Proteins

 The basis of protein structure is the amino acid, of which 20 have been recognized as constituents of most proteins 

All Amino acids have amino group(NH2) and Carboxylic Group(COO2)

But, they are differentiated by the remainder of the molecule (R) as shown in the figure.

  

 

 Those amino acids that cannot be synthesized in the body and need to be taken from food are essential (indispensable) amino acids. 

Absence Essential amino acids from the diet leads to poor growth performance by a growing animal. 

Essential amino acids are labelled by (**) sign in the following table .

WHY DO WE NEED PROTEIN TO SURVIVE ?

We need protein for:

• Growth (especially important for children, teens, and pregnant women) 

• 10% - 35% of calories should come from protein.

• Tissue repair 

• Immune function 

• Energy when carbohydrate is not available 

• Preserving lean muscle mass 

• Synthesis of enzymes, hormones all antibodies

• Control Fluid movement in the body

• Buffer(PH control): Due to the carboxyl or acid group (-COO) and amino or basic group (- NH2)

Classification of proteins

Based on chemical composition.

 

1. Simple protein - yield amino-acids upon complete hydrolysis

           E.g.: - albumin - in eggs, zein of corn

2. Compound/conjugated proteins

           Protein + Non protein

           E.g.: - Hgb         (Protein + hem)  - Blood

II. Based on Nutritional Value:- This classification depends on the essential amino acids content of the protein.

1. Complete proteins: Contain all the essential amino acids in the proportion that is required to support growth and maintain tissues.

E.g. Almost all animal proteins except gelatine (lack two essential A.As.). 

They are denoted as complete because they resemble body protein (Egg & Milk).

    b. Incomplete Proteins: This refers to proteins that do not contain all essential amino acids in the proportion that is required to maintain growth and tissue repair

III.  Based on Conformation of the Protein: This refers to the three dimensional shape of the protein in its natural state. Based on this proteins are classified as:

1. Globular proteins

-Tightly folded poly peptide chain - spherical or globular shape

-Mostly soluble in water & salt solution

      E.g.:  - Enzymes, antibodies, and many hormones, Hgb

2. Fibrous proteins

-Polypeptide chains arranged in parallel manner along an axis

-Tough & insoluble in water 

E.g.:  - Collagen of tendons & bone matrix

-    Keratin of hair, skin, nails and 

-    Elastin of blood  vessels

IV.     Based on their Chemical Structure

1. Primary structure :    refers to the sequence of amino acids in the polypeptide chain of proteins held by peptide bond. 

Eg. Ala---gyc---Phenala---histd---tyr---trp

 

2. Secondary Structure: This refers to the folding of the polypeptide chain upon itself resulting in alpha helix (right twisted or left twisted) and or B-pleated sheet. This structure is held strong by intra molecular hydrogen bonding.

1.  Alpha helices                            OR

 

 

2. B-Pleated Sheath

 

 

c. Tertiary Structure: - This refers to the three dimensional arrangement of the protein structure (whether it is folded upon itself giving rise to globular proteins or whether its straight chain of poly peptides resulting in fibrous protein). This structure is maintained by the sulfide bond.

Biological Value of protein: If a protein contains all the indispensable amino acids it is said to have a high biological value.

 If a protein does not contain all the indispensable amino acids it is said to have a low biological value. 

High biological value (HBV) proteins include: meat, fish, eggs, cheese, and milk. 

Low biological value (LBV) proteins include: cereals, pulses, some nuts and vegetables. 

In general, animal protein sources have a higher biological value, than vegetable sources.

Nitrogen Balance

Definition: Nitrogen balance refers to the situation where nitrogen intake from food is equal to nitrogen excretion. 

This occurs in a healthy non-growing adult person taking adequate amount of energy from carbohydrates. 

In some situations Nitrogen excretion may be greater than nitrogen intake, this is called negative nitrogen balance. 

Other situations where nitrogen excretion is less than nitrogen intake from food are called positive nitrogen balance

Factors affecting nitrogen equilibrium

Positive nitrogen balance 

• Pregnancy 

• Lactation 

• Growth 

• Recovery from illness (Convalescent Stage)

Nitrogen intake = Nitrogen excretion (nitrogen balance)

Negative nitrogen balance 

• Starvation

• Devastating illness 

• Protein Energy Malnutrition 

Food sources of proteins

1. Animal source foods:-

• Beef, Lamb, pork 

• Fish, chicken  

• Poultry meat substitutes, cheese, milk, 

2. Plant source foods

• nuts, legumes, Soya bean cereals and legumes. 

Protein Requirements

• RDA average = .8 g/kg/day

• RDA athlete   = 1.2-1.6 g/kg/day

• High levels of protein intake above 2 g/kg/day can be harmful to the body 

 

Protein utilization

• Digestion

• Absorption

• Metabolism of protein