This presentation contains a brief introduction of Proteins, their structure, classification, their nutritional importance, their utilisation inside the body, protein RDA, nitrogen balance and classification of amino acid. Presentation is designed with simple words added with different image and tabular illustrations to make learning easy. This is helpful for the Medicine students, allied health science, Nutrition and dietetics students and also for a general nutrition science.

1. - PROTEINS -
Presented by: Syed Wajid Ali
Department of Nutrition & Dietetics
ERA UNIVERSITY LUCKNOW

2. TABLE OF CONTENTS
1. NTRODUCTION TO PROTEINS
2. STRUCTURE OF PROTEINS
3. CLASSSIFICATION OF PROTEINS
4. CLASSIFICATION OF AMINO ACIDS
5. NUTRITIONAL IMPORTANCE OF PROTEINS
6. PROTEIN UTILISATION
7. NITROGEN BALANCE
9. REQUIREMENTS OF PROTEIN

3. INTRODUCTION
◦ The name protein is derived from the Greek word proteios “of prime importance or
the first kind” this is because proteins are the basis of the cytoplasm of cells and are
present in all living organisms.
◦ Proteins are the most complex materials produced in nature.
◦ Proteins are the most abundant macromolecules in living cells and constitute 50% or
more of their dry weight.
◦ They are consisting of α-amino acids linked together by peptide linkages

4. STRUCTURE OF PROTEIN
◦ Proteins are complex organic nitrogenous compounds and composed of varying amounts of
carbon, hydrogen , oxygen and sulphur.
◦ Nitrogen is a characteristic component of proteins forming about 16% of their weight i.e. 100 g
of protein contains 16 g of nitrogen.
◦ They are polymers of α-amino acids and synthesized from only 20 amino acids, known as the
common amino acids. Polypeptides are made by polymerization of amino acids through peptide
bonds.
◦ Big polypeptide chains containing more than 50 amino acids are called proteins.

5. CONT…
◦ PRIMARY STRUCTURE: determined by the
sequence of amino acids
◦ SECONDARY STRUCTURE: occurs when the
amino acids are linked by hydrogen bonds.
◦ TERTIARY STRUCTURE: formed when alpha
helices and beta pleated sheets are held
together by week interactions.
◦ QUARTERNARY STRUCTURE: consists of
more than one polypeptide chains.

6. CLASSIFICATION OF PROTEINS
◦ The knowledge about the grouping of proteins helps in better understanding of its importance
in making dietary choices. Proteins can be classified based on the following factors.

8. CLASSIFICATION BASED ON THE
SOURCES
◦ Based on the origin, the proteins are conventionally divided into two groups- animal
proteins and plant proteins.
◦ The animal foods like eggs, milk, meat (white and red) and fish contribute to animal
protein sources. These are proteins with higher biological value as they are completely
utilised and of higher quality as they supply all the essential amino acids in adequate
amounts Whereas, plant proteins obtained from plant foods like cereals, millets,
legumes, nuts, seeds green leafy vegetables are of lower-quality and lower biological
value as they lack or limit in one or more of the essential amino acids.
◦ Methionine, lysine, threonine and tryptophan are the most common limiting amino
acids.

9. Classification based on nutritive value
On nutritional standpoints, proteins are classified in to three types as follows:
◦ Complete Proteins: The proteins that contain all the nine essential amino acids in adequate
proportion to promote growth are termed as complete proteins. Egg albumin, milk casein and
meat proteins are complete proteins.
◦ Partially Incomplete proteins: The proteins which partially lack one or more essential amino
acids and promote moderate growth are known as Partially Incomplete proteins. Wheat and
Rice proteins are limiting in amino acids Lysine and Threonine and hence are classified as
partially incomplete proteins.
◦ Incomplete Proteins: The Proteins that completely lack one or more essential amino acids and
do not promote growth are incomplete proteins. Gelatin lacks trypsin and maize protein zein
lacks trypsin and lysine are incomplete proteins.

10. CLASSIFICATION OF AMINO ACIDS
◦ There are about 200 amino acids found in nature. There are about 20
amino acids in a protein hydrolysate. The amino acids are classified
based on the chemical structure (chemical classification), based on
nutritional value (Biological classification) and based on their metabolic
fate in the body (Metabolic classification).

11. BIOLOGICAL CLASSIFICATION
◦ Based on nutritional values amino acids are classified into essential or non-essential
amino acids.
1. Non essential amino acids: Amino acids that could be synthesized in the body, so
they are not needed in the diet. They are 10 amino acids.
2. Essential amino acids: Amino acids that could not be synthesized in the body, so they
have to be taken in the diet, and their deficiency results in diseases.

12. LIST OF ESSENTIAL AND NON
ESSENTIAL AMINO ACIDS
NON ESSENTIAL AMINO ACID ESSENTIAL AMINO ACID
GLYCINE VALINE
ALANINE LEUCINE
SERINE ISOLEUCINE
TYROSINE THREONINE
CYSTEINE METHIONINE
ASPARAGINE TRYPTOPHAN
ASPARTIC LYSINE
GLUTAMIC ACID HISTIDINE
GLUTAMINE PHENYLALANINE
PROLINE
ARGININE

14. NUTRITIONAL IMPORTANCE OF PROTEINS
◦ Protein plays a significant role in overall well-being of an individual.
◦ The importance of protein in all spheres of life from conception till death cannot be underrated.
Source of energy: When the diet is inadequate in carbohydrates and fats, proteins are the
anaerobic fuel for energy. Each gram of protein provides around 4kcal.
Growth and tissue maintenance: The proteins play a pivotal role of in the cell division process. The
tissues are repaired and maintained in a good state only by the action of proteins. Further, new
tissue formation and building of muscles will happen only if the good quality proteins are in
constant supply to the body

15. ◦ Structural maintenance: The Collagen, main protein of very high tensile strength which is found
in connective tissue such as tendons, cartilage, structural matrix of bones and teeth and cornea
of the eye is the intercellular substance that binds the cells together. Collagen helps in
maintaining the organs in its inherent form and position. The cell wall and primary fibrous
constituents of the cell have structural proteins. Elastin, another important structural protein
seen in ligaments has the elastic property-quality of stretching in either dimensions.

16. ESSENTIAL BODY COMPOUND FORMATION
◦ Enzymes: Essentially, it is the enzymes that catalyze all the biochemical reactions within the
body. o The enzymes are proteins mostly globular in structure and highly specific in its
functions. Enzymes like urease, catalase, dehydrogenases and the digestive enzymes are some of
the enzymic proteins.
◦ Hormones: Hormones are proteins which regulate many metabolic functions and essential for
reproduction as well as growth. As known pancreatic hormones like insulin and glucagon
regulate carbohydrate metabolism. Growth hormone regulates bone growth. Adrenal hormone
epinephrine is a derivative of amino acid tyrosine.

17. ◦ Neurotransmitters: The photoreceptors of the eye which transmit the nerve signals responsible
for vision are proteins. The amino acid tryptophan serves as a precursor for formation of an
important neurotransmitter Serotonin.
◦ Keratin: Healthy maintenance of skin, hair and nails need protein keratin.
◦ Nutrient transport: The transport of essential nutrients from the intestine across the intestinal
wall to the blood and the tissues is carried out by the proteins. These transport or carrier
proteins are specific for each nutrient. The protein in Hemoglobin binds and carries oxygen to
the peripheral tissues as blood passes through the lungs, where it is released to participate in
the process of oxidation. Lipids from liver is carried to other organs by lipoproteins in blood
plasma. Copper in blood is transported by the metalloprotein Ceruloplasmin. Retinol is bound to
and carried by retinol binding protein.

18. ◦ Detoxification: The toxins from food are detoxified in the liver by the enzymes which are
proteins.
◦ pH maintenance: Proteins serve as buffers by combining with both hydrogen and hydroxide
ions in the blood and thus maintaining an optimum ph. Both acidosis and alkalosis prove fatal

19. FACTORS AFFECTING PROTEIN
UTILIZATION
◦ In addition to the quantity of protein consumed other factors that interfere with the utilization
of protein and make their consumption of limited value are listed below:
◦ Sex: The RDA for men is about 20% higher than that of women. Iron is an exception as the
requirement is greater in menstruating women. Additional requirements (20-30% above normal)
are needed for pregnant & lactating women.
◦ Age: In general, the nutrient requirement is much higher in the growing age. For instance, the
protein requirement for a growing child is about 2 g/kg body wt/day compared to 1 g/kg body
wt/day for adults.
◦ Amino acid ratio: The sequence or pattern of amino acids in the particular food consumed will
have an impact on the growth promotion based on the individual’s age.

20. ◦ Physical inactivity: The body’s ability to utilize protein decreases with immobility as seen in
elderly who are disabled and as in case of astronauts who lose protein due to immobility and
weightlessness in space.
◦ Stress: Physical stress such as injury and emotional stresses like fear, anger, and anxiety
increases nitrogen loss.
◦ Anti-nutritional factors: The protein digestibility and utility is adversely reduced by the anti-
nutritional or toxic factors such as trypsin and chymotrypsin inhibitors, phytates, cyanogenic
compounds, lathyrogens, saponins, anti-vitamins and allergens present in pulses. Cooking and
Processing of pulses before cooking improves utilization

21. NITROGEN BALANCE
◦ Nitrogen balance is a measure of nitrogen input minus nitrogen output.
◦ Nitrogen Balance = Nitrogen intake - Nitrogen loss
◦ Sources of nitrogen intake include meat, dairy, eggs, nuts and legumes, and grains and cereals.
◦ Examples of nitrogen losses include urine, feces, sweat, hair, and skin.

22. ◦ Positive nitrogen balance: This is a state in which the nitrogen intake is higher than the output.
◦ Some amount of nitrogen is retained in the body causing a net increase in body protein. Positive
nitrogen balance is observed in growing children, pregnant women or during recovery after
serious illness

23. Negative nitrogen balance
◦ In this, the nitrogen output is higher than the input.
◦ Some amount of nitrogen is lost from the body depleting the body protein.
◦ Prolonged negative nitrogen balance may even lead to death.
◦ Observed in children suffering from kwashiorkor or marasmus.
◦ Negative nitrogen balance may occur due to inadequate dietary intake of protein (deficiency of
a single essential amino acid) or destruction of tissues or serious illness.
◦ Growth hormone & insulin promote positive nitrogen balance while corticosteroids result in
negative nitrogen balance.
◦ Cancer & uncontrolled diabetes cause negative nitrogen balance.

24. ASSESSMENT OF NUTRITIONAL VALUE
OF PROTEINS
◦Protein efficiency ratio (PER)
◦Biological value (BV)
◦Net protein utilization (NPU)
◦Chemical score

27. REQUIREMENTS OF PROTEINS
◦ The requirement of protein is dependent on its nutritive value, caloric intake & physiological
states (growth, pregnancy lactation) of individual.
◦ For an adult, 0.8g protein/kg body weight/day is adequate.
◦ The requirement is nearly double for growing children, pregnant & lactating women.
◦ The protein content of foods is variable,
◦ Cereals have 6-12%; pulses 18-22%; meat 18-25%, egg 10-14%; milk 3-4% and leafy vegetables
1-2%.
◦ In general, the animal proteins are superior than vegetable proteins as the dietary source