Protein Flashcards
What happens to dietary proteins in the stomach?
Proteins are denatured by hydrochloric acid and hydrolyzed by pepsin, breaking them into smaller polypeptides and free amino acids
Why do we need a dietary supply of proteins?
To maintain protein balance, replace obligatory losses through urine and feces, support growth, and synthesize non-protein substances
List the nine essential amino acids.
Histidine, isoleucine, lysine, threonine, leucine,methionine,phenylalanine, tryptophan, and valine
HILTLMPTV
What are conditionally essential amino acids? What types of reasons make an AA conditionally essential?
Amino acids that become essential under specific conditions, such as illness or stress
-insufficient synthesis
-increased requirement
-decreased synthesis
-defective synthesis
Why are cysteine and tyrosine considered semi-essential?
They require methionine and phenylalanine, respectively, as precursors and become essential if these precursors are limited
Which AA are conditionally essesntial?
Cysteine, glutamine, glycine, arginine and tyrosine
what AA becomes conditionally essential with stress, injuries and surgeries? why?
Glutamate
-important for immune function and is a precursor for glucose
what AA becomes conditionally essential during decreased synthesis? why?
Arginine
-important in DNA synthesis, urea cycle and regulation of blood flow (NO formation)
What condition affects tyrosine synthesis, and what are the consequences? what is the function of tyrosine?
Phenylketonuria (PKU) results from a lack of the enzyme phenylalanine hydroxylase, requiring dietary tyrosine supplementation
tyrosine synthesizes catecholamines, NT, melanin and thyroid hormones
Outline the major phases of protein digestion
Mechanical digestion in the mouth, gastric hydrolysis in the stomach, enzymatic breakdown by pancreatic proteases, and further digestion at the brush border.
What occurs in the mouth during protein digestion?
Mechanical digestion: chewing and crushing moisten food, preparing it for further breakdown
Describe the role of the stomach in protein digestion
The stomach uses hydrochloric acid to denature proteins and activates pepsinogen to pepsin, breaking proteins into smaller peptides
What triggers pancreatic enzyme secretion in the small intestine?
The presence of polypeptides and amino acids stimulates the release of cholecystokinin (CCK), prompting pancreatic enzyme secretion
How are pancreatic zymogens activated?
Enteropeptidase activates trypsinogen to trypsin, which in turn activates other zymogens like chymotrypsin and carboxypeptidase
where is CCK released from? where does it act on?
released from the SI in response to polypeptides and small AA and will stimulate bile acids and pancreatic enzymes from the pancreas
what is the role in CCK on pH?
release of CCK will stimulate the release of HCO3- to decrease acidity and make an optimal environment for enzyme activity in the intestine
what enzyme activates trypsinogen?
enteropeptidase
what do the enzymes in the SI break proteins down into?
di-,tri, and oligopeptides
What is the significance of brush border peptidases?
They complete the digestion of oligopeptides into free amino acids and di/tripeptides for absorption
what are incretins and where are they released from? provide a specific example
hormones released from the intestine that stimulate insulin secretion prior to blood glucose spikes to prepare the body for larger insulin repsonses
Glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP)
what is DPP-4? what is its effect on metabolism?
An inactivator of incretins which prevents the lowering of blood glucose
what are endogenous vs exogenous proteins?
endogenous proteins are produced within the body and often recycled, while exogenous proteins are consumed through diet
what are examples of endogenous proteins?
digestive secretions, sloughed-off epithelial cells and recycled proteins
What happens to amino acids after they are absorbed by enterocytes in the small intestine?
they are used for protein synthesis within the enterocyte, converted into other metabolites, or transported across the basolateral membrane into the bloodstream
What is the role of sodium in amino acid absorption? why is this important?
Sodium creates a gradient that helps drive the active transport of amino acids into enterocytes, this is needed because we need to be able to uptake proteins at all times wihtout relying on a [gradient] for absorption
Describe the transport of di- and tripeptides across the brush border, what occurs to the peptides after this?
Di- and tripeptides are absorbed via a proton-dependent transporter called PepT1, once inside the enterocyte, they are usually hydrolyzed into free amino acids by intracellular peptidases
explain the impact of having diffent types of amino acids that need to be transported
differing AA compete for transport which affects absorption
How is the sodium gradient maintained in enterocytes for amino acid transport?
The sodium gradient is maintained by the Na⁺/K⁺ ATPase pump on the basolateral membrane of enterocytes, which actively pumps sodium out of the cell and potassium in, creating a gradient
How does the sodium-dependent amino acid transporter work?
Sodium binds to the transporter first, inducing a conformational change that increases the transporter’s affinity for amino acids. Once the amino acid binds, the transporter moves both sodium and the amino acid into the cell
Are all amino acids absorbed via sodium-dependent transport mechanisms?
No, not all amino acids rely on sodium-dependent transport. Some amino acids are transported via other mechanisms, such as facilitated diffusion or proton-dependent transport systems.
What factors affect the rate of protein digestion and absorption?
Protein structure, the food matrix, peptide length, and the relative essentiality of amino acids
How does the small intestine adapt to changing protein absorption needs?
By increasing surface area through hyperplasia or altering transporter and enzyme expression in response to demand
Why aren’t intact proteins usually absorbed by adults?
Due to the action of brush border proteases and the lack of transporters for whole proteins
How do newborns absorb whole proteins from colostrum?
Through leaky junctions that allow immunoglobulins and growth factors to pass into the bloodstream, aiding immune development
What is celiac disease, and how does it affect protein digestion?
An autoimmune disorder where gliadin protein permeates tight junctions, leading to an immune response and intestinal damage
How does the small intestine utilize dietary amino acids?
Amino acids serve as a major fuel source, particularly for mucosal cells, affecting dietary protein utilization efficiency.
Why do some amino acids have lower portal concentrations? what are some proteins that you would not expect to be seen in portal circulation?
Because they are heavily used by enterocytes for energy and mucin synthesis before entering the bloodstream
-Glutamine, glutamate and aspartate
What is protein malabsorption, and what can cause it?
It refers to inadequate protein digestion/absorption, caused by pancreatic dysfunction, intestinal resection, or malnutrition
What is refeeding syndrome, and why is it dangerous?
A condition where malnourished individuals cannot properly digest and metabolize nutrients, leading to potentially fatal imbalances
how does the anount of dietary protein vs endogenous protein differ within the pool size?
there is slightly more dietary protein ~100g as compared to endogenous ~75
What are the major fates of amino acids in the body?
1) Protein synthesis
2) Precursors for non-protein nitrogenous molecules
3) Catabolism for energy with nitrogen excretion
what major roles does the liver play in AA metabolism?
1) synthesizes proteins
2) regulates gluconeogenesis using alanine
3) activates / deactivates AA metabolism based on hormonal signals.
What non-protein molecules will be impacted by a deficiency for each arginine and tyrosine?
arginine: nitric oxide
tyrosine: catecholamines
What is transamination? why is it important? what catalyzes this rxn?
The transfer of an amino group to an α-keto acid in order to make new amino acids
-forms non-essential
catalyzed by aminotransfereases (AST and ALT)
what enzyme is needed for aminotransferases to work?
PLP
-acts as a coenzyme
What is oxidative deamination, and where does it occur?
the process by which an amino group is removed from an amino acid as ammonia, with the remaining carbon skeleton converted to a keto acid. It primarily occurs in the liver and kidney
Why is oxidative deamination important in nitrogen metabolism?
it releases ammonia, which can be converted to urea in the liver for safe excretion. It also regenerates α-ketoglutarate, a key intermediate in the citric acid cycle
What are the effects of insulin, glucagon, and cortisol on protein metabolism?
Insulin promotes synthesis, while glucagon and cortisol stimulate protein breakdown and energy production from AAs
How is urea synthesis regulated?
Urea cycle activity increases with high protein intake and decreases during fasting, with regulation at the enzyme level
what AA do not participate in transamination rxns?
Argininge, Tryptophan, Lysine, Threonine and Proline
LATT-Pull
where is ALT found? what do high levels signal? what about in alcoholic hepatitis? why?
1)found in the liver (kidney, heart and muscle)
2)high levels signal damage
3)typically AST:ALT ratio is lower in most diseases as the liver deals with majoryity of the issues; but in alcohol related hepatitis AST:ALT is higher (>2) because serum ALT activity is decreased
what is the major AA deaminated in the liver?
glutamate
what makes ammonia toxic?
-it raises pH to damaging levels (inteferes with ETC)
-neurotoxicity
what makes an AA gluconeogenic? what are examples?
Their C-skeleton gets converted to pyruvate or CAC intermediates
-Ala,Gly,Cys,Ser,Asp,Asn,Glu,Gln,Arg,Met,Val,His,
Pro
What makes an AA ketogenic? what are examples?
Their C skeletons make acetyl coA or Acetoacetate
-Lys, Leu
What is the glucose-alanine cycle? why is it important?
Muscles form alaline from pyruvate which is transferred to the liver where it is converted back to pyruvate to make glucose
-Alanine is converted to glucose and nitrogen is excreted as urea in order to remove excess nitrogen and provide glucose for energy or maintain [blood glucose]
what AA is primarily used for transamination in the kidneys? what abt the liver?
Glutamine in the kidneys
Glutamate in the liver
what can affect translational efficiency?
-mRNA structure
-Ribosomal availibility
-nutrient / enegry status
-stress conditions
what are 3 common reasons for alterations in protein structure?
1) abnormal genes
2) abnormal processing
3) inability to degrade abnormal proteins
how is sickle cell anemia caused?
substitution of glutamate for valine causing decreased O2 carrying capacity
what is familial hypercholesterolemia?
abnormal synthesis of LDL receptors affects uptake of cholesterol from blood, increasing serum cholesterol and atherosclerosis risks
what are connective tissue disorders that affect collagen? why is this an issue?
Alterations in collagen structure result in many diseases due to it being the most abundant protein in the body
How does neonatal diabetes result? what does it impact?
alterations in the folding of insulin during synythesis interfere with other cellular processes in cells that make insulin
what is the importance of protien degradation?
1) regulation of protein abundance
2) elimination of abnormal proteins
what is the major pathway for protein degradation? how does it work? when does the activity of the UPS system increase?
Ubiquitin-proteasome systnem (UPS)
-proteins that need to be degraded are tagged multiple times to signal removal
-activity increases pathological conditions and starvation
what us the affect of inflammation on the UPS system?
cytokines are involved with activation of the UPS system, meaning inflammation can upregulate this system
what can cause abnormal UPS function? what are some examples that result from this?
abnormal protein accumulation
-neurodegenerative diseases (alzheimer’s)
-cardiovascular diseases (atherosclerosis)
what is autophagy-lysosomal pathway (ALP)? what 3 steps are involved? where does it mostly occur? what increases activity of this pathway?
self-digestion of lysosomes
1) sequestration
2) fusion
3) acidification & digestion
occurs mainly in the liver and minimally in the muscle cells, increased by glucagon and decreased by insulin and AA
How is Parkinson’s disease related to protein degradation systems?
Accumulation of unfolded proteins causes dysfunction of the UPS and ALP pathways resulting in protein aggregation and the death of dopamine neurons
*improper brain signaling
What are the 4 pathways of protein degradation in the body?
1)Ubiquitin-Proteasome System (UPS): Responsible for 80-90% of protein degradation, targeting damaged or short-lived proteins
2)Autophagy-Lysosomal Pathway (ALP): Degrades large protein aggregates and organelles
3)ER-Associated Degradation (ERAD): Removes misfolded proteins from the endoplasmic reticulum
4)Calcium/Calpain-Dependent System: Degrades specific proteins in response to calcium signals
What is protein turnover, and why is it important?
the continuous process of protein synthesis and degradation, necessary for maintaining homeostasis and adapting to changes in energy or nutrient availability
What are the two components of nitrogen metabolism?
1) Protein turnover (synthesis and degradation) and 2) Nitrogen balance (nitrogen intake versus excretion)
What is nitrogen balance, and what does it indicate?
the difference between nitrogen intake and loss. Positive balance indicates growth, while negative balance indicates muscle wasting or protein loss
what is the energetic cost of protein turnover, how does this change during starvation?
Protein turnover uses 10-25% of basal metabolic rate energy. During starvation, the turnover rate decreases to conserve energy
Where does most amino acid degradation occur, and what other tissues does this occur?
The liver is the primary site for amino acid degradation
-Muscles oxidize branched-chain AAs, and the small intestine handles glutamine, glutamate, and aspartate
What happens to amino acids not used for protein synthesis?
They are partially oxidized, with carbon skeletons converted into glucose, glycogen, triglycerides, cholesterol, ketone bodies, or non-protein derivatives like creatine and heme
How do skeletal muscles function in amino acid storage and metabolism?
Skeletal muscles act as a reserve of gluconeogenic precursors and regulate protein turnover to support protein synthesis and gluconeogenesis when needed
when does uptake of AA by skeletal muscle occur?
occurs after ingesting protein
What are the characteristics of amino acids with a low Km for degradation enzymes?
they have a high affinity for their enzymes, often undergo irreversible reactions, and include essential amino acids like branched-chain and sulfur-containing amino acids
What types of reactions are associated with amino acids that have a medium Km (10-20 mM)?
Amino acids like Thr and Lys with a medium Km undergo irreversible reactions
-they cannot be easily regenerated
With AA with a high Km, what would you expect in regards to their reactions and availability physiologically? what is a potential risk?
They have a low affinity for their enzymes and undergo reversible rxns
-more flexibility in utilization / replenishment
-they can be found in [high] and cross the blood brain barrier, leading to PKU
What is the significance of the free amino acid pool, and why must it be regulated?
The free amino acid pool must be continually replenished. Excess amino acids can be toxic and interfere with processes like transport into the brain
What are the main sources of the amino acid pool?
Dietary proteins, tissue protein breakdown, and de novo synthesis of non-essential amino acids
what are catabolic signals for protein regulation?
thyroid hormones, glucagon, glucocorticoids, catecholamines, cytokines
what are anabolic signals for protein metabolism?
insulin and amino acids
what kind of proteins do you expect to have a rapid turnover? what are examples?
regulatory proteins
-Incretins, insulin
what kind of proteins do you expect to have a slower (not slowest) turnover? what are examples?
non-regulatory proteins
-hemoglobin, albumin
what kind of proteins do you expect to have the slowest turnover? what are examples?
structural proteins
-collagen
what causes decreases in muscle mass with aging?
lower protein intake and little change in degradation leads to loss of lean body mass through decreased resistance activity
how do hypermetabolic (catabolic) states affect muscle wasting? what are some causes of this and why?
it increases muscle watsing
-burns, malnutrition, cancer and sepsis increase BMR which may result in muscle wasting if the higher energy needs are not met
how can bulding muscle be an issue in hypermetabolic states even when the proper amount of AA are consumed?
increases in catecholamines, and inflammatory cytokines will interfer with insulin signalling causing issues in building muscle even with proper AA intake
what is the EAR for protein? what does this indicate?
0.66g/kg/day
-the amount of protein needed to maintain N balance
where are majority of obligatory protein losses seen?
urine and feces
How does age affect protein requirements?
Protein needs per weight decrease with age. For infants and children, requirements are higher to support growth and development
Why is dietary energy important for protein utilization?
Insufficient energy intake leads to amino acids being used for energy rather than protein synthesis, reducing protein utilization efficiency
What is the AMDR for protein intake?
10-35% of total energy intake
what cells does acute protein deficiency have the greatest effect on?
cells with rapid turnover
what 3 ways can protein status be evaluated by?
1) measuring plasma proteins in blood
2) assessment of rapidly growing tissues such as hair and skin (visual signs)
3) assessment of lean body mass (failure to grow in children)
What is marasmus, and what are its symptoms?
Energy + protein deficiency
-muscle wasting, fat loss, and growth retardation in infants
(starvation)
What characterizes kwashiorkor, and who is affected?
adequate energy + protein deficiency
-growth retardation, fatty liver
-associated with poverty , in developing countries, kids ages 1-5
-more serious than marasmus
What are some concerns with high-protein diets?
High-protein diets can cause weight loss, calcium loss, and in extreme cases, ketoacidosis or “rabbit starvation” through displacement of other macros (protein > 45% of energy)
What factors determine protein quality?
the amino acid pattern or score, digestibility, and the availability of amino acids
What is true digestibility versus apparent digestibility?
True digestibility accounts for endogenous protein losses, while apparent digestibility only measures ingested and fecal nitrogen
How can food processing affect amino acid availability?
High temperatures and acid can alter amino acids, making them less bioavailable or metabolically unusable
Why do plant proteins generally have lower digestibility than animal proteins?
Plant proteins may be encapsulated in cell walls or contain trypsin inhibitors, reducing their digestibility
What is the limiting amino acid concept?
The determinant of protien quality based on the most deficient essential amino acid in relation to the requirement is known as the limiting AA
Which amino acids are commonly limiting in food sources?
Lysine, methionine, threonine, and tryptophan are common limiting amino acids in various foods, especially plant sources
How is protein quality assessed using bioassays?
Bioassays like Protein Efficiency Ratio (PER) and Biological Value (BV) measure nitrogen utilization and weight gain per gram of protein
-useful for diets but not foods
What is PDCAAS?
Protein Digestibility-Corrected Amino Acid Score (PDCAAS) evaluates protein quality by combining amino acid composition and digestibility
-more readily used
How is protein content on food labels determined?
Food labels use PDCAAS to calculate the % Daily Value of protein, reflecting both quality and digestibilit
when is %daily value needed on food lables
if making a high proteihn claim or if made for children < 4 years
How can amino acid oxidation be used to assess protein requirements?
Excess amino acids are oxidized, indicating their availability relative to needs and guiding determination of dietary requirements
All animal sources contain all essential amino acids except for what source? what AA does it lack?
Gelatin
-lacks Trp
What are the benefits and challenges of vegetarian diets?
Benefits include lower risk of CHD and diabetes, but challenges involve meeting protein, iron, B12, and calcium needs
What claims are allowed for protein on food labels?
“high protein” - protein rating 40+
“More protein / higher protein” - protein rating 20+
“High quality protein” / “source of AA”- not permitted
