Biochemistry of Proteins: function and classification

What are proteins?

• Proteins are necklaces of amino acids – long chains molecules.

• Proteins are the basis of how biology get this done. As enzymes, they are the driving force behind all the biochemical reactions which make biology work.

• As structural elements, they are main constituents of our bones, muscles, hair, skin, and blood vessels.

• As antibodies, they recognize invading elements and allow the immune system to get rid of the unwanted invaders.

• For these reasons, scientists have sequenced the human genome – the blueprint for all of the proteins in biology.

General Characteristics of Proteins• They are the most complex and most diverse in chemical

composition, conferring upon the different tissues.• Protein molecule contains elements of C, H, O,N, S, and P

together with traces of Fe, Cu, I, Mn, and Zn.• It has a molecular weight of 5,000 to 3,000,000• They are the most important of the biologic substances

being the fundamental constituent of cell cytoplasm.• They supply not only heat and energy but also material for

building and repair.• Unlike carbohydrates and lipids, only small amounts of

protein is temporarily stored in the body, and which can be quickly used up upon demand.

Classification of ProteinsBased on Axial Ratio: Axial ratio is the ratio of the length to the breath.• Globular proteins – with axial ratio less than 10 but

not below 3 or 4. They are compactly folded and coiled.

Ex. Insulin, plasma albumin, globulin, enzymes• Fibrous proteins – with axial ratio greater than 10.

They are spiral and helical and are cross linked by disulfide and hydrogen bonds.

Ex. Keratin, myosin, elastin, collagen

Globular and fibrous proteins• On the basis of structural shape, proteins can be classified into

two major types: fibrous proteins and globular proteins. • А fibrous protein is а protein that has а long, thin, fibrous

shape. Such proteins are made up of long rod-shaped or string-like molecules that can intertwine with one another and form strong fibers. They are water-insoluble and generally have structural functions within the human body.

• А globular protein is а protein whose overall shape is roughly spherical or globular. Globular proteins either dissolve in water or form stable suspensions in water, which allows them to travel through the blood and other body fluids to sites where their activity is needed.

Simple and Conjugated Proteins • Proteins are classified as either simple

proteins and conjugated proteins. • А simple protein is made up entirely of

amino acid residues. • А complex protein has other chemical

components in addition to amino acids. These additional components, which may be organic or inorganic, are called prosthetic groups.

Globular Proteins

• Globular proteins have their axial ratio less than 10 but not below 3 or 4. They are compactly folded and coiled.

• Examples are insulin, plasma albumin, globulin, enzymes

Hemoglobin

Fibrous Proteins

• Fibrous proteins are spiral and helical and are cross linked by disulfide and hydrogen bonds

• Examples are keratin, myosin, elastin, collagen

CollagenThe connective tissue protein

Biologically Important Proteins

Protein No. of AA

Function

Insulin 51 Enzyme for sugar metabolism

Cytochrome C 104 Enzyme for cell respiration

Growth hormone 191 Used as anti-aging treatment

Hemoglobin 574 Oxygen transport in blood

Hexokinase 730 Enzyme for glycolysis

Gamma globulin 1320 Part of immune system in blood

Myosin 6100 Muscle action

Blood ProteinsAlbumins Create osmotic pressure and

transport other molecules

Immunoglobulins Participate in immune system

Fibrinogens Blood coagulation

Alpha-1-Antitrypsin Neutralize trypsin that has leaked from the digestive system

Regulatory proteins Regulation of gene expression

GlycoproteinsGlycoproteins are proteins that contain carbohydrate. Proteins destined for an extracellular location are characteristically glycoproteins. For example, fibronectin and proteoglycans are important components of the extracellular matrix that surrounds the cells of most tissues in animals. Immunoglobulin G molecules are the principal antibody species found circulating free in the blood plasma. Many membrane proteins are glycosylated on their extracellular segments.

•

LipoproteinsBlood plasma lipoproteins are

prominent examples of the class of proteins conjugated with lipid.

The plasma lipoproteins function primarily in the transport of lipids to sites of active membrane synthesis.

Serum levels of low density lipoproteins (LDLs) are often used as a clinical index of susceptibility to vascular disease.

Nucleoproteins

Nucleoprotein conjugates have many roles in the storage and transmission of genetic information.

Ribosomes are the sites of protein synthesis. Virus particles and even chromosomes are protein-nucleic acid complexes.

Phosphoproteins

• These proteins have phosphate groups esterified to the hydroxyls of serine, threonine, or tyrosine residues.

• Casein, the major protein of milk, contains many phosphates and serves to bring essential phosphorus to the growing infant.

• Many key steps in metabolism are regulated between states of activity or inactivity, depending on the presence or absence of phosphate groups on proteins.

• Glycogen phosphorylase a is one well-studied example.

Metalloproteins

• Metalloproteins are either metal storage forms, as in the case of ferritin, or enzymes in which the metal atom participates in a catalytically important manner.

Hemoproteins

• These proteins are actually a subclass of metalloproteins because their prosthetic group is heme, the name given to iron protoporphyrin IX.

• Because heme-containing proteins enjoy so many prominent biological functions, they are considered a class by themselves.

Flavin is an essential substance for the activity of a number of important oxidoreductases.

Flavoproteins