PROTEINS IN NUTRITION Structure Amino Acids Basic Structure 20

proteins in nutrition structure amino acids basic structure 20 amino acids in natural proteins; also in proteins of human tis

PROTEINS IN NUTRITION
Structure
Amino acids
Basic structure

20 amino acids in natural proteins; also in proteins of human tissues
Peptides
2 – 10 000 amino acid units; oligopeptides (2 – 100 units);
polypeptides (100 – 10000 units); dipeptide; tripeptide....
Peptide bound:
+ H2O
 synthesis of peptide bond – peptide and protein formation
*
hydrolysis of peptide bond (e.g. during digestion)
Proteins
Structure
Primary – sequence of amino acids in chain
Secondary – mutual position of two or more chains to relevant area
(helixes, composite list ...) – 2nd dimension
Tertiary – mutual position of secondary structures to space – 3rd
dimension
Quarter – tertiary structure with other bonds (hydrogen bonds, S-S
bonds, gravitation powers etc.) or with bounded non-proteins compounds
– metals, phosphate etc.
Natural proteins have usually quarter structure – 2 base types:
*
globular – soluble in water – function proteins of tissues,
proteins dissolved in blood, enzymes etc.
*
fibrilar – insoluble in water – structural proteins – fibre or net
structure
Requirements of protein intake
Minimal intake: 0,6 – 0,8 g / kg of body weight / day
This is amount of proteins, which corresponds to number of deceased
cells, which must be compensated. Proteins from deceased cells are
used as energy source.
Recommended intake:
Adults – about 1,0 g proteins / kg of body weight / day
Children:
quick growth phase (till 2 years) : about 1,5 g / kg / day
other: about 1,2 – 1,5 g / kg / day
Pregnancy and breastfeeding: about 1,5 g / kg / day
Sportsmen in training: max. 1,3 – 1,5 g / kg / day (higher amount do
not use for proteosynthesis, but only as energy source)
Excessive intake: The human body is able to use for proteosynthesis
only limited amount of amino acids (see above). Higher amount is
utilized as energy source after deamination:

Deamination and urea forming are reaction, which are very demanding to
liver function. The urea must be consecutive removed from the organism
by kidneys.
The excessive intake of proteins entails problems for liver and
kidneys.
Denaturation
Denaturation of proteins is very important reaction from nutrition
point of view. During food processing occurs to denaturation by high
temperature and / or micro-wave heating.
Denaturation lead to destruction of higher structures, protein
molecule is unpacks to area structure, which is better attacked by
digestive enzymes.
Denaturation can to lead also to the inactivation of some
anti-nutritional factors, e.g. trypsine inhibitors
Side reactions
can to lead to decreasing of biological value of proteins:
*
Maillard reaction (reaction of carbohydrates with amino group)
*
Decomposition of unstable amino acids (tryptophan, lysine etc.)
*
changing of amino acid configuration (L – to D – amino acids)
*
formation of non-digestible compounds (e.g. lysinoalanin).
Influence of browning reactions (Maillard) to the nutritional value of
foods
*
Destruction of essential amino acids (Lysine, Cysteine,
Methionine, Tryptophan, Tyrosine)
*
Destruction of some vitamins (ascorbic acid, pyridoxin, thiamine)
*
Worsened the digestibility of proteins
*
Deactivation of enzymes
Maillard reaction
*
Meat muscle glycogen & blood glucose – reacts with amino acids in
protein.
*
Bread – crust is formed by Maillard reaction between gluten and
whatever sugars are available (maltose, lactose, sucrose etc.,).
*
Flavour of beer generated by roasting of malted barley, kilning of
malt, wort boiling and boiling fermentable sugars in the presence
of ammonia (bitter caramel).
Digestion of proteins
Stomach: pH 1,5; pepsin; protein chain  more polypeptide chains
Small intestine: pH about 7;
*
proteases from pancreas – trypsin and chymotrypsin: polypeptide
chain  more oligopeptide chains
*
peptidases from pancreas or intestine mucous membrane:
oligopeptide chain  amino acids.
Free amino acids are resorbed and by blood circulation transported to
liver.
Metabolism and using of amino acids
All metabolic processes are localised in liver.
1.
Proteosynthesis: amino acids  globular proteins  blood  tissues
proteins; 1st reaction is peptide bond synthesis (see above)
2.
Formation of other amino acids (which are not sufficient number
for proteosynthesis); essential amino acids cannot formed;
transamination of oxocarboxylic acids with enzymes transaminases
3.
Formation of nitrogen compounds, which are necessary for living; 1st
reaction is decarboxylation with amine compounds formation:
*
porfyrines – in organisms haeme – haemoglobin, myoglobin
*
pyrimidines, purines – nucleic acids
*
creatine (energy reserve of working muscle)
4.
Deamination and energy utilization; organism use above all the
oxidative deamination.
Essential amino acids
Organism cannot synthesised it and must it intake from food.
1.
Amino acids with ramified chain: Val, Leu, Ile
2.
Amino acids with other functional group:
*
hydroxy group: Thr
*
sulphur group: Met
*
-amino group: Lys
3.
Aromatic and heterocyclic amino acids: Phe, Trp.
Semiessential amino acids
Two amino acids with especial structure – histidine and arginine – are
essential for young children, which have insufficient enzyme systems
for synthesis of imino- group (Arg) and imidazole ring (His), resp.
Biological value of proteins
Biological value of proteins is now evaluated according to essential
amino acid content (because non-essential acids organism can to
synthesised) by means of two criteria:
Amino Acid Score - AAS
Limiting amino acid:
*
AA which limited proteosynthesis range in liver;
*
AA which is in relative (for the human requirements) smallest
amount in protein source;
*
when during proteosynthesis all molecules of limiting AA are used,
proteosynthesis is stops, and other rest amino acids are used as
energy source;
*
limiting AA has smallest AAS.
AAS1 = C EA1 / C EA1ref * 100%
AAS2 = C EA2 / C EA2ref * 100%
.........
AAS8 = C EA8 / C EA8ref * 100%
C EA1 = content of amino acid No. 1 (e.g. leucin) in evaluated protein
C EA1ref = content of amino acid No. 1 (e.g. leucin) in reference
protein
Reference protein: Protein which have of essential amino acid
composition optimal for human requirements; the protein of all eggs is
used as reference protein
8 amino acids – 8 values; L EAA has smallest value;
AAS of protein corresponds with AAS of L EAA
AAS  100 (AAS of reference protein = 100)
Essential Amino Acid Index - EAAI
EAAI conveys dependency of proteosynthesis range on the relative total
content of EAA.

Evaluation of criteria
Fully-valuable proteins: AAS  65; EAAI  75
proteins of meat (muscle proteins); egg proteins (collected
fractions); milk proteins (collected fractions)
limitation by any amino acid is insignificant
Deficiency proteins: AAS < 60; EAAI < 65
ligament proteins from meat (e.g. collagen) – limiting AA Trp, Val
all plant proteins:
cereals – limiting AA Lysine
legumes – limiting AA Methionine, Cysteine
Note:
1.
Acceptable mixture of plant proteins give collected protein with
relative high values of criteria – this is important for
vegetarian nutrition
2.
Higher intake of fully-valuable proteins is necessary for organism
in the growth phase – children, teenagers
Plant proteins as substitution of animal proteins
*
Legumes
*
Defatted oilseeds
*
Protein concentrates (about 70 – 90 % of protein)
*
Protein isolates (above 90 % of protein)
*
Soya milk; soy cottage cheese
*
Fermented soy products
Disorders on the metabolism of protein and amino acids
Coeliac disease
*
Intolerance to the gluten - disorders in small intestine – ulcers,
inflammations, disorders of nutrient resorption
*
Gluten sources: cereals – wheat, barley, rye; smaller amount in
rice
Phenylketouria
*
Deficiency of enzyme phenylalanine hydroxylase (necessary for
phenylalanine metabolism)
Accumulation of:
*
Phenylalanine
*
Phenyl pyruvate
*
Phenyl lactate
Consequences
*
Disorders of CNS
*
Disorders of blood composition – destruction of erythrocytes
Similar as in the case of Diabetes mellitus