kelompok 2 - protein

8/3/2019 Kelompok 2 - Protein

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Proteins are polymers of amino acidslinked together by peptide bonds.They can also be referred to as

polypeptides.Proteins are key constituents of food,contributing towards organolepticproperties (particularly texture) andnutritive value.Proteins participate in tissue building andare therefore abundant in muscle andplant tissues.



Amino acids in solution at neutral pH arepredominantly zwitterions.

The ionization state varies with pH:at acidic pH , the carboxyl group is un-ionized and the amino group is ionized;at alkaline pH , the carboxyl group is ionizedand the amino group is un-ionized.

consists of:an amino group (NH2)a carboxyl group (COOH)a hydrogen atoma distinctive R group all bonded to

a single carbon atom ( α-carbon)



The peptide bond is the covalent bond betweenamino acids that links them to form peptidesand polypeptides.

A peptide bond is formed between the α-carboxyl group and the α-amino group of twoamino acids by a condensation (or dehydration synthesis) reaction with the loss ofwater.



Peptides are compounds formed bylinking small numbers of amino acids (upto 50).A polypeptide is a chain of 50 – 100 aminoacid residues.A protein is a polypeptide chain of 100+amino acid residues and has a positivelycharged nitrogen-containing aminogroup at one end ( N-terminus) and anegatively charged carboxyl group at itsother end ( C-terminus).



The primary structure of aprotein is simply thesequence of amino acidslisted from the N-terminalamino acid.

If we compare the primarysequence Leu-Val-Phe-Gly-Arg-Cys-Glu-Leu-Ala-Ala withGly-Leu-Arg-Phe-Cys-Val-Ala-Glu-Ala-Leu, these two

peptides have the samenumber of amino acids , thesame kinds of amino acids ,but have different primarystructures .



The secondary structure of aprotein describes thearrangement of theprotein backbone(polypeptide chain) due

to hydrogen bondingbetween its amino acidresidues.

Hydrogen bonding canoccur between an amidehydrogen atom and alone pair of electrons on acarbonyl oxygen atom, asshown in Fig. 2.13.



Two kinds of hydrogen bonded secondarystructures occur frequently with featuresthat repeat at regular intervals.

These periodic structures are the α – helixand the β- pleated sheet .

The β -pleated sheet can give a two-

dimensional array and can involve morethan one polypeptide chain.



The tertiary structure of aprotein is the threedimensional arrangementof all atoms within themolecule, and takes into

account the conformationsof side chains and thearrangement of helical andpleated sheet sections withrespect to each other.



There are two categories of tertiary structures:

Fibrous proteins• overall shape is a long rod;• mechanically strong;• usually play a structural role in nature;• relatively insoluble in water and unaffected by moderate

changes in temperature and pH

Globular proteins• helical and pleated sheet sections fold back on each other;• interactions between side chains important for protein

folding;• Polar residues face surface and interact with solvent;• non-polar residues face interior and interact with each

other;• structure is not static;• generally more sensitive to temperature and pH change

than their fibrous counterparts.



The tertiary structure of a protein is heldtogether by interactions between theside chains.

These can be through non-covalentinteractions or covalent bonds.

The most common non-covalentinteractions are electrostatic (ionicbonds, salt bridges, ion pairing),hydrogen bonds, hydrophobic

interactions and van der Waalsdispersion forces .



Not all proteins possessquaternary structure; it is only aproperty of proteins that consistof more than one polypeptidechain.

Each chain is a subunit of theoligomer (protein), which iscommonly a dimer, trimer or tetramer.

Haemoglobin has quaternarystructure. It is a tetramer

consisting of two α- and two β-chains.The chains are similar to

myoglobin and haemoglobin isable to bind four oxygen atomsthrough positive cooperativity.



Denaturation is a change in a protein which causesan alteration in its physical and/or biologicalproperties without rupture of its peptide bonds.

Denaturation is accompanied by a loss of nativebiological activity, but also affects physicalproperties. Some important consequences of proteindenaturation are:Loss of biological activity (e.g. enzyme activity).Loss of solubility and changes to water-binding

capacity.Increased intrinsic viscosity.Increased susceptibility to proteolysis.



Denaturation can be reversible , but ifdisulphide bonds are broken thedenaturation process is often consideredirreversible.

Different proteins have different susceptibilitiesto denaturation since their individualstructures are different.

There are various denaturing agents that candestabilize protein structures that arecategorized as physical agents or chemicalagents .



Post-translational modification of a proteinis a chemical change that has occurredafter the protein was synthesized by thebody.

Post-translational modification can createnew functional families of proteinsEnzymes also cause post-translationalmodification,such as peptide bondcleavage by specific proteases.



Food proteins have an important nutritionalrole and are primarily used by the body tosupply nitrogen and amino acids fromwhich the body synthesizes its own proteins.

Within the gastrointestinal tract, hydrolyticenzymes break down food proteins intotheir component amino acids, which arethen used by the body to synthesize other substances.The liver balances the pattern of aminoacid supply against the needs of synthesis.

kelompok 2 - protein

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