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
MEMBRANE TRANSPORT
Basolateral Membrane
Only free amino acids enter the circulation – Peptides hydrolyzed by cytoplasmic peptidases in enterocyte – Amino Acids transported across BLM via carriers • Some carriers the same as at the BBM • Some are unique
AA Absorption into Extraintestinal
Tissues
• Liver
– Carriers similar to basolateral membrane
– + other systems
• Extrahepatic Tissue (e.g., kidney)
– Carriers similar to basolateral membrane
• Other (e.g., renal tubular cells, erythrocytes, neurons ?)
– The
γ-glutamyl cycle
PROTEIN AND AMINO ACID
METABOLISM
ANABOLISM CATABOLISM • ANABOLISM: 1) protein (e.g., tissue, plasma ptn) synthesis 2) synthesis of nonprotein, Ncontaining molecules • CATABOLISM: energy (glucose, fatty acids, ketones) 1) exogenous AAs (i.e., dietary) 2) endogenous ptn (e.g., tissue ptn)
SYNTHESIS AND CATABOLISM
OF TISSUE PROTEIN
CONTROL OF ENDOGENOUS PROTEIN
TURNOVER (REGULATION)
• Turnover rates: visceral > skeletal muscle
– 15 - 20% of RMR
• syn & degrad are controlled independently
– e.g., growth: syn > degrad
fever: syn < degrad
ANABOLISM:
Protein (e.g., tissue) synthesis
SYNTHESIS (biochemistry)
- ↑ syn: insulin, growth hormone, testosterone
CATABOLISM:
Degradation of endogenous proteins
DEGRADATION (biochemistry) - research ongoing Cellular Protein Degradation Systems: lysosomes: - contain proteinases (cathepsins) - mostly extracellular ptn cytoplast: - contain proteinases - intracellular ptn - ↑ degrad: glucagon, catecholamines, glucocorticoids
ANABOLISM:
Synthesis of nonprotein, N-containing molecules
Synthesized in liver (and often other sites) – Functions of amino acids, Figure 6.22 – Some described in aa metabolism section – Other examples • Glutathione - cys, gly, glu • Carnitine - lys, met • Creatine - arg, gly, met • Carnosine - his, ala • Choline - ser
Glutathione - cys, gly, glu
Found in most cells • Functions – transports aas as part of the γ-glutamyl cycle – involved in the synthesis of leukotriene (LT) • Mediates body’s response to inflammation – protects cells as an antioxidant • Reacts with hydrogen peroxides (H 2 O 2) and lipid hydroperoxides (LOOHs) • Scavenger of free radicals, O 2· and OH· transports • Glut synthesis sensitive to ptn intake & pathological state – ↓ [conc] with ↓ intake, inflammation & disease (when you need it
Carnitine - lys, met
• Synthesized in liver & kidney
• Found in most tissues - primary pool is muscle
• Major dietary sources - beef & pork ( ~ 70% intake is absorbed)
• Functions
– transport of FAs, especially LC-FAs (C
≥10), across the inner
mitochondrial membrane for [Ox]
– Also forms acylcarnitines from SC-acylCoAs to buffer the free
coenzyme (Co) A pool
• Carnitine deficiency (rare) results in impaired nrg metabolism
• Carnitine supplementation (
↑ plasma & muscle carnitine)
– Claims to “burn” fat, supply nrg,
↑ metabolism are not proven
– Claims to improve physical performance are not proven
• Based on the “role” carnitine plays in the body
Creatine - arg, gly, met
Key component of energy compound creatine phosphate
(phosphocreatine)
• Synthesis starts in kidney
→ end in liver
→ released in blood
• 95% in muscle, 5% in kidneys and brain as creatine or phosphocreatine
• Major dietary sources - meat & fish
• Functions
– Phosphocreatine (~50% at rest) is a high-nrg phosphate storehouse
• Functions
– Phosphocreatine replenishes ATP in contracting muscle
• Excretion
– Creatine & phosphocreatine
→ creatinine (indicator of muscle mass)
• Supplementation
–
↑ [conc] ~ 20-50% & amt of short duration exercise
– Depends on supplementation practice and subject characteristics
(e.g., creatine stores) etc.
Carnosine (β-alanyl histidine) - his, ala
Found in skeletal & cardiac muscle, brain, kidney, stomach
• Major dietary sources - meat
• Functions
– Not all identified
– Exhibits antioxidant activity
– Regulates intracellular calcium & contractility in muscle
Choline - methylation of ser
Found commonly as part of the phospholipid
lecithin (phosphatidyl choline)
– eggs, liver, organ & muscle meats, wheat germ,
legumes, added as emulsifier
• Functions
– part of acetylcholine (neurotransmitter);
phosphatidyl choline; spingomyelin
– methyl donor
Choline - cont’d
Excretion – Oxidized in liver & kidneys → → CO 2 + NH 4 \+ • Deficiency – Severe ↓ [conc] → fatty liver • AI for choline, 425 & 550 mg for adult females & males, respectively • Easily obtained through diet – animal products and fats
Purine & Pyrimidine Bases
Nitrogen-containing bases of nucleotides
• Synthesis in liver involves several amino acids
– Pyrimidines – uracil, cytosine, thymine
– Purines – adenine, guanine
• Degradation
– Pyrimidines
→ CO
2 + NH
4
+ (to urea) + malonyl CoA
– Purines →→→ uric acid
• Excreted in urine, some into GI tract (~200 mg/d)
• In gout / renal failure, uric acid accumulates in blood and joints
” Amino acid metabolism is a
REGULATED process “
Free AAs in pools (e.g., plasma and within cells) are from both
dietary and endogenous AA sources
• Pools serve as a link between 2-cycles of N metabolism:
a) balance: intake v output of N (i.e., AAs)
b) turnover: synthesis v degradation (i.e., protein)
• Ptn intake and rate of tissue degradation differs
- free amino acid pool remains constant
• AAs are not stored: 0.16% in free form; of that, 2.8% in plasma
(most in muscle)
• CONTROL: AAs in excess needed for ptn synthesis are OXIDIZED
FREE AMINO ACIDS IN POOLS
1) DAA > IAA
2) IAA:»_space; LYS THR (ie. totally indispensable)
3) DAA:»_space; ALA ASP GLU GLN
- conserve IAA
- role in reamination and nitrogen
metabolism
CATABOLISM OF AMINO ACIDS
• Why does it occur?
1. as a normal part of "tissue turnover" (20-25% of AAs from tissue proteolysis) 2. as a means of disposing of excess AAs from dietary ptn 3. catabolism enhanced: a. absence of CHO intake b. insufficient NRG intake c. a "catabolic state“ eg, burns, diabetes mellitus
AMINO ACID DEGRADATION (N BALANCE)
1) Removal of NH 2 (amino group) • amino transferases & deaminases catalyze removal • products are other DAAs or NH 2 and Cskeletons • NH 2 urea (urea cycle) • GLU / α-keto GLU - key to make DAA, N \+ H 4, urea
2) Catabolism of C-skeleton (specific to each AA) C-skeleton (of part thereof) can be used for: a) SYNTHESIS OF GLUCOSE (contribute Cs) glucogenic = pyruvic acid → glucose i.e., ALA GLY CYS SER THR ASP ASN GLU GLN ARG MET VAL HIS PRO (exptn: LEU, LYS) ↓ glu = malate →oxaloacetate → PEP → glu ↑ glu = malate → pyruvate → acetyl-CoA → FA b) SYNTHESIS OF FATTY ACIDS (i.e., LEU, LYS) ketogenic = only acetyl-CoA → FA synthesis (also LEU catabolism directly makes HMG-CoA → cholesterol c) SYNTHESIS OF GLUCOSE AND FATTY ACIDS both: i.e., PHE ILE TRP TYR d) USED FOR ENERGY (all AAs) e) SYNTHESIS OF NON-ESSENTIAL AAs e.g., oxaloacetate → ASP pyruvate → ALA, α-keto GLU → GLU
A. Intestinal Cell Amino Acid Use
1. Peptide hydrolases in brush borders / cytosol exist - free AAs only pass to portal vein and liver 2. AA used for nrg 3. Synthesis • Apoproteins necessary for lipoprotein formation • New digestive enzymes • Hormones • Nitrogen-containing compounds 4. GLU ASP transaminated to ALA GLU GLN → ALA (ORN CIT PRO also produced) - ↓ toxicity of ↑intakes of GLU GLN ASP 5. Glutathione also formed - reduce reactive oxygen species 6. Urea cycle enzymes present - ORN CIT produced
- Peptide hydrolases in brush borders / cytosol exist -
free AAs only pass to portal vein & liver - AA used for nrg (> GLN ?)
- Synthesis
• Apoproteins necessary for lipoprotein formation
• New digestive enzymes
• Hormones
• Nitrogen-containing compounds
4. GLU ASP transaminated to ALA GLU GLN → ALA (ORN CIT PRO also produced) - ↓ toxicity of ↑intakes of GLU GLN ASP
- Urea cycle enzymes present - ORN CIT produced
- Glutathione also formed
- reduce reactive oxygen species
B. METABOLISM & SPECIAL ROLE OF
THE LIVER
Absorbed AAs
→ interstitial fluid
→ capillaries of villi
→ portal vein
→
LIVER
• Liver monitors incoming dietary AAs: Determines fate according to
body needs (ie. maintain homeostasis)
1) anatomical position
2) site of IAA catabolism (except BCAA)
3) site of urea synthesis
4) synthesis of plasma ptn (secretory)
5) synthesis of fixed ptn (non-secretory)
6) high rate of ptn synthesis and degradation
ie. liver
↑ in size after a meal
7) very sensitive to changes in AA supply
i.e., capable of rapidly changing rate of metabolic processes
(liver ptn: short-term storage form)
Average disposition of AAs
entering liver
ptn synthesis in liver 20% – plasma ptn / non-protein compounds 6% – liver ptn 14% • free AAs released to systemic circulation 23% • >70% are BCAAs • catabolism 57% • 1-2% BCAA • variable DAA (eg. some GLN GLU) • mostly IAA
C. METABOLISM IN SKELETAL
MUSCLE
1) BCAA → α-keto acid (abundant BCAA transferase) 2) INDIVIDUAL AA METABOLISM: – ALA GLN synthesis de novo (ie. glucose-ALA cycle) – LEU (ketogenic) as energy source (esp. starvation) – ASP ASN GLU ILE VAL → GLN, energy, lactate, intact, keto acid – All other AAs → not oxidized, intact, keto acid
MEASURES OF MUSCLE MASS &
DEGRADATION
1) creatine (0.3-0.5% muscle)
→ phosphocreatine
creatinine (urine - 1.7% of total creatine pool)
- variation of creatine in muscle - not accurate
2) 3-methylHIS (actin) released during proteolysis
6
urine (index of degradation)
- actin widely distributed in diet
- depends on dietary intake
(nonreusable – i.e., meat)
D. METABOLISM IN KIDNEY
1) removal of nitrogenous (ptn) wastes
(ie. urea, creatinine, ammonia, trace amts AAs)
2) metabolism of all reabsorbed AAs
(especially DAAs: GLY GLU GLN ALA ASP)
3) kidney has (like liver) enzymes: AAs
→
glucose
E. METABOLISM IN THE BRAIN AND
THE CNS
1) NEUROTRANSMITTERS: – PHE / TYR → catecholamines • dopamine, norepinephrine, epinephrine – TRP → serotonin • active transport; competition for uptake (TRP/LNAA) 2) GLU - removal of excess ammonia – α-ketoglutarate for transamination – gamma-aminobutyric acid (GABA): modulation of nerve impulses 3) PEPTIDES (CNS) – ACTH [adrenocorticotrophic hormone], the enkephalins, somatotropin [growth hormone]
GENETIC DISORDERS OF AAM
EXAMPLES
Phenylketonuria (PKU) • Absence/defect of enzyme phenylalanine hydroxylase or enzyme cofactors (PHE → TYR) in the liver • high plasma levels of PHE and metabolites • mental retardation if not treated
GENETIC DISORDERS OF AAM
(EXAMPLES)
C. CONTRIBUTING CONDITIONS (both)
Maple Syrup Urine Disease (MSUD) • defect in (lack of) branched-chain α - ketoacid dehydrogenase complex (BC α-KA → CO 2+product) • accumulation of BCAAs and BCKAs • mental retardation
- poverty, food shortages, lack of animal ptn, lack of land or utilization of agricultural production - "web" of political, economic and marketing conditions in "developing" countries - when infants/children begin vomiting/diarrhea given watery, starchy gruel → ↑ nutr def.
ENERGY-PROTEIN MALNUTRITION
both KWASHIORKOR and MARASMUS are diseases of
inadequate intake of ptn and nrg
A. KWASHIORKOR
- seen in children 3-5 y of age
- mainly in “developing” countries
- a disease in which ptn def > nrg def
B. MARASMUS
- seen in children less than 1 y of age (usually)
- condition in which nrg deficit is predominant
which desease is energy defecient
marasmus
which desease is pro deficient
kwashiorkor
ENERGY-PROTEIN MALNUTRITION
D. SYMPTOMS
- Failure to grow (both)
- Dry, peeling and blotchy skin (K)
- Dry, brittle hair, loss of pigment (K)
- Enlarged, fatty liver (K)
- Loss of appetite, vomiting, diarrhea (both)
→ loss of H
2O, Na, K
6.
↓ level of serum ptns (less severe in M)
7.
↑ susceptibility to infections [measles] (both) - Edema (K)
- May reduce brain growth (0-2 y), brain fn (M)
- In practice, observe a condition between K and M
EFFECTS OF STARVATION
(VERSUS STRESS)
stress as in inflmation, infection, missing limb, diabetes
- Body ptn flux rates vary: min to mths Influenced by many factors: 1. immediate food intake 2. previous diet 3. overall nutritional status 4. stress 5. infection (additional problem in starvation)
Starvation
• ptn syn ↓ (ptn-> 6 CHO to brain etc)
• ptn bkd ↓ (N losses small)
- adaptative mechanism (loss of 3-4 g ptn/d)
- critical ptn maintained (eg, heart, kidneys)
Infection/Stress
ptn bkd ↑ – stress could be part of starvation Eventually, organs weaken, ↓ immune fn, ↑ infections/disease, death
Protein Deficiencies
• Protein deficiency rarely isolated, other nutrients
↓
– e.g., ptn/nrg K+M
• stunting, poor musculature, edema, thin hair, skin
lesions,
↓ albumin, hormonal imbalance
• poor countries (PEM)
• Cdn, pathologic conditions / poor management of ill
