Chapter 5: Proteins Flashcards
Importance of protein
critical to growth and development
role can be overstated if it is elevated above other nutrients
protein functions optimally only when energy intake from CHO and fat is sufficient
Amino acid (AA)
basic component of proteins
contain carbon, hydrogen, oxygen, and nitrogen
20 AA’s are used by the body to make various proteins - 9 are indispensable (essential/body can’t make them); 11 are dispensable (non-essential/can be made by liver or found in food) with 6 being conditionally indispensable (during periods of stress/starvation body can’t make enough)
Issue of protein deficiency
in western countries where few healthy adults are at risk for amino acid deficiencies
those at risk for low protein intake include those with eating disorders, the frail elderly, those with food insecurity, and people with liver or kidney disease
Protein quality
based on the amounts and types of amino acids and the extent to which the amino acids are absorbed
measure by the digestible indispensable amino acid score
Complete proteins
animal sources
contain all the indispensable amino acids in the proper amounts and proportions to each other to prevent amino acid deficiencies and to support growth
Incomplete proteins
plant sources
lack one or more of the indispensable amino acids or the proper concentrations
doesn’t have all the essential amino acids
Complementary proteins
combining two incomplete proteins
when consumed during the same day they can be nutritionally equal to a complete (animal) protein
Peptide
two or more amino acids that are combined
dipeptide = two amino acids
tripeptide = three amino acids
polypeptide = four or more amino acids
most proteins are polypeptides and are made up of many amino acids
Functions of protein in the body
1) Enzymes - necessary to catalyze (speed up) reactions; unique structure of each enzyme interacts with its substance; specific to molecule that it must interact with
2) Hormones and other regulators - hormones are compounds that act as chemical messengers to regular metabolic reactions; AA’s regulate critical metabolic pathways
3) Structural proteins - proteins of muscle and connective tissue; proteins found in skin, hair, and nails
4) Transport proteins - like delivery trucks of the body
5) Immune system proteins - immune system is protein-based; deficient in protein = decrease immune response
Digestion
begins in the stomach
protein is denatured by gastric juices containing HCl which allows other enzymes to break them down further
HCl activates pepsin which is an enzyme that will breakdown the proteins into smaller unites
generally breaks down very large polypeptides into smaller units that are still quite large
Absorption
takes place in the small intestine
2/3 of amino acids are absorbed in the form of dipeptides or tripeptides, and the other 1/3 are absorbed as individual amino acids
different protein carriers have affinity with certain amino acids
indispensable amino acids are absorbed more rapidly than dispensable amino acids
once absorbed, he body does not distinguish between the amino acids originally obtained from exogenous or endogenous sources
once absorbed, the majority of amino acids will remain in the liver for metabolism with a small amount circulating in the blood and being transported to other parts of the body
amino acids that are not absorbed are excreted in feces
some amino acids will not leave the small intestine cells because they will be used to make cellular proteins with those not used being released into the blood via the portal vein
Transportation and Storage
after absorption, some amino acids are transported to the liver whereas others circulate in the blood
liver monitors the supply of amino acids and dictates which amino acids will be transported to which tissues (with the exception of BCAA’s due to low amounts of BCAA transferase in liver)
50-65% of amino acids found in liver with the remaining being in amino acid pool
10-25% of resting metabolism is expended each day to synthesize and degrade proteins
Amino acid pool
free amino acids that are circulating in the blood or in the fluid found within or between cells
always in flux as a result of food intake, exercise, and the breaking down or building of tissues, especially skeletal muscle
constant flux known as protein turnover
consist of amino acids that are absorbed from food but do not remain in the liver
amino acids in this pool are involved in anabolic and catabolic processes
Livers role in metabolism of protein and AA’s
major site for amino acid metabolism by monitoring the body’s AA needs and responds accordingly with anabolic or catabolic processes
Anabolic
building complex molecules from simple ones
Catabolic
breakdown of complex molecules into simple ones
Deamination
comes from first removal of amino group from the amino acids
part of catabolism
Transamination
comes second
transfer of the amino group to another carbon skeleton
part of both catabolism and anabolism
Amino acid pool
anabolic process = synthesis of skeletal muscle protein that uses amino acids from the pool to synthesize the new proteins
catabolic process = when skeletal muscle proteins are degraded and amino acids are returned to the amino acid pool
Anabolic functions (occurs in the liver)
liver enzymes (amino acids are incorporated), plasma proteins (where the remaining amino acids go), synthesis of muscle protein, albumin (protein that circulates in the blood and is made in the liver), manufacturing of compounds (ie. creatine)
many of the proteins made in the liver are synthesized and released in response to infection or injury
Catabolic functions
amino acids are not stored for future use like CHO and fats, but the so-called storage site protein is skeletal muscle
in extreme circumstances, protein can be removed from skeletal muscle but the removal of a large amount of AA’s has a negative effect on muscle’s ability to function so the body tries to protect skeletal muscle from being used in this way
Amount of energy in protein
protein yields about 4 kcal/g
CHO and fats are used first, then proteins last if needed
known as protein sparing effect
Protein metabolism during exercise
proteolysis (muscle breakdown) occurs when the body is stressed (ie. endurance exercise) and is stimulated by cortisol
in endurance exercise as muscle glycogen stores decline substantially, the skeletal muscle uses some amino acids for energy
metabolic response is influenced by the CHO content of athlete’s diet (ie. low CHO diet and CHO depleted = oxidation of amino acids is increased and vice versa)
body prefers to use CHO when exercising, amino acids are back up energy source
3-5% of energy comes from amino acids, but during prolonged exercise that amount is higher
amino acids are also used for gluconeogenesis (convert to glucose for energy) - 18 of 20 amino acids can be converted to glucose
Ammonia
by-product of the breakdown of amino acids
must be filtered out in the urine
nitrogen is also lost in urine every day
Resistance training
results in muscle protein breakdown at time of exercise, but eating protein during rest and recovery is extremely important as it results in a positive protein balance