Protein Flashcards
What is Protein?
- Linear chains of amino acids – polypeptides
- Protein = proteos meaning “primary” or “taking first place”
- Not a single entity but a complex mix of many different proteins –
each with its own amino acid composition & sequence - A single protein can have from 50 to 1000 amino acids
- The particular sequence of amino acids confers varying roles &
functions for different amino acids
Essential (indispensible) aa
not synthesized by mammals and are therefore dietarily essential or indispensable nutrients
1. Isoleucine
2. Leucine
3. Valine
4. Lysine
5. Methionine
6. Phenylalanine
7. Threonine
8. Tryptophan
9. Histidine
Amino Acids can be grouped on the basis of:
- Side chain structure
E.g. Aromatic, acid, basic, sulfur containing etc. - Net electrical charge
Neutral, positively charged, negatively charged - Polarity
Polar (interact with water) and non-polar - Essentiality
conditionally essential (indispensible) aa
usually not essential, except in times of illness and stress
1. Arginine
2. Cysteine
3. Glutamine
4. Proline
5. Tyrosine
Non-essential (dispensible) aa
- Alanine
- Aspartic acid*
- Asparagine
- Glutamic
Acid† - Glycine
- Serine
Protein Digestion occurs where?
stomach
* HCL denatures proteins
* HCL converts pepsinogen to pepsin
* Pepsin acts on protein→ large polypeptides
small intestine
Pancreas
- Secretes zymogens (proenzymes) which are activated in the SI to
* Trypsin
* Chymotrypsin
* Carboxypeptidases A & B
Enterocytes
- Secrete
* Aminopeptidases
* Dipeptidyl amino peptidases
* Tripeptidases
Attack specific bonds → smaller polypeptides →
amino acids, dipeptides, tripeptides
no digestion in oral cavity
Protein Absorption
- where?
- uses?
- dependent on?
- absorbed through?
- also needs a ______ system
- Occurs in the Small Intestine, especially duodenum
and upper jejunum - Uses energy
- Specific transport systems, mainly Na dependent
- Absorbed through brush border into enterocyte
- Then absorbed through basolateral membrane of
enterocyte by transport systems and taken into
circulation for distribution around the body - Absorption in the body systems also needs a carrier
system
Protein Absorption
- capacity
- peptides have their own _______
- which aa are absorbed more easily?
- supplements with high amounts of one aa may?
- Capacity for absorption is much greater than dietary intake as the body
secretes protein into the bowel ~ 70 – 300g/day - Peptides have their own transporters and are absorbed more quickly than
amino acids - Essential amino acids are absorbed more quickly than non-essential amino
acids - Supplements with high amounts of one amino acid may impair absorption
of others with the same carrier
Protein Synthesis
- increases in tissues following _______?
- fast proteins
- slow proteins
- All AAs needed to synthesise a particular ___?
- Synthesis of a particular protein in the body involves transcription of a ____ into _____ followed by its ________-
- Increases in tissues following food intake
- Amino acid use in the body influenced by the type of dietary proteins
- Fast proteins – whey, soy, amino acid mixtures, protein hydrolysates
- Slow proteins - casein
- Slow – lower and prolonged blood amino acid [ ] and better retention than fast, used more for skeletal muscle
- All AAs needed to synthesise a particular P must be available at point of synthesis
- Synthesis of a particular protein in the body involves transcription of a gene into mRNA followed by its translation
Hormones with a role in protein synthesis
- Protein degradation predominates over synthesis
- Epinephrine
- Cortisol
- Higher glucagon:insulin
e.g. Overnight, fasting, infection, injury, trauma - Protein synthesis predominates over degradation
- Higher insulin:glucagon
- Growth hormone
e.g. after eating, greatest if CHO & protein eaten together
Protein (P) Content in the Body
- Protein content per 60-70 kg adult
- aa pool
- only a small ______ protein store or buffer (_% total body protein) with no energy storage form of protein
Protein content per 60-70 kg adult:
* 10-11kg P i.e. ~16% body weight
* muscle ~43%
* Organs ~25%
* skin ~15%
* blood ~16%
* amino acid (AA) pool
* small pool of free AAs in all tissues
* supplies AAs for P formation
* receives AAs from P degradation
* only a small ‘labile’ protein store or buffer (1% total body
protein) with no energy storage form of protein
Structural role of protein
- Contractile proteins – actin + myosin
- Cardiac, skeletal and smooth muscle
- Fibrous proteins
- Collagen, elastin and keratin
- Bone, teeth, skin, hair, tendons, cartilage, blood vessels, hair and
nails
buffering role of protein
- Proteins contribute to acid base balance by accepting/releasing H+ ions
- Works with the phosphate system (in cells) and bicarbonate system (in
blood) to maintain pH
fluid balance role of protein
- Attract and keep water in a particular location by contributing to osmotic pressure
- Imbalance contributes to oedema or ascites
catalyst role of protein
- Enzymes
- Bind with substrate to generate a product
- Often require a cofactor or coenzyme
- Multiple physiological processes depend
on enzymes
messenger role of protein
- Hormones – chemical messengers in the body
- Synthesized and secreted by endocrine organs
- Transported in blood to other locations where
they bind with protein receptors
immunoprotection role of protein
- Immunoproteins – immunoglobulin (Ig) or antibodies (Ab)
- Produced by plasma cells from B-lymphocytes
- Bind and inactivate antigens
- Immunoprotein-antigen complex destroyed by
complement proteins or cytokines
transport role of protein
- Transport various substances in
the blood, into, out of or within
cells - Cell Membranes
- Uniporters, symporters and
antiporters - Blood
- 100s
- Lipoproteins
- Albumin
- Transthyretin/ Prealbumin
- Retinol binding protein
- Globulins
acute phase responder role of protein
- Synthesised in the liver in
response to a sudden critical
illness – infection, injury or
inflammation - E.g. C-reactive protein
This Photo by Unknown Author is licensed under CC BY-SA
N-Containing Nonprotein Compounds
- Glutathione
- Antioxidant
- Transports amino acids in some tissues
- Participates in synthesis of leukotrienes
- Carnitine
- Transports fatty acids across the inner
mitochondrial membrane for oxidation - Role in ketone catabolism for energy
- Creatine
- Component of creatine phosphate
(phosphocreatine) - Source of energy for the muscle
- Purine and Pyrimidine Bases
- Components of DNA and RNA
Protein turnover
All body proteins are in ‘dynamic state’ i.e. constant synthesis & degradation
* absolute rates vary in different tissues (minutes, months)
enzymes, skin or mucosa, structural proteins (bone, muscle)
* relative rates are determined by N balance experiments
in adults: synthesis = degradation → N balance
* synthesis: ~4 g P/ kg body weight/ day
in children: synth > degradation → +ve N balance
* ~12 g P/ kg bw /d in newborn
* ~6 g P/kg bw /d by 1 year
Nitrogen balance is a measure of the _____ ________ status of a person - _____ vs _______
- it refers to overall relationship between _ intake (diet) & _____
- N is mainly derived from _____ __ (NH2 group) but also other compounds e.g. _____ ____, _______ from meat, ________
- N balance reflects _______ not absolute rates of protein synthesis
versus degradation
- protein nutritional and anabolic
vs catabolic - N and excretion
- protein N and nucleic acids, creatine from meat, vitamins
- relative
Factors affecting N balance:
Positive balance
Physiological state
* Growth
* Pregnancy
* Muscle building
* Intense physical activity
* Convalescence
Nutritional state
* incr. in E intake with weight gain
* switch from low to high P intake
Factors affecting N balance:
Negative balance
Physiological state
* injury, surgery
* burns
* bleeding
* proteinuria
* diarrhoea
Nutritional state:
* not enough E
* not enough P
* switch from high to low P diet
To synthesize an AA requires ability to make its _________ _______ from endogenous sources
* E.g. Pyruvate, α-ketoglutarate & oxaloacetate (all derived from the citric acid cycle)
carbon skeleton
- Transamination is the transfer of amino group from one AA to an _____ ______ ____
- Forms _________ AAs as required eg to match dietary AA with body needs for protein synthesis
- Requires vitamin __ as coenzyme
- AAs able to be formed in this way are _______, _______ acid and _______ acid
- Most transamination reactions are catalysed by ___________
- Reversible
alpha keto acid
different
B6
alanine, glutamic acid and aspartic acid
aminotransferases
Protein Degradation
i. Deamination to ammonia:
* removal of amino gp (mainly from glutamate)
* to form alpha-keto acid ‘C skeleton’ with release of NH4+
ii. Oxidation to energy
* C skeleton can enter TCA cycle
So, the C skeleton of AAs can be:
→ glucose (overnight fast)
→ ketones (long term fasting, or diabetes)
→ fat biosynthesis? (xs P intake)
Protein Detoxification
NH3 is the toxic end product of P metabolism
* transported to liver for urea synthesis via incorporation
into glutamine (most tissues) or alanine (muscle)
* safely excreted via kidneys (2/3rds) or GIT (1/3rd):
N excretion as urea varies according to P requirement:
* urea can be reabsorbed from kidney tubules, or
* degraded by bacteria in colon to reform -NH2 which
can be reabsorbed back to liver i.e. urea N is salvaged
Factors that increase protein requirements
Increased breakdown of muscle (stress response)
* injury, surgery, burns
* cancer cachexia
Increased losses of protein
* bleeding, diarrhoea
Increased retention of protein
* growth, pregnancy
* muscle building plus exercise
How do we maintain N balance
with low P intake?
- ↓ N retained in labile P pool
- ↓ P turnover in tissues i.e. synthesis + degradation.
- ↓ P oxidation:
* C skeleton → ATP, glucose
* NH2 group → urea synthesis
urea excretion - ↑ salvage of urea N
How do we maintain N balance
with high P intake
- ↑ N retained in labile P pool
- ↑ P turnover in tissues i.e. synthesis + degradation
- ↑ P oxidation:
* C skeleton → ATP, glucose
* removal of NH2 gp
→ urea synthesis & excretion - Conversion to fat?
- ↓ Salvage of urea N
Is a high protein diet harmful?
- increase urea excretion:
* obligatory H2O loss may → dehydration
* risk groups
* infants
* elderly
* athletes - increase Ca excretion
* ? may contribute to osteoporosis - ‘Rabbit’ starvation’
* inadequate fat & carbohydrate. in early pioneers
Consequences of Inadequate Protein 1
Marasmus – a severe
deficiency of energy
* Frail, emaciated appearance,
impaired growth, weakness,
apathy, thin, dry hair, low body
temperature & BP, prone to
dehydration & infections
Consequences of Inadequate Protein 2
- Kwashiorkor
- Historically considered a severe deficiency
of protein. This has been challenged. - Oedema in the legs, feet, and stomach,
diminished muscle tone and strength,
rashes, lesions, brittle hair, prone to
infection, sadness, apathy, excess fluid in
the lungs, septicaemia, pneumonia, water &
electrolyte imbalance, death
Consequences of Inadequate Protein 3
Protein-energy malnutrition (PEM) – a lack of sufficient dietary protein &/or
energy
* In Australia most malnutrition is disease related
Multiple health problems including
GI tract breakdown
Impaired immune function
Muscle loss & reduced strength
Fluid imbalance
Reduced mental function and QOL
Assessment of Protein Status -
Anthropometry
- Mid-upper arm muscle area
- Estimate of changes in skeletal muscle
- In vivo neutron activation analysis (research)
- Allows direct estimation of the amount of N in the body
- Bioelectrical impedance analysis, Computerized tomography,
magnetic resonance imaging or dual energy X-ray
absorptiometry - Estimate skeletal muscle mass
Assessment of Protein Status –
Biochemistry – Serum Proteins
Total serum protein
* Insensitive as a nutrition marker. Only depleted when clinical signs of malnutrition apparent. Influenced by
many factors besides nutrition
Serum albumin
* Half-life of 14-20d so not sensitive to short-term changes. Influenced by many other factors e.g.
inflammation, disease Not a useful indicator of protein status or malnutrition in most circumstances
Serum transferrin
* Transports iron. Half-life 8-19d so responds more rapidly to change. Influenced by many factors e.g.
Inflammation, disease, Fe status. Not a useful indicator of protein status or malnutrition in most
circumstances
Serum retinol-binding protein (RBP)
* Carrier for retinol. Half life 12 hours so changes rapidly in response to dietary intake. However influenced
by multiple other factors e.g. Liver disease, catabolic states, vitamin A status. Not a useful indicator of
protein status or malnutrition in most circumstances
Serum transthyretin (also called prealbumin or thyroxin binding prealbumin)
* Transports thyroxine, carrier for RBP. Half life 2d. More sensitive indicator of protein energy malnutrition
than albumin. Responds rapidly to short-term effects of nutrition therapy. Influenced by many diseases,
although not as quickly as other serum proteins. Not used in most malnutrition screening/assessment tools.
Useful as part of assessment in some circumstances alongside other data e.g. Clinical and dietary.
Assessment of Protein Status –
Biochemistry (selected other)
- Urinary creatinine excretion
- Derived from breakdown of creatine phosphate in muscle.
If creatine in muscle is constant urinary creatine indicates
muscle mass - Requires at least 3d plus low creatine diet. Not practical
in the clinical setting - Nitrogen Balance
Assessment of Protein Status – Functional
Tests – Selected
- Handgrip strength
- Uses a handgrip dynanometer
- Subjects perform a maximal contraction for a few seconds
- Results compared to interpretative criteria
- Have been found to be signficant predictors of
malnutrition/malnutrition related complications - Immunological tests
- Changes have been observed in protein-energy malnutrition
- Influenced by many other factors besides nutrition
- Markers include lymphocyte count, thymus-dependent
lymphocytes, delayed cutaneous hyper-sensitivity, cytokines
Assessment of protein status – malnutrition
assessment tools
- Tools available for screening and assessment
- Multiple tools available worldwide.
- Malnutrition Screening Tool, Subjective Global Assessment (SGA)
and Patient Generated SGA (PG-SGA) are commonly used tools
in Australia - Assess multiple types of data including weight change, dietary
intake, nutrition impact symptoms, functional status, disease and
physical assessment of muscle and fat mass and wasting
Assessment of Protein Status – Other
methods
- Dietary intake – usual limitations apply
- Clinical – used in the interpretation of other data
high protein foods (10-30g)
gelatine (84g)
meat, fish, cheese, egg
wheat germ, yeast
soybean, nuts
moderate protein foods (3-10g)
breakfast cereals, milk
bread, lentils & beans
rice, pasta
low protein foods (less than 3g)
beverages, fats & oils
potatoes & other veg’s
fruit, juices
