Protein Flashcards
what is the average nitrogen content?
Average nitrogen of proteins = 16%
Convert nitrogen cont into protein content:
1/0.16 = 6.25
N x 6.25 = protein content
What is the significance of protein?
Acceptable Macronutrient Distribution Range: 10-35%
Protein in the human body:
- 40% in skeletal muscle
- 25% in body organs
- 35% in skin and blood
Proteins Functions:
1. structural
2. regulatory
3. energy
What are the structural functions?
- contractile proteins => actin, smooth muscle
- fibrous protein => collagen, elastin, certain
- globular proteins=> spherical shape e.g. myoglobin
What are the regulatory functions?
Enzymes/catalysts: changing rate of reactions
- cofactors => minerals (metalloprotein), folate, B-vitamins
Hormones/Messagners: acting as chemical messengers
Transporters:
- in blood - hemoglobin, albumin (fatty acids), prealbumin = transthyretin (thyroxine and RBP), transferring (iron), ceruloplasmin (copper), lipoprotein
Immunoglobins/antibodies
Buffers=> amino acids act as acids or bases
Fluid balance:
- ascites => fluid balance is impaired in protein deficiency - abnormal accumulation of fluid in the abdominal
Acute Phase Reactant Proteins:
- formed in the liver - in response to infection, injury/inflammation
functions:
- stimulate the immune system, wound healing, chelate and remove iron from circulating
C-reactive protein = clinical indicator for inflammatory status
What are other functions?
Storage proteins:
Conjugated proteins: Glycoproteins
What is the storage protein for glucose storage?
glycogenin
Which glycoprotein has a role in clinical diagnosis?
glycated hemoglobin (A1C, blood glucose level)
What are the sources of energy at excessive intake?
Deaminination of amino aids; enters into:
- TCA cycle -> formation of ATP
- glucogenesis -> energy storage
- fatty acid synthesis -> energy storage
Describe the protein structures.
Primary: polypeptide backbone does not differ btw polypeptide chains but side chains differ
Secondary: alpha (cylindric) and beta-pleated sheets (stretched out backbone) => both stable
tertiary: interaction among the amino acids residues or side chains
Quaternary: oligomers = 2 or 4 polypeptides chains => held together by hydrogen bonds and electrostatic attraction
What are the essential amino acids?
phenylalanine, valine, threonine, methionine, tryptophan, histidine, isoleucine, leucine, lysine
What is phenylketonuria (PKU)?
missing phenylalanine hydroxylase => converts phenylalanine to tyrosine
- instead into phenylpyruvic acid
- interferes with neurotransmitter synthesis saying retaration
Is an essential amino acid
- classic diet therapy
what are exogenous sources of amino acid?
Animal-sourced foods: meats, poultry, fish, dairy
plant-sourced foods: almond milk, tofu, grains, legumes
Protein supplements/protein bars
amino acids supplements
in GI tract:
- desquamated mucosal cells
- digestive enzymes and glycoproteins
In the case of proteins, the small intestine can only absorb
amino acids, dipeptides, tripeptides
How is protein digested in the stomach?
- release of hydrochloric acid (HCl) from parietal cells, which is stimulated by: gastrin, acetylcholine, histamine
- protein degradation => protein unfolding; disrupts hydrogen and electrostatic bonds (quaternary, tertiary, secondary) - Pepsinogen
- released from chief cells => pepsin
- hydrolysis of peptide bonds
- protein -> large polypeptides, oligopeptides, free amino acids
How is protein digested in the small intestine?
the major site of protein digestion, absorption
acidic chyme enters the duodenum
- stimulation of secretion of secretin and cholecystokinin
- stimulate pancreatic juice secretion from acinar cells -> containing digestive enzymes (zymogens)
- stimulates the secretion of enteropeptidases released from brush border
trypsinogen => trypsin
chymotrypsinogen => chymotrypsin
proelastase => elastase
procollagenase => collagenase
procarboxypeptidases A and B => Carboxypeptidases A and B -> only exopeptidase => release of amino acids from C-terminal end of a peptide chain
Aminopeptidases
hydrolyze AA from the N-terminal end of oligopeptides
Dipeptideylaminopetidases
hydrolyze AA from the N-terminal and of dipeptides
tripeptidase
hydrolyze specific AA to yield dipeptide and 1 free AA
How much amino acids aborbed in different parts of the body?
60% => form of di- or tripeptides
pept1 => transporter into enterocytes
40% of absorbed AA as free amino acids
30-50% of AA used by intestine
50-70% of amino acids transported vis basolateral membrane to portal blood
Amino acid transport
y+ => passive
ASC => active transport; binding of Na+
- amino acids need to be transported out of the enterocytes -> reach blood
How is the uptake of polypeptides or proteins directly into the bloodstream bad?
- leaks btw epithelial cell junctions
- transport by vesicles to the submucosa
potential consequences: - acute immune response
- development of sensitivity to polypeptides
- food allergy, asthma
- idiosyncratic drug reaction
vulnerable population:
- atopic heredity
- infants => feed formula solely -> resolution breastfeed for 6 months or use partially or extensively hydrolyzed infant formula (smaller protein fragments)
- GI tract diseases
How are amino acids used in enterocytes?
synthesis of proteins, nitrogen-containing compounds and energy
- new digestive enzymes
- hormones
-structural proteins and nucleotides for new intestinal cells
Retained amino acids include: glutamine, glutamate, aspartate, arginine, methionine
What is the role of glutamine?
- the primary source of energy
- formation of alanine
- trophic effects -> stimulates GI mucosal cell proliferation
-> prevents atrophy of gut mucosa
-> creates a barrier against bacterial translocation
w/ threonine => synthesis of mucin
-> produced in skeletal muscles => taken up by intestinal cells
-> Enteral and parental nutrition products are enriched with glutamine
What is the role of glutamate?
90% of absorbed glutamate used by the intestine
- energy
- formation of alanine
- formation of proline => released into portal blood for delivery to the liver
- with aspartate -> ornithine -> citrulline (urea cycle)
- with glycine and cysteine -> glutathione (antioxidant)
What is the role of arginine?
- used in intestinal cells; source of energy
- little found in portal blood
- transaminated to => oxaloacetate (TCA cycle); ornithine (urea cycle)
what is the role of arginine?
- critulline (formation) and urea
- intestinal injury => conditionally indispensable amino acid; arginine or citrulline supplementation in the diet
what is the role of methionine?
50% -> metabolized in the intestine
- precursor for S-adenosylmethionine (SAM) -> one-carbon donor => formation of neurotransmitter, DNA methylation
- transformed to cysteine via tran-sulfuration pathway (vit B-6 dependent) => used for formation of glutathione and taurine (bile salts)
Role of the liver
- monitor the passage of AA from the gut to the peripheral circulation
- monitors AA concentration in plasma
- uses AA for protein synthesis
- primary site of AA catabolism
Which hormone regulates protein anabolism?
Insulin
- promotes cellular AA uptake ‘
- simulates transcellular movement of AA transporter to membrane
- inhibition of AA oxidation
- stimulates protein synthesis
After/during meal: inc. protein synthesis and dec. degradation of proteins
Protein formation in liver
For release into plasma:
Albumin
- most abundant plasma proteins
- critical for fluid balance
- indicator of visceral protein status
- transports proteins: nutrients (Ca, Zn, B-6, FA) and drugs & hormones
Transthyretin:
- transports thyroid hormones (T4) and vit A
- a more sensitive indicator of visceral protein status -> rapid protein turnover
More transport proteins:
VLDL -> lipid transport
HDL-> reverse cholesterol transport
ceruloplasmin -> transport of Fe
others:
prothrombin -> blood coagulation
immunoglobins -> immune response
acute phase proteins -> infection, injury, inflammation
purines and pyrimidines are synthesized in the liver
interorgan amino acid transport
provide amino acids for protein synthesis and other functions to all tissues
- happens in all states
Dietary proteins: whey vs. casein
whey protein:
- ‘fast protein’ => digested faster, amino acid reached, quicker
- fast, high, transient
- inc, protein synthesis and amino acid oxidation
- no change in protein breakdown
casein protein:
- ‘slow proteins’ => slow digestion; amino acids in blood deposited slowly
- slow, lower, prolonged
- protein synthesis slightly inc,
- amino acid oxidation is moderately stimulated
- protein breakdown is marked inhibited by 30%
When do protein and AA catabolism occur?
- During starvation
- low CHO => TCA cycle intermediates, brain glucose
Protein intake > needs for growth and maintenance
hormones involved in catabolism
glucagon
catecholamines
glucocorticoids
thyroxin
Transamination
transfer of -NH2 group from one amino acid to alpha-keto acid (carbon skeleton of amino acids)
- involved in alanine, glutamate, aspartate
Deamination
release of -NH2 group form of ammonia
serine to pyruvate
Deamidation
loss of an amide functional group -NH2 in the side chain of amino acids glutamine and asparagine in the form of ammonia
glutamine to glutamate
Pathways for ammonia removal in the skeletal muscle:
glutamate and glutamine formation
alanine-glucose cycle
Glutamine and glutamate formation:
disposal pathway in extrahepatic tissues
- glutamate is generated in the muscle -> deaminate by glutamate dehydrogenase to alpha-ketoglutarate and ammonia -> in the form of glutamine carried out of the cells to the liver, kidney, and intestine;
- glutamine-> also made in the brain and adipose tissues; greated formation in muscle and lungs
Alanine-glucose cycle
in the fed state: glutamate => form glutamine for disposal of ammonia
in fasting state:
- between meals and excessive glucose needs of the muscle
- glutamate -> alanine -> released into blood -> liver takes up alanine and catabolizes it and releases ammonia to urea cycle
Hyperammonemia [NH3]
impaired urea cycle
- liver damage, liver disease
- urea cycle enzyme defects
Consequences -> brain malfunction and coma
Treatments:
- nitrogen-restricted diet, drugs that acidify the GI tract, antibiotics
What can the carbon skeleton be used for?
energy, glucose (TCA cycle), ketone bodies (pyruvate acetyl-CoA, acetoacetate), cholesterol (directly from leucine), fatty acids
Dietary proteins are required for:
- supply of nitrogen for the synthesis of non-essential AA
- supply of essential AA
What factors do the quality of protein depend on:
1) amount and proportion of essential amino acids
2) Protein digestibility
3) amino acid availability
high-quality or complete proteins
proteins containing all essential AA in approximate amounts as needed by humans
- animal sources, soy proteins, quinoa
Low-quality or incomplete proteins
proteins having too low amounts of one or more essential AA than needed by humans
- plant sources
limiting amino acid
the essential amino acid present in the lowest amount in the food relative to the requirement for this AA in the human body
- mutual supplementation
ex. legumes are high in lysine but low in methionine and cysteine
grain products are low in lysine but high in methionine and cysteine
Protein digestibility
amount of amino acids being absorbed after ingestion of given proteins
Amino acid availability
diminished during food processing because of:
- extensive heating => creates protein side chains that cannot be hydrolyzed during protein digestion
- Maillard reaction => presence of sugars -> reduced availability of lysine
- oxidation of amino acids
How can you evaluate protein quality?
- chemical or amino acid score
- protein digestibility corrected amino acid score
- protein efficiency ratio
- Biological value
- net protein utilization
Chemical or amino acids score
analytical determination of AA composition of the
test protein
Procedure:
1. Quantification of essential AA conc. in the protein
2. comparison to the value of reference proteins considered to have a value of 100
the essential amino acid with the lowest score (in %) in the test protein = limiting amino acid of the test protein determines the amino acid score
protein digestibility corrected amino acid score
= amino acid score (in %) x true digestibility (in %)
Nitrogen => indirect method to quantify protein content
protein efficiency ratio
= gain in body weight/grams of protein consumed
measurements: body weight before and after diet total grams of test proteins consumed
Biological value
measure for the amount of retained amino acids => part of amino acids that were absorbed + being utilized
= Nitrogen retained/nitrogen absorbed x 100
high biological value -> provides amino acids in amounts consistent w/ the individual amino acid requirements
net protein utilization
measure how well the ingested proteins are digested and absorbed and how well the amino acids absorbed are utilized
= nitrogen retained/nitrogen consumed x 100
