Biochemistry

1. PROTEINS
Proteins are complex, organic compounds composed of many
amino acids linked together through peptide bonds and cross-
linked between chains by sulfhydryl bonds, hydrogen bonds and
van der Waals forces.
There is a greater diversity of chemical composition in proteins
than in any other group of biologically active compounds.
The proteins in the various animal and plant cells confer on
these tissues their biological specificity.

2. CLASSIFICATION
Proteins can be classified as:
(a) Simple proteins- On hydrolysis they yield only the
amino acids and occasional small carbohydrate
compounds. Examples are: albumins, globulins,
glutelins, albuminoids, histones and protamines.
(b) Conjugated proteins- These are simple proteins
combined with some non-protein material in the
body. Examples are: nucleoproteins, glycoproteins,
phosphoproteins, haemoglobins and lecithoproteins.
(c) Derived proteins- These are proteins derived from
simple or conjugated proteins by physical or chemical
means. Examples are: denatured proteins and
peptides.

4. SIMPLE PROTEINS
Albumins
• Albumins are readily soluble in water, dilute acids
and alkalies, coagulated by heat.
• Albumins may be precipitated out from solution
using high salt concentration, a process called 'salting
out'.
• They are deficient in glycine. Serum albumin and
ovalbumin (egg white) are examples

5. Globulins
• Globulins are insoluble or sparingly soluble in water,
but their solubility is greatly increased by the addition
of neutral salts such as sodium chloride.
• These proteins are coagulated by heat.
• They are deficient in methionine.
• Serum globulin, fibrinogen, myosin of muscle and
globulins of pulses are examples.

6. Prolamins
• Prolamins are insoluble in water but soluble in
70-80% aqueous alcohol.
• Upon hydrolysis they yield much proline and
amide nitrogen, hence the name prolamin.
• They are deficient in lysine.
• Gliadin of wheat and zein of corn are
examples of prolamins.

7. Glutelins
Glutelins are insoluble in water and absolute alcohol but soluble
in dilute alkalies and acids.
They are plant proteins e.g., glutenin of wheat, oryzenin (rice)
Histones
Histones are small and stable basic proteins
They contain fairly large amounts of basic amino acid, histidine.
They are soluble in water, but insoluble in ammonium hydroxide.
They are not readily coagulated by heat.
They occur in globin of hemoglobin and nucleoproteins.

8. Scleroproteins
Insoluble in water, dilute acids and alkalies
Animal proteins
Present in hair, horn, nails, cartilage, bone
Collagen and keratin are the examples

9. Conjugated or compound proteins
These are simple proteins combined with some non-protein
substances known as prosthetic groups.
The nature of the non-protein or prosthetic groups is the basis for
the sub classification of conjugated proteins-
1. Nucleoproteins
simple basic proteins (protamines or histones) in salt
combination with nucleic acids as the prosthetic group.
They are the important constituents of nuclei and chromatin.

10. 2. Glycoproteins
A glycoprotein is a compound containing carbohydrate
(or glycan) covalently linked to protein
Eg. luteinizing hormone a follicle-stimulating hormone
3. Chromoproteins
These are proteins containing coloured prosthetic
groups e.g., haemoglobin, flavoprotein and
cytochrome.
4. Lipoproteins
These are proteins conjugated with lipids such as
neutral fat, phospholipids and cholesterol

11. 5. Metalloproteins
These are metal-binding proteins.
A globulin, termed transferrin is capable of combining
with iron, copper and zinc.
This protein constitutes 3% of the total plasma protein.
Another example is ceruloplasmin, which contains
copper.
Calcium as prosthetic group in protein Calmodulin

12. 6. Phosphoproteins
These are proteins containing phosphoric acid.
Phosphoric acid is linked to the hydroxyl group of
certain amino acids like serine in the protein e.g.,
casein of milk.

13. Derived proteins
These are proteins derived by partial to complete
hydrolysis from the simple or conjugated proteins by
the action of acids, alkalies or enzymes
They include two types of derivatives, primary-derived
proteins and secondary-derived proteins.

14. 1. Primary-derived proteins
These protein derivatives are formed by
processes causing only slight changes in the
protein molecule and its properties.
There is little or no hydrolytic cleavage of
peptide bonds.
e.g. coagulated albumin, fibrin , fibrinogen

15. Secondary-derived proteins
These proteins are formed in the progressive hydrolytic cleavage
of the peptide bonds of protein molecule.
They are roughly grouped into proteoses, peptones and peptides
according to average molecular weight.
e.g. Proteoses are hydrolytic products of proteins, which are
soluble in water and are not coagulated by heat.
Peptones are hydrolytic products, which have simpler structure
than proteoses. They are soluble in water and are not
coagulated by heat.
Peptides are composed of relatively few amino acids. They are
water-soluble and not coagulated by heat

16. Classification based on Biological function
1. Catalytic protein:
• They catalyze biochemical reaction in cells. Eg.
Enzymes and co-enzymes
2. Structural protein;
• They make various structural component of living
beings.
• Eg. Collagen make bone, Elastin make ligamnets and
keratin make hair and nails
3. Nutrient protein:
• They have nutritional value and provide nutrition
when consumed.
• Eg. Casein in milk

17. 4. Regulatory protein:
• They regulate metabolic and cellular activities in cell and
tissue.
Eg. Hormones
5. Defense protein:
• They provide defensive mechanism against pathogens.
• Eg. Antibodies, complement proteins
6. Transport protein:
• They transport nutrients and other molecules from one
organ to other.
Eg. Haemoglobin
• 7. Storage protein:
• They stores various molecules and ions in cells.
Eg. Ferritin store Iron

18. 8. Contractile or mobile protein:
• They help in movement and locomotion of various
body parts.
• Eg. Actin, myosin, tubulin etc
9. Toxic protein:
• They are toxic and can damage tissues.
• Eg. Snake venom, bacterial exotoxins etc

19. Classification based on shape of protein molecule
Fibrous proteins
They have primarily mechanical and structural
functions, providing support to the cells as well as the
whole organism.
These proteins are insoluble in water as they contain,
both internally and on their surface, many
hydrophobic amino acids.
The presence on their surface of hydrophobic amino
acids facilitates their packaging into very complex
supramolecular structures.

20. Globular proteins:
Most of the proteins belong to this class.
They have a compact and more or less spherical structure,
more complex than fibrous proteins.
In this regard, motifs, domains, tertiary and quaternary
structures are found, in addition to the secondary
structures.
They are generally soluble in water but can also be found
inserted into biological membranes (transmembrane
proteins), thus in a hydrophobic environment.

21. Properties
• Colour and Taste
Proteins are usually colourless and tasteless.
These are homogeneous and crystalline.
• Shape and Size
The proteins range in shape from simple crystalloid spherical
structures to long fibrillar structures. Two distinct patterns ofshape
have been recognized
A. Globular proteins- These are spherical in shape and occur
mainly in plants, esp., in seeds and in leaf cells. These are bundles
formed by folding and crumpling of protein chains. e.g., pepsin,
edestin, insulin, ribonuclease etc.
B. Fibrillar proteins- These are thread-like or ellipsoidal in shape
and occur generally in animal muscles. Most of the studies
regarding protein structure have been conducted using these
proteins. e.g., fibrinogen, myosin etc.

22. • Molecular Weight
The proteins generally have large molecular weights.
It might be noted that the values of molecular weights of
many proteins lie close to or multiples of 35,000 and
70,000.
• Colloidal Nature
Because of their giant size, the proteins exhibit many
colloidal properties, such as; Their diffusion rates are
extremely slow and they may produce considerable
light-scattering in solution, thus resulting in visible
turbidity

23. • Denaturation
Denaturation refers to the changes in the properties of a protein.
In other words, it is the loss of biologic activity.
In many instances the process of denaturation is followed by
coagulation
• Ion Binding Capacity
The proteins can form salts with both cations and anions based on
their net charge.
• Solubility
The solubility of proteins is influenced by pH.
Solubility is lowest at isoelectric point and increases with
increasing acidity or alkalinity.
• Optical Activity
All protein solutions rotate the plane of polarized light to the left,
i.e., these are levoratotory.

24. Hydrolysis
Proteins are hydrolyzed by a variety of hydrolytic agents.
A. By acidic agents: Proteins, upon hydrolysis with conc. HCl yield
amino acids in the form of their hydrochlorides.
B. By alkaline agents: Proteins may also be hydrolyzed with 2N
NaOH.