117 Class Note for FD SC 400 at PSU
117 Class Note for FD SC 400 at PSU
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Date Created: 02/06/15
4 Food Proteins Proteins are synthesized by plants and animals to play a role in their physiology The functions a protein has from the organism s point of view include communication e g insulin structural e g collagen in skin or keratin in hair biochemical catalysts eg enzymes transportation e g hemoglobin to transport oxygen in the blood defense e g antibodies and storage e g globulins in seeds Very rarely do our needs from the protein match those of the plantanimal we are going to eat Our needs are firstly nutritional 7 of the 20 amino acids common in nature we can synthesize ll of them and must gain the rest from dietary sources and secondly functional 7 we rely on proteins to change the texture of our food for example by forming gels or by stabilizing foams and emulsions The only area where the proteins evolved function overlaps with our needs is when we use proteins as enzymes in bioprocessing e g making corn syrup from the action of amylase enzymes on starch We are also interested in how proteins interact with other food components eg avor binding or have toxic or antinutritional properties e g botulinum toxin is a protein In this section we will start by brie y reviewing the biochemical structure of proteins We will then examine how they can be modified postmortem by processing and through interactions with other ingredients and how this leads to some functional roles We will finally look at some examples ofthe uses of enzymes in food processing 1 Protein structure A protein is a linear sequence of amino acids linked together by O R peptide bonds There are 20 amino acids prevalent in nature and the C C HH sequence in which the cell assembles them is set in the DNA code H ie the primary structure of the protein The 20 amino acids all have distinct structures and unique properties but with a couple of important exceptions we can classify them as o Chargeduncharged Some amino acids have functional groups eg carboxylic acid amine phenol thiol that can carry a charge depending on the pH 0 Hydrophobichydrophilic Amino acids may be more or less water soluble depending on the polarity of their structure Charged amino acids tend to be more watersoluble Some important example amino acids include The side chain Phenylalanine has 0 O 0 Ju of glutamrc ac1d a bulky nonpolar HO Wax3amp0 is a very polar benzene ring as L0H 5 carboxylic acid part of its side Si H NH which may carry chain H NH2 a negative charge depending on the pH Cysteine contains a l functional thiol group l7 EH Thiols can dimerize under HENquot F I oxidizing conditions to 1 form disulfide bonds a di cysteine is called cystine During their RNA mediated polymerization process the amine group of one amino acid reacts W DWI 0M Lysine contains an amine group that is an unusually strong ll lng nucleophile The 4 reactivity of this amine with reducing sugars 1 in the Maillard reaction is particularly HANa H xav strong and so lysine is El quickly destroyed 39 during nonenzymatic browning Tlu sugummc UHF 2H gamble side chains nhc pnum39y smmuru I E sell 113 Rglnaimngjcnqtivwc ends allow Further Kal39llrri liinl with the carbo lic acid I R T r o R group of anoth r and Q Qmf 1 R wE NH water is eliminated The 0 H P dipeptide formed still has 3 HO H i a reactive amino and I I Dullhlu Imml lt S lI39rII39L S carboxylic acid group Ic ETBH I iqu chain backbone In which can continue to quot9 limin exibility react to form sequentially 1 0quot larger polypeptides and qmdmn 39 eventually a complete 4 N protein The number of amino acids required to H form a protein varies widely e g lysozyme is egg white is a simple globular protein with 129 residues while collagen is a triple helix of chains each containing more than 1400 residues A freshly synthesized polypeptide is no real use to the organism It must first spontaneously fold into a characteristic shape necessary for it to function properly Protein folding is a spontaneous process but incredibly precise process that remains one of the wonders of the natural world The first step in understanding protein folding is to think about the preferred conformation of a polymer A exible chain will not stand straight in solution the random conformation of the movable bond angles means instead it will take on a random coil conformation In the case of a protein there are a number of hydrophobic groups on the chain that would prefer to remove themselves from the aqueous cell matrix by being buried in the core of the protein This hydrophobic effect will tend to fold the random coil somewhat more tightly however folding too tightly would have an entropy cost Incroast aliai I I 3mg r a Inarem s olvemt entropy 0n nailm stain Many denatured slam Another way of looking at protein folding is as a competition between the chain entropy and the solvent entropy The highest entropy a chain can take is to form a random coil Each bond angle is set randomly and can ex randomly Note that a random coil does not imply a single structure for the protein instead a number of interchanging shapes all based on the principle that the bond angles are random However a random coil protein still has many hydrophobic amino acid residues exposed to the aqueous solvent Water molecules organize themselves into a structured clathrate cage around hydrophobic groups ie lowering their entropy The lowered solvent entropy is a problem that can be overcome by more tightly folding the protein but that would create a problem by increasing the degree of ordering of the chain The basis of protein folding then is a balance of forces argument between chain entropy seek to unfold to maximize the disorder of the polymer itself and solvent entropy seek to fold up to remove the ordering effects of hydrophobic amino acid residues on water This simple argument allows us to image a protein in solution spontaneously taking on a tightly folded conformation but the details distinguishing proteins lead to other structural intricacies A whole range of other noncovalent interactions are important in supporting and refining the shapes taken Important amongst these are i steric restraints 7 the chains exibility is restricted to movement about the acarbon and may be further restricted by bulky side chains ii strong interchain hydrogen bonding may strengthen certain configurations and iii permanent charges on the chain will lead to interchain electrostatic repulsion or attraction these forces will be strongly pH dependent Finally covalent disulfide bonds between two cysteine residues are very strong and can readily stabilize structure There are some common structural features that occur across a wide range of protein types These are the secondary structures 7 a local folding of the polypeptide chain over a part of its length A protein may contain several types of different secondary structure or it may contain none Examples of secondary structure include the xhelix and sheet In a B sheet polypeptides line up either parallel or antiparallel with one another There are strong hydrogen bonds between the nitrogens on one chain and the oxygen on a second chain However probably a more important driving force is half the side chains line up above the plane of the sheet and half below The half above tend to be hydrophobic and the half below hydrophilic The overall structure is amphipillic and held together by hydrophobicity An a helix is a coil formed over a region of the polypeptide chain There are strong hydrogen bonds between the nitrogen on the chain and the oxygen on the forth proceeding amino acid in the helix However probably more important is the radial positioning of the side chains sticking out at angles from the helix One side of the helix tends to accumulate hydrophobic residues and one side hydrophilic The overall helix is therefore amphiphilic and can easily be used to build tertiary structure by hiding the hydrophobic half in the core of the protein a It it wig Till l lel Opl lDbi 3 Q ifs hydrophilic r or K Tertiary structure is the bulk folding of the chain to make a defined threedimensional structure As we have seen the primary driving force for protein folding is solvent entropy Interchain hydrogen bonding electrostatic interactions and disulfide bonds may reinforce the structure formed Most proteins are at least approximately spherical blobs in solution as this allows them to minimize their surface to volume ratio and hide more hydrophobic amino acids in the core away from water More hydrophilic polymers will tend to be more open and more hydrophobic more densely packed Nonspherical proteins e g collagen need a strong secondary structure to maintain an extended shape normally for structural reasons 2 Protein Denaturation The native structure of a protein is the energetic minimum under physiological protein Any change in conformation away from this shape will represent an energy cost The protein is not completely static 7 it will ex and bend in response to thermal energy or binding ligands but will always tend to return to the same shape When the protein is moved out of the physiological state the balance of forces acting are changed and the protein may respond to change shape to minimize its energy under the new conditions For example a globular protein folds up tightly in water to hide its hydrophobic residues If the same molecule was moved into an organic solvent it would wan to unfold to hide its more hydrophilic groups in the core away from the nonpolar solvent The most common way to denature proteins is through heat Consider how the energy balance holding the protein together change with temperature for the reaction of native protein transforming to denatured protein PNlt gtPD The reaction will proceed spontaneously if the Gibbs free energy for the reaction is negative The two major contributions to Gibbs free energy here are solvent entropy opposes unfolding and chain entropy favors unfolding AG ACTsolvent ACkhain 39TAssolvent39 TAS chain If a protein unfolds the chain becomes less organized so its entropy increases so iTDS becomes negative and the reaction is favored On the other hand if the chain unfolds more clathrate water is formed the solvent becomes more organized so its entropy AS decreases so iTAS becomes positive and the reaction is opposed Under physiological conditions these factors balance each other with AGmal slightly positive and the reaction stays in the native state As temperature increases the T multiplier on both entropy terms increases and the value of AGED th and AGchain increase in parallel AGED th becomes increasingly positive and AGchain increasingly negative Their sum is unchanged and the protein remains stable However at about 700C the structure of the clathrate water starts to break down and the hydrophobic opposition to chain entropy stops being able to keep up and their sum the free energy for denaturation becomes negative and the protein unfolds In practice the various other interactions e g interchain hydrogen bonds electrostatic interactions disulfide bonds etc will oppose or favor denaturation to different extents depending on their prevalence and strength in a given protein and the denaturation temperature will vary between types of protein Other factors can also cause denaturation Any factor or combination of factors that increase the forces forcing a protein apart or reduces those holding them together may cause denaturation Examples include 0 Changing the pH so the protein is U highly charged positive below the pI or negative above can favor protein denaturation because the like charges on the chain repel one another and favor expansion of the aqueous Illa SA 1 non pmar plume 3 fulded strueture Makrng the sulventmurenunrpular e g by addrng eertan a1enhn1s reduees the energy enst ufexpusmg thehydrnphnhre amnu aerds tn snlyent and ean favur denaturatann when there rs a surface present 3 g a eshly prepared fuam nr ernu1srnn 1 an aqueuus prntern wru 1 adsnrh at the rnterfaee tn all nw any hydrnphnhre surface patehes tn rnnye nut quhe aqueuus enyrrnnrnent The surface prntern may then 3unfn1d at the surfaeetn a1an rnnre ufthehydmphubm amnu ands tn rnnye rntn the nunrpular enyrrnnrnent prntern denaturatann rs m pnnclple reyersrhle Onee the denatunng stress rs rernnyed the prntern null try tn get back rntn rts energy rnrnrrnurn whteh underphysmlugmal cundmuns rs thenataye state In praetaeeprnternregeneratrnnrs drfdeultheeause the prense sequence nfstepsneeded tn fuld the prntern rs hard tn repeat xtrs very easy fur the prntern tn try tn rnrnrrnrze rts energy by rnrsfnldrng The rnrsfnlded strueturersnnt rdea1 natrye hut m nrder tn get nut nfthrs state rtrnustunfnld agatn befure rt ean haye annthertry at refnldrng currently Gettang nut nfthe rnrsfnlded state rs a enst and the prntern rs 39equen y stuek seenndary energy rnrnrrnurn and never regenerates Addrtannahy denaturatrnn 1eads tn the Expusure nfhydrnphnhre arnrnn ands atthe prntern surface These ammu aerds wuuld rdeanyrepaekrntn the enre quhe prntern hut 39equendy ear bele stranded atthe surface The surface hydrnphnhrepatehes ean 1ead tn prnterns aggregatang tn hrde 39um water a The nataye prntern rs the rnnst stahle anwest energy state under physrnlngeat enndrtrnns when the cundmuns are ehangedtheha1anee uf fumes ehanges and h the prntern denatures when the denatunng cundmuns are rernnyed the prntern enu1d eregeneratehutthrs ean he nurte drfdeult m praetaee as the rnany d denatured fnrrnsrnust re n1d m preersely the sarne sequence tn rehudd Lhexdeal natrye strueture In rnany eases the prnterns wru rnnye therr hydrnphnhre ammu aerds away 39um water by e aggregatrng Aggregated prnterns t earnntregenerate and may ertherprenprtate nr e1 3 Protein Functionality Protein structure describes the chemical and physical shape of a protein Functionality describes what the protein does from our point of view As noted above an important functional role of protein that depends on their native state functionality ie biological functionality depends on their behavior as enzymes Enzymes are biological catalysts that increase the rate of a chemical reaction by providing a lowerenergy pathway between starting conditions and end point Frequently the reaction accelerated by the enzyme may be not apparent in its absence and it may seem as if a completely new pathway is being opened up but in fact the only effect is catalytic Enzymes are unusually specific catalysts in that they will work on a very defrned starting material to make a single often chirally pure form Because the catalyst is an enzyme any factor leading to denaturation may quickly destroy its activity A detailed treatment of enzyme structurefunctionality kinetics and associated biotechnology is beyond the scope of this course and we shall instead focus on some illustrative examples 0 Polyphenoloxidase catalyzes the oxidation of a wide range of phenolic compounds to diphenols and hence to quinones Quinones spontaneously ie nonenzymicaly I h m polymerize to brown melanin pigments Il quot 1 quot V Polyphenoloxidase needs oxygen and a copper 39 39 cofactor to function phenol diplrun 3H 0H CH cu Polyphenoloxidase activity is a plant defense mechanism DH designed to turn natural plant phenols into l I MELANMS antimicrobial products in the case of M quot tissue injury In plant processing this is a problem because freshly cut fruit and vegetables will spontaneously brown quickly unless steps are taken to i denature the enzyme ie blanching ii remove the oxygen e g store the vegetables under water or add ascorbic acid which reacts preferentially with the oxygen or iii inhibit the enzyme eg drop the surface pH with acid Sulfur dioxide is a very effective inhibitor of enzymatic browning by binding to the quinnones and preventing the nonenzymatic polymerization step that leads to melanins The products of polyphenoloxidase are nontoxic and avorless but extensive browning quickly leads to perceived spoilage on r 0 diphxmol calculus o Lipoxygenase catalyzes the formation of lipid peroxides from polyunsaturated fatty acids Lipid peroxides are a crucial step in the oxidation of fats to form rancid avors and peroxide activity can quickly lead to off avors e g if the germ is not adequately separated before milling grain In other cases lipoxygenase is added to facilitate avor formation The radicals forms as the peroxides break down radicals are formed that can cooxidize pigments and destroy vitamins Soy lipoxygenase is often added to bleach flour o Amylase is an important starchdegrading enzyme Amylase enzymes are used to produce deXtrins small glucose oligimers and glucose from suspensions of starch Initially a heatsensitive endoamylase ccamylase is added to a hot suspension of starch to rapidly reduce the average molecular weight The starch is then cooled but is less viscous because ofthe lower MW and a more heat sensitive exoamylase glucoamylase is added to further reduce the starch polymers to smaller units that can be partially puri ed and used as food ingredients see carbohydrates section below The other major functional roles of proteins depend more on their properties as polymers rather than their evolved biological functionality and frequently denatured proteins can be as or more functional than the native form While there is a wide range of these functions we will categorize them as hydrodynamic and surface properties Hydrodynamic functionality depends on the physical size and shape of the protein as an object suspended in a uid In this case the protein can be seen as a very small colloidal particle or neutral buoyancy It will build viscosity by forcing uid streamlines to de ect around it and may aggregate to form a gel The analogy between a polymer molecule and a colloidal particle is pursued further in the polysaccharides section below and at this stage we can settle for the general rule that interprotein cross links cf occulation in emulsions tend to lead to rst an increase in viscosity and second the formation of a gel The mechanisms of interchain bond formation include disulfide bonds but the most common are the attractions between the hydrophobic groups on the surface of denatured proteins Hydrophobic attraction can be mediated by strong electrostatic repulsion cf the DVLO potential if the protein is far from its isoelectric point and the salt concentration not too high Another important interaction is the strong affinity between certain amino acid residues and calcium If two separate proteins attempt to bind the same calcium it can form a strong link between the Calciummediated aggregation is particularly important in forming a tofu gel from soy proteins or setting caseinate gels The second group of functionalities important in proteins depends on their surface hydrophobicity Because a native protein can o en not fold adequately to protect all of their hydrophobic amino acids there are frequently hydrophobic regions on their surfaces A denatured protein has even more exposed surface hydrophobicity Either of these cases can allow a protein to adsorb at the surface of an emulsion droplet or a foam bubble and protect them against occulation and coalescence In addition small hydrophobic molecules can bind to the hydrophobic regions on the protein Importantly a protein can bind up avor molecules and stop them becoming volatile and thus perceived
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