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Secondary structure

The local folding of the polypeptide in some regions gives rise to the secondary structure    of the protein. The most common are the α -helix and β -pleated sheet structures ( [link] ). Both structures are the α -helix structure—the helix held in shape by hydrogen bonds. The hydrogen bonds form between the oxygen atom in the carbonyl group in one amino acid and another amino acid that is four amino acids farther along the chain.

The illustration shows an alpha helix protein structure, which coils like a spring, and a beta-pleated sheet structure, which forms flat sheets stacked together. In an alpha-helix, hydrogen bonding occurs between the carbonyl group of one amino acid and the amino group of the amino acid that occurs four residues later. In a beta-pleated sheet, hydrogen bonding occurs between two different lengths of peptide that are antiparallel to one another.
The α -helix and β -pleated sheet are secondary structures of proteins that form because of hydrogen bonding between carbonyl and amino groups in the peptide backbone. Certain amino acids have a propensity to form an α -helix, while others have a propensity to form a β -pleated sheet.

Every helical turn in an alpha helix has 3.6 amino acid residues. The R groups (the variant groups) of the polypeptide protrude out from the α -helix chain. In the β -pleated sheet, the “pleats” are formed by hydrogen bonding between atoms on the backbone of the polypeptide chain. The R groups are attached to the carbons and extend above and below the folds of the pleat. The pleated segments align parallel or antiparallel to each other, and hydrogen bonds form between the partially positive nitrogen atom in the amino group and the partially negative oxygen atom in the carbonyl group of the peptide backbone. The α -helix and β -pleated sheet structures are found in most globular and fibrous proteins and they play an important structural role.

Tertiary structure

The unique three-dimensional structure of a polypeptide is its tertiary structure    ( [link] ). This structure is in part due to chemical interactions at work on the polypeptide chain. Primarily, the interactions among R groups creates the complex three-dimensional tertiary structure of a protein. The nature of the R groups found in the amino acids involved can counteract the formation of the hydrogen bonds described for standard secondary structures. For example, R groups with like charges are repelled by each other and those with unlike charges are attracted to each other (ionic bonds). When protein folding takes place, the hydrophobic R groups of nonpolar amino acids lay in the interior of the protein, whereas the hydrophilic R groups lay on the outside. The former types of interactions are also known as hydrophobic interactions. Interaction between cysteine side chains forms disulfide linkages in the presence of oxygen, the only covalent bond forming during protein folding.

This illustration shows a polypeptide backbone folded into a three-dimensional structure. Chemical interactions between amino acid side chains maintain its shape. These include an ionic bond between an amino group and a carboxyl group, hydrophobic interactions between two hydrophobic side chains, a hydrogen bond between a hydroxyl group and a carbonyl group, and a disulfide linkage.
The tertiary structure of proteins is determined by a variety of chemical interactions. These include hydrophobic interactions, ionic bonding, hydrogen bonding and disulfide linkages.

All of these interactions, weak and strong, determine the final three-dimensional shape of the protein. When a protein loses its three-dimensional shape, it may no longer be functional.

Quaternary structure

In nature, some proteins are formed from several polypeptides, also known as subunits, and the interaction of these subunits forms the quaternary structure    . Weak interactions between the subunits help to stabilize the overall structure. For example, insulin (a globular protein) has a combination of hydrogen bonds and disulfide bonds that cause it to be mostly clumped into a ball shape. Insulin starts out as a single polypeptide and loses some internal sequences in the presence of post-translational modification after the formation of the disulfide linkages that hold the remaining chains together. Silk (a fibrous protein), however, has a β -pleated sheet structure that is the result of hydrogen bonding between different chains.

Questions & Answers

what is an organ
rab Reply
a part of a organism
Devinayasha
A group of cell makes organ
Hamza
Organ is part of the body.
MR
a group of tissues that perform a specific function
Divya
What is pseudopodia
Mmesoma Reply
a temporary protrusion of the surface of an ameboid cell for movement and feeding.
Black
This help the animals to move from one place to another
Francess
what is the live ?
AZHKIR Reply
what
Hamdi
awkward🤒
bix
What is biology?
khan Reply
what is alliminatary canal
Shaaibu
i don't know but i wana to laern please tell me
Hamdi
there is something called googling as far as i know
bix
biology_it's science that study of living things
Black
alimentary canal_the whole passage along which food passes through the body from mouth to anus. It includes the esophagus, stomach, and intestines.
Black
is a branch of science whith deals the study of living things
AZHKIR
what element in colors purple
Mikaela Reply
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Ubon Reply
what is preganglionic
Ubon
don't know
Sweety
what is reproduction
Aben
reproducing specifically : the process by which plants and animals give rise to offspring and which fundamentally consists of the segregation of a portion of the parental body by a sexual or an asexual process and its subsequent growth and differentiation into a new individual.
Black
is the process where by organism produce their new organism of the same species from those who a ready in existence.
Prudent
what is unicellular cell
Abigail
unicellular orgnism* meaning consisting of a single cell
Black
How does reproduction take place in human being
Aben
Is a single celled organism
Brian
What is the Antibiotic
Tamara Reply
antibiotics is any medication that stop the growth of bacteria
onuoha
what is biomolecules
Lawerence Reply
This is a group of molecules produced by a living organism
Odion
thank alot,I had a hard time getting the answer
Lawerence
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It Reply
Now Mr. Niger Dac M;movement R;respiration N;nutrition G;growth E;excretion R;reproduction D;death A;adaptation C;competition
Odion
Sorry.... I;irritability
Odion
GOOD MORNING MY NAME IS MESUMBE PRECIOUS WHAT IS YOUR NAME
Mesumbe Reply
ccc
Peace
hi
Emmanuel
Emmanuel Daniel
Emmanuel
what is the meaning of Mr Niger?
Emmanuel
the meaning of Mr Niger, movement , reproduction , nutrition, inspiration , growth, excretion ,reproduction
Shonde
Emmanuel Daniel it's Mr NIGER D
debby
and d is for death
Shonde
Brown Jones ug
Anguyo
state all the elements and their symbols
Taiwo Reply
what types of muscles are found in the heart
Diamond Reply
Cardiac muscle
Divya
cardiac or myocardia muscle
onuoha
cardiac muscles
Elvis
cardiac muscle
Jemima
Cardiac muscle
Dr
Thank you dear!
yimam
please can you tell me the meaning of Mr Niger?
Emmanuel
It is an initial to represent life processes of organisms. M means movement, R for reproduction, N stands for nutrition, I for irritability or sensitivity,G means growth , E for excretion and R stands for respiration.
Quartey
cardiac muscle
Stanisla
what is digestion
Dolla Reply
it is the chemical break down of insoluble food substances such as fatty acid to soluble substances which are then used for body processes
Lawerence
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sadeeq Reply
levels of ecological study
sadeeq
no
Dolla
human sperms contains acid
Dolla
what is an artery
Dolla
It is one of the organs of the circulatory system that carries oxygenated blood from the heart to other parts of the body.
Quartey
artery is type of blood vessel which carries blood away from heart...
shams
how liver destroy red blood cells?
shams
big bang theory was discovered by
Sweety Reply
Georges Lemaître
Mr

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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