Protein Flashcards
What are the nutritionally essential amino acids?
Phenylalanine, isoleucine, leucine, lysine, methionine, threonine, tyrptophan, valine, and histidine
Describe the main functions of proteins and amino acids
- Growth, maintenance, and movement: proteins form integral parts of most body structures (skin, tendons, membranes, muscles, organs, bones). Support growth and repair of body tissues
- Enzymes: facilitate chemical reactions
- Hormones: some are made of proteins, functioning as messengers and signals and regular body processes
- Immunity: antibodies, cytokines, and chemokines are proteins; regulate gene transcription and translation through mTOR signaling pathway
- Fluid and electrolyte balance: help maintain fluid volume and the composition of body fluids
- Acid-base balance: act as buffers
- Transportation and storage: transport substances (lipids, vitamins, minerals, and oxygen); some store a specific micronutrient
What are the 4 levels of protein structure?
Primary structure, secondary structure, tertiary structure, quaternary structure
Name some examples of transport proteins
Hemoglobin (carries oxygen from lungs to the cells)
Lipoprotein (transports lipids around the body)
Retinol-binding protein (transports retinol from liver to target tissues)
Albumin
Prealbumin
What are the secretory sites, main target sites, and functions of Gastrin?
Secreted in pyloric antrum, duodenum, and pancreas
Target site is parietal cell
Acid secretion, gastric contraction
What are the secretory sites, main target sites, and functions of Secretin?
Secreted in duodenum
Main target site is pancreas
Water and bicarbonate secretion
What are the secretory sites, main target sites, and functions of Cholecystokinin?
Secreted in duodenum and jejunum
Main target sites are pancreas and gallbladder
Pancreatic enzyme secretion, gall bladder contraction
What are the secretory sites, main target sites, and functions of Gastric inhibitory peptide?
Secreted in duodenum and jejunum
Main target site is pancreas
Insulin secretion
What are the secretory sites, main target sites, and functions of Glucagon-like peptide-1?
Secreted in small intestine (L cell)
Main target site is pancreas
Insulin secretion
What are the secretory sites, main target sites, and functions of Glucagon-like peptide-2?
Secreted in small intestine (L cell)
Main target site is small intestine
Intestinal growth (crypt cell)
What are the secretory sites, main target sites, and functions of Peptide YY?
Secreted in small intestine (L cell)
Main target site is brain stem
Slowing the gastric emptying, reducing appetite
What are the secretory sites, main target sites, and functions of Somatostatin?
Secreted in pyloric antrum, hypothalamus
Main target sites are GI tract, pituitary gland
Suppressing the release of GI hormones
What are endogenous sources from which protein enters the GI tract?
Desquamated mucosal cells, digestive enzymes, other glycoproteins such as mucus
Which amino acids are the most rapidly absorbed?
Methionine, leucine, isoleucine, and valine
What percent of dietary protein is absorbed as dipeptides and tripeptides? What percent is absorbed as free amino acids?
67% and 33%
What are the limitations of of free amino acid formulas in the clinical setting?
Higher osmolality, less palatable, cost more. Elemental formulas vs polymeric did not show and advantage with the former for patients with Crohn’s disease
What factors affect the accuracy of nitrogen balance measurement?
Renal dysfunction, errors in estimating intake, or incomplete collection of urine, stool, fistula, or ostomy losses may affect balance results
What is the formula for calculating nitrogen balance (NB)?
NB = Nitrogen Intake - Nitrogen Output
What is the formula for calculating Nitrogen Output (g/day)?
Nitrogen Output (g/day) = [Urinary Urea Nitrogen (mg/100 ml) x Urinary Volume (L/d)/100] + 20% of Urinary Urea Losses + 2 gm
What are the best ways to evaluate protein adequacy after traumatic injury?
Nitrogen Balance; made easier when the sole nutrition source is PN or EN. Clinical guidelines help define protein requirements in critical illness, but it is often necessary to adjust protein goals depending on the clinical condition of the patient (eg, worsening renal function where protein may need to be decreased).
Describe the characteristics of arteriovenous amino acid differences across organs as a measure of protein metabolism
Invasive method
A tracer amino acid should be infused
Samples of blood are obtained from both the arterial and venous catheter
The measurements are made across muscle beds
Describe the characteristics of tracer methodology as a measurement of protein metabolism
Constant infusion method used in human studies
No requirement for a steady state
Ideal for measuring acute changes in patients
Important research tool
Describe the characteristics of urinary creatinine excreted over 24 hours as a measurement of protein metabolism
Indirect measurement of muscle mass
Has high variation
Describe the characteristics of urinary 3-methyl histidine excretion as a measurement of protein metabolism
Can be used as a measure of muscle degradation
Compromised accuracy with organ dysfunction
What functions are amino acids used for within the intestinal cell?
Synthesis of nucleic acids, glutathione, apoproteins as components of lipoproteins, or other nitrogen-containing compounds
Secretory protein synthesis
Act as an important energy source to the GI tract
Name the gluconeogenic amino acids
Alanine, glycine, cysteine, serine, threonine, asparagine, arginine, aspartic acid, histidine, glutamic acid, glutamine, isoleucine, methionine, proline, valine, and phenylalanine
In critically ill patients, hepatic glucose production ___ which can contribute to what condition?
Increases; hyperglycemia
What is the key role of alanine and glutamine in muscle BCAA metabolism?
They participate in the process to remove nitrogen wastes that are produced during BCAA oxidation
Which amino acid is the only one that undergoes a complete oxidation pathway in muscle (via the formation of acetyl coA)?
Leucine
Describe the role of the liver in regards to protein
Regulates the flow of dietary amino acids and other nitrogenous compounds derived from tissue degradation
Synthesizes plasma proteins
Metabolism of toxic nitrogenous wastes that have accumulated from exogenous protein intake or from protein breakdown during catabolic states (sepsis, trauma)
Carbon skeletons from gluconeogenic acids can be metabolized to glucose
Site of gluconeogenesis
Describe the role of the kidneys in regards to protein
Involved in interorgan exchange and synthesis of a variety of amino acids
Major site for arginine, histidine, and serine production
Conversion of phenylalanine to tyrosine and glycine to serine
Site of gluconeogenesis
Remove nitrogenous waste from the body (urea)
What is the hepatic metabolite of ammonia?
Urea
Describe the role of the brain and central nervous system in regards to protein
Amino acids are required for neurotransmitter synthesis (tyrosine used to synthesize catecholamines)
Inhibitory neurotransmitters (glycine and taurine)
Glutamine helps rid the brain of ammonia
Neuropeptides act as potent neurotransmitters (mediate sensory and emotional responses
Protein is the primary substrate for which metabolic process during starvation metabolism?
Gluconeogenesis
How soon does glycogenolysis (breakdown of hepatic glycogen stores) occur after fasting? How long until stores are depleted?
2-3 hours; 24 hours
Which organs are the site of gluconeogenesis and begins how soon after a meal?
Occurs in liver and kidneys, starts within 4-6 hours after last meal
After 2 days of starvation, what does the brain switch its fuel source to?
Switches from glucose to ketone bodies
What is the PDCAAS?
Protein digestibility-corrected amino acid score
represents the relative adequacy of a protein’s most limiting amino acid
What is the PDCAAS equation?
PDCAAS (%) = [mg of limiting amino acid in 1 g of test protein)/(mg of same amino acid in 1 g of reference protein)] x fecal true digestibility percentage
What would a PDCAAS of 100% mean?
That 100% of the amino acids of a protein after digestion can be used for protein synthesis, or that the protein contains all essential amino acids in adequate proportions
The nitrogen content of proteins varies from __% to __%, and nitrogen conversion factors range from __ to __
13-19%
5.26-7.69
How is the protein content for enteral and parenteral formulations determined?
Amino acid analysis; the protein content can be calculated from its amino acid concentrations
The total amount of protein provided from an IV amino acid mixture is about __% less than what would be assumed. Why?
17%
When a peptide bond is made, a molecule of water is released; thus, the weight of a peptide bond is 18 mass units less than its molecular weight
Example: to provide 1.0-1.5 gm of protein per kg of body weight via PN, how many gm of a mixed amino acid solution needs to be administered?
1.2-1.8 gm
Formula for converting nitrogen to protein
Protein = Nitrogen x 6.25
Why is there negative nitrogen balance after minor elective surgery?
Due to a decrease in protein synthesis
Why would there be a negative nitrogen balance with severe sepsis and injury?
Both protein synthesis and catabolism are elevated and feeding further increases turnover
What are the risks of feeding too much protein in critical illness?
Prerenal azotemia (with excess protein increasing the burden to the kidneys to work to excrete excess urea nitrogen). Excess protein feeding over long term can result in kidney stones and increased risk for osteoporosis
Why might metabolic rate and protein requirements for an underweight individual be higher per kg of body weight?
The more metabolically active central protein compartments (liver, heart, lungs, brain) make up a greater proportion of the total body weight; and there is a loss of both fat and muscle
What is the most abundant amino acid in the body?
Glutamine - accounts for more than 50% of the intracellular free amino acid pool in the muscle
Which amino acids serve as the body’s primary nitrogen shuttle between muscle and visceral organs?
Glutamine and alanine
Choline can be manufactured in the liver from ___
Methionine
Examples of conditionally essential amino acids under certain clinical conditions
Tyrosine, cysteine, glutamine, histidine, taurine
Amino acids with pharmacological potential
Arginine, carnitine, choline, creatine, beta-hydroxy methylbutyrate (HMB - a metabolite of leucine)
