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
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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
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]
how is sickle cell anemia caused?
substitution of glutamate for valine causing decreased O2 carrying capacity
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 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 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
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 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
All animal sources contain all essential amino acids except for what source? what AA does it lack?
Gelatin
-lacks Trp
