral acids 2) Enzyme: pepsin
DIGESTION IN INTESTINE
1) Pancreatic Enzymes (trypsin activated): trypsin, chymotrypsin, elastase, carboxypeptidase A, B ? - trypsin inhibitors [soybean]: ↓ ptn digestion 2) Intestinal Enzymes: aminopeptidases
- Exo –
- Endo –
- Exogenous ptn
- Endogenous ptn
- Endopeptidases
- Exopeptidases
out
in
– diet
– ptn syn in body
– cleave peptide bound within the primary structure
– cleave peptide bond at the ends of the primary structure (C & N
terminals)
PROTEIN AND AMINO ACID ABSORPTION
DAILY PROTEIN (AS AAs) ABSORBED (e.g., Adult human) g/d
• Endogenous G.I. protein: saliva 3
gastric juice 5
bile 1
pancreatic juice 8
mucosal cells 50
Total: 67
• Dietary ptn consumed: 100% digestion 100
• Fecal loss: e.g., NAs, bacteria etc. 10
Total absorbed: 157
- Efficiency of digestion and absorption of both dietary and
endogenous protein is high (10 g/d feces)
- Dietary factors may
↓ absorption
ABSORBED PRODUCTS
Absorption occurs along the entire small intestine, but differs for
certain AAs and peptides
1) Free amino acids: 1/3 of total ptn being absorbed
Site: ileum and jejunum
2) Peptides: 2/3 of total ptn being absorbed
Large peptides (6 AAs) hydrolysed
Di, tri peptides move intact
3) Intact proteins: e.g., Immunoglobulins from milk after
birth remain intact:
1) trypsin inhibitor in colostrum
2) no pepsin secretion
3) high pH in stomach
4) immature
MEMBRANE TRANSPORT (Mucosa)
Modes of transport depicted in Figures 6.7, 6.8
- Various possible mechanisms (not known?), not all identified
- Literature continuously changing
1) Free amino acids: L-AAs by active transport
e.g., carriers: a) B – AAA, BCAA
b) L system - neutral
c) y+ system - dibasic
Affinity of AA to carrier exists (competition) – i.e., IAA > DAA
2) Peptides: 1) Passive diffusion
2) Carriers: one shown to exist, but affinity differs
between peptides
3) Intact proteins: Pinocytosis
