best4online.com

Home

Site Map

Blog

Privacy

Add our site

Contact

 

Search the web

 

 External links

beautyworlds.com

BeautyCare.com

The Female Form: 1900-2000 One Hundred Years of Dips and Curves

Face of the Year International Beauty Contest

The Stirring of Sleeping Beauty

Modern Standards of Beauty: Nature or Nurture

Pheromones: The Smell of Beauty

Different Place Different Beauty

Evolutionary Psychology

Beauty and the Menstrual Cycle

The Question of Beauty

Babyness and Sexual Attraction

Female Pheromones and Male Physiology

Face Values

Revolting Bodies: The Monster Beauty of Tattooed Women

Piercing and the Modern Primitive

We must stop glorifying physical beauty

Click Here to Get Gorgeous

BeautynBrains

When Was the Last Time You Looked Glamorous?

Facial Beauty and Fractal Geometry

The Impact of Family Structure and Social Change

The Reality of Appearance

Sexual Selection and the Biology of Beauty

Venus, From Fertility Goddess to Sales Promoter

Why We Fall in Love

The Science of Attraction

The Biology in the Beholder's Eye

The Science of Attraction by Rob Elder

Your Cave or Mine

All Ah We is One Family

Skin Texture and Female Facial Beauty

 

 
Proteins

       Like water, proteins have a central place in the cell. Proteins constitute the structure of living organisms and catalyze cellular reactions. The genetic information present in the DNA segment (gene) is expressed in the form of a protein. Proteins are thus among the most fundamental biological macromolecules and have versatile functions. Cells contain thousands of different proteins, each with a specific function or biological activity, including enzymatic catalysis (biological catalysts), molecular transport, nutrition, cell or organismal defense and motility, structural roles, regulation of metabolism, and many others.

Proteins are composed of very long polypeptide chains having 100 to 2000 amino acid residues joined together by peptide linkage. Some proteins have more than one polypeptide chains (subunits). Whereas simple proteins yield only amino acids on their hydrolysis, conjugated proteins in addition have some other non-protein component such as a metal ion or organic prosthetic group like a carbohydrate or lipid.

Proteins differ in their size, shape, binding affinities, charge, etc. ; these properties are used in their analysis and purification. Proteins can be separated and visualized by electrophoretic methods. All proteins found in the natural world are made by the same set of about 20 amino acids. Their differences in the functions result from the variations in the composition and sequence of the amino acids in the polypeptide chain.

The structures of 20 amino acids differ as the result of their different side chain, as well as variation in their size and charge. Whereas some groups in the branched chain and aromatic amino acids are hydrophobic, in most other amino acids they are hydrophilic. These side chains have an important bearing on the stabilization of protein structure, and are closely involved in many other aspects of protein function. Attractions between positive and negative charges pull different parts of 

proteins

 

the molecule together. Hydrophobic groups tend to cluster together in the center of globular proteins, while hydrophilic groups remain in contact with water on the periphery. The thiol groups in cysteine form sulfur-sulfur bonds, which are crucial in the formation of primary structure. The hydroxyl and amino groups of amino acids can get attached to oligosaccha-ride side chains which are a feature of many mammalian proteins. Histidine and the dicarboxylic amino acids (glutamate and aspartate) are critical ion-binding proteins, such as the calcium-binding proteins and iron - binding proteins .

Certain amino acids achieve their final structure only after their precursors are incorporated into the polypeptide, e.g. hydroxyproline and hydroxylysine residues of the collagens, and methy-lated histidines and lysines of actin and myosin proteins. Hydroxyproline and hydroxylysine are critical components of the cross-linking of collagen chains, leading to their rigid and stable structures. The role of the methylated amino acids in contractile protein function remains to be elucidated. Eight of the 20 amino acids found in proteins are nutritionally essential because the structures of these amino acids cannot be made in the body of animals, and therefore must be provided in the diet.

The amino acid sequences of proteins can be determined by fragmenting them into smaller pieces using specific reagents, and then establishing the amino acid sequence of each fragment by the Edman degradation method. By placing the peptide fragments in the correct order by finding sequence overlaps between fragments generated by different methods, the amino acid sequence of the original polypeptide chain can be established.

Homologous proteins from different species of organisms show sequence homology, i.e. certain positions in the polypeptide chains contain the same amino acids, regardless of the species, though in other positions the amino acids may differ. The invariant residues of amino acids are evidently essential to the function of the protein. The degree of similarity between amino acid sequences of homologous proteins from different organisms correlates with the evolutionary relationship of the species .

Each protein has a unique three-dimentional structure, reflecting its function. The protein structure is stabilized by multiple weak interactions. Primary protein structure is established by amino acid sequence and location of disulfide bonds, secondary structure refers to the spatial relationship of adjacent amino acids, and tertiary structure is the three-dimentional conformation of an entire polypeptide chain. The quaternary structure of protein involves the special relationship of multiple polypeptide chains.

Proteins are classified as fibrous and globular proteins. Fibrous proteins primarily have structural roles. The stability of structural proteins, forming alpha-helix or beta-conformation, is established by their amino acid content and by their relative placement in the sequence. In structural proteins such as keratin and collagen, a single type of secondary structure ( alpha-helix, beta-conformation, or beta bend) predominates. The polypeptide chains are supertwisted into ropes and combined in larger bundles to provide the needed strength. The structure of elastin also per-mits stretching.

Globular proteins have more complicated tertiary structures and may contain several types of secondary structure in the same polypeptide chain. Globular proteins are compact, with their hydrophobic amino acids located in the protein interior. Different proteins often differ in their tertiary structure. Three-dimensional protein structure can be destroyed by treatments ( e.g., heat) that disrupt weak interactions, a process called denaturation, which also destroys protein function. Thus, protein structure and function have a very close relationship. Certain denatured proteins can renature spontaneously to produce functional protein ( e.g., ribonuclease ), which shows that the tertiary structure of a protein is determined by its amino acid sequence.

The quaternary structure refers to the interaction between the oligomeric subunits of proteins or large protein assemblies, e.g. four subunits of hemoglobin exhibit cooperative interac-tions on oxygen binding, these effects being mediated by subunit interactions and conformational changes .

 

 

 

 

 

Home | Privacy | Contact Us

Copyright 2004 Best4online.com. All Rights Reserved.