Proteins are polymers made of amino acids linked by peptide bonds. There are 20 standard amino acids, some of which are essential to obtain from diet as the body cannot synthesize them. Amino acids are classified based on essentiality and presence of functional groups. Proteins are digested into amino acids in the stomach by pepsin and further in the small intestine by pancreatic enzymes like trypsin. Amino acids and small peptides are absorbed into the bloodstream and transported to tissues where they are used for growth, repair, energy and synthesis of other proteins and molecules. Excess amino acids are converted to urea which is excreted in urine.
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protein.pdf
1.
2. INTRODUCTION
❑ It’s a heteropolymer of alpha amino acid that are attached through peptide bond.
❑ Each amino acid has a central C called alpha carbon to which 4 different groups are attached
these are –COOH, -NH2, -H and –R groups.
R
NH3 C COOH
H
❑ The are group is variable and responsible for the type of amino acid.
R= H , glycine (Simplest A.A)
R= CH3 , Alanine
R= CH2OH, Serine
❑ Amino acid both have acidic and basic group therefore amphoteric in nature.
❑ Consuming protein in the diet provides A.A used to make cellular substances.
❑ Dipeptides, Tripeptides and A.A are digested products of protein.
3. ESSENTIAL A.A NON ESSENTIAL A.A SEMI ESSENTIAL A.A
• Leucine
• Isoleucin
• Lysine
• Valine
• Threonine
• Methionine
• Phenylalanine
• Tryptophan
• Glycine
• Alanine
• Aspartic acid
• Asparagine
• Glutamic acid
• Glutamine
• Cystine
• Serine
• Proline
• Tyrosine
• Argentine
• Histidine
Both are semi essential in children
Threonine is semi essential in
adults
These A.A can be synthesized in
body but in adequate amount.
CLASSIFICATION OF AMINO ACID
On the basis of essentiality
4. On the basis of functional group
i. Neutral Amino Acid
like glycine
alanine
Leucine
Isoleucine
Valine
ii. Acidic Amino Acid
It has additional –COOH group in the Carbon chain
Aspartic acid
Asparagine
Glutamine
Glutamic acid
iii. Basic Amino acid
It has additional –NH2 group in the carbon chain.
Like Arginine
Lysine
5. iv. Aromatic Amino Acid
It has ring structure
Like Phenylalanine
Tryosine
Tryptophan
v. Alcoholic Amino acid
It has –OH group in the chain
Like Serine
Threonine
vi. Sulphur containing amino acid
It has S atom in a ring structure.
Like cysteine
Methionine
vii. Heterocyclic Amino acid
It has a N atom in a ring Structure.
Like histidine
Proline
6. TYPES OF PROTEIN
i. Simple protein
These protein on hydrolysis only yields Amino acid.
ii. Complex Protein
These protein on hydrolysis yields Amino acid and other compound.
a) Haemoglobin- haem pigment
b) Lipoprotein – lipid
c) Glycoprotein – Carbohydrate
d) Nucleoprotein – Nucleic acid
Globular protein Fibrous Protein
• Albumin
• Globulin
• Glutelin(wheat)
• Zeatin(corn)
• Keratin(scleroprotein)
• Collagen(most
abundant protein)
• Elastin
7. DIGESTION OF PROTEIN
1. IN STOMACH
▪ Only 10-20% of the protein digestion takes place in stomach.
▪ Chief cell of the stomach secrete pepsin in its zymogen form as pepsinogen.
▪ Gastric HCL triggers the conversion of pepsinogen into pepsin.
▪ Protein are hydrolyzed by pepsin into peptides and proteoses.
▪ Pepsin denatures when the pH exceeds 5, HCO3 thereby increases pH in the duodenum and
inactivates pepsin in intestine.
▪ most important features of pepsin digestion is its ability to digest the protein Collagen(major
constituent of meats)
▪ Person who lacks pepsin, the ingested meats are less well penetrated by the other digestive
enzymes.
2. IN SMALL INTESTINE
Pancreatic proteolytic enzymes- trypsin, chymotrypsin, pro carboxypeptidase and elastase cause
digestion of protein.
8. i. Endopeptidase:- Hydrolyzes the interior peptide bonds of proteins
Trypsinogen Trypsin
Chymotrypsinogen Chymotrypsin
Protein Proteoses, polypeptides, some A.A
ii. Exopeptidase
Hydrolyzes one A.A at a time from the C-terminus of proteins.
Procarboxypeptidase carboxypeptidase
Polypeptides A.A
Small % of the protein constituent A.A by the pancreatic juices. Most remain as dipeptides and
tripeptides.
Peptidase present in the brush border of enterocytes splits larger polypeptides into tripeptides and
A.A.
9. ABSORPTION OF PROTEINS
• Proteins are absorbed in the intestine as dipeptide, tripeptide and A.A
• A.A and peptides are transported by 7 different types of transport system.
• 5 systems require Na ions and 2 system requires Cl ion.
• A.A is Co transport with Na and in Two system, transport is independent of Na
• Dipeptide and tripeptide are transported into enterocytes by peptide transporter 1. This system
require H ion instead of Na ion.
• Absorption of A.A occurs mainly at duodenum and jejunum.
• Cytoplasmic peptidases hydrolyze the dipeptide and tripeptides into A.A after being
transported into intestinal cells.
• Facilitated diffusion transports the A.A from the cells to the blood.
10. Maximum amino acid
absorbed
Some amino acid reaches large
intestine and by bacterial action
changes into ammonia and
excreted out
Via systemic circulation
Liver Kidney Tissue
Amino
Acid
Ammonia
Urea
Amino acid helps in
Growth and repair
Energy
Enzymes
Hormones
Amino Acids
1. Gluconeogenesis
2. Plasma Protein
formation
Ammonia
Urea
Amino Acid
