浙大食化考研.docx

1、浙大食化考研Chapter 3 Proteins5.1 IntroductionProteinsProteins are molecules of great size, complexity, and diversity. They are the source of dietary amino acids, both essential and nonessential. At the elemental level, proteins contain 50-55% carbon, 6-7% hydrogen, 20-23% oxygen,12-19% nitrogen, and 0.2

2、-3.0% sulfur. Protein synthesis occurs in ribosomes. After the synthesis, some amino acid constituents are modified by cytoplasmic enzymes. This changes elemental composition of some proteins. Proteins that are not enzymatically modified in cells are called homoproteins, and those that are modified

3、or complexed with nonprotein components are called conjugated proteins or heteroproteins. The nonprotein components are often referred to as prosthetic groups. Examples of conjugated proteins include nucleoproteins (ribosomes), glycoproteins (ovalbumin, k-casein), phosphoproteins (a- and b-casalns,

4、kinases, phosphorylases), lipoproteins (proteins of egg yolk, several plasma proteins), and metalloproteins (hemoglobin, myoglobin, and several enzymes). Glyco- and phosphoproteins contain covalently linked carbohydrate and phosphate groups, respectively, whereas the other conjugated protein noncova

5、lent complexes containing nucleic acids, lipids, or metal ions. These complexes can be dissociated under appropriate conditions. Proteins also can be classified according to their gross structural organization. Thus, globular proteins ate those that exist in spherical or ellipsoidal shapes, resultin

6、g from folding of the polypeptide chain(s) on itself. On the other hand, fibrous proteins are rod shaped molecules containing twisted linear polypeptide chains (e.g., tropomyosin, collagen, keratin, and elastin). Fibrous proteins also can he formed as a result of linear aggregation of small globular

7、 proteins, such as actin and fibrin. A majority of enzymes are globular proteins, and fibrous proteins invariably function as structural proteins. The various biological functions of proteins can be categorized as enzyme catalysts, structural proteins, contractile proteins (myosin, actin, tubulin),

8、hormones (insulin, growth hormone), transfer proteins (serum albumin, transferrin, hemoglobin), antibodies (immunoglobulins), storage proteins (egg albumen, seed proteins), and protective proteins (toxins and allergens). Storage proteins are found mainly in eggs and plant seeds. These proteins act a

9、s sources of nitrogen and amino acids for germinating seeds and embryos. The protective proteins are a part of the defense mechanism for the survival of certain microorganisms and animals. All proteins are essentially made up of the same primary 20 amino acids; however, some proteins may not contain

10、 one or a few of the 20 amino acids. The differences in structure and function of these thousands of proteins arise from the sequence in which the amino acids are linked together via amide bonds. Literally, billions of proteins with unique properties can be synthesized by changing the amino acid seq

11、uence, the type and ratio of amino acids, and the chain length of polypeptides. All biologically produced proteins can be used as food proteins. However, for practical purposes, feed proteins may be defined as those that are easily digestible, nontoxic, nutritionally adequate, functionally useable i

12、n food products, and available in abundance. Traditionally, milk, meats (including fish and poultry), eggs, cereals, legumes, and oilseeds have been the major sources of food proteins. However, because of the burgeoning world population, nontraditional sources of proteins for human nutrition need to

13、 be developed to meet the future demand. The suitability of such new protein sources for use in foods, however, depends on their cost and their ability to fulfill the normal role of protein ingredients in processed and home-cooked foods. The functional properties of proteins in foods are related to

14、their structural and other physico-chemical characteristics. A fundamental understanding of the physical, chemical nutritional, and functional properties of proteins and the changes these properties undergo during processing is essential if the performance of proteins in foods is to be improved, and

15、 if new or less costly sources of proteins are to compete with traditional food proteins. Amino acidsAmino acids are the building blocks of proteins which contain both basic amino groups and acidic carboxyl groups. -amino acids are the basic structural units of proteins. These amino acids consist of

16、 an -carbon atom covalently attached to a hydrogen atom, an amino group, a carboxyl group, and a side chain R group. Natural proteins contain up to 20 different primary amino acids linked together via amide bonds. These amino acids differ only in the chemical nature of the side chain R group. The ph

17、ysicochemical properties, such as net charge, solubility, chemical reactivity, and hydrogen bonding potential, of the amino acids are dependent on the chemical nature of the R group.At neutral pH values in aqueous solutions both the amino and the carboxyl groups are ionized. The carboxyl group loses

18、 a proton and obtains a negative charge, while the amino group gains a proton and hence acquires a positive charge. As a consequence, amino acids possess dipolar characteristics. When the amino group of one amino acid reacts with the carboxyl group of another amino acid, a peptide bond is formed and

19、 a molecule of water is released. This C-N bond joins amino acids together to form proteins。Protein structure Proteins are macromolecules with different levels of structural organization. The primary structure of proteins relates to the peptide bonds between component amino acids and also to the ami

20、no acid sequence in the molecule. Researchers have elucidated the amino acid sequence in many proteins. The secondary structure of proteins involves folding the primary structure. Hydrogen bonds between amide nitrogen and carbonyl oxygen are the major stabilizing force. These bonds may be formed bet

21、ween different areas of the same polypeptide chain or between adjacent chains. The tertiary structure of proteins involves a pattern of folding of the chains into a compact unit that is stabilized by hydrogen bonds, van der Waals forces, disulfide bridges, and hydrophobic interactions. The tertiary

22、structure results in the formation of a tightly packed unit with most of the polar amino acid residues located on the outside and hydrated. Large molecules of molecular weights above about 50,000 may form quaternary structures by association of subunits. These structures may be stabilized by hydroge

23、n bonds, disulfide bridges, and hydrophobic interactions. 5.2 Denaturation of Proteins Denaturation is a process that changes the molecular structure without breaking any of the peptide bands of a protein. The process is peculiar to proteins and affects different proteins to different degrees, depen

24、ding on the structure of a protein. Denaturation can be brought about by a variety of agents, of which the most important are heat, pH, salts and surface effects. Considering the complexity of many food systems, it is not surprising that denaturation is a complex process that cannot easily be descri

25、bed in simple terms. Denaturation usually involves loss of biological activity and significant changes in some physical or functional properties such as solubility. The destruction of enzyme activity by heat is an important operation in food processing. In most cases denaturation is nonreversible; h

26、owever, there are some exceptions, such as the recovery of some types of enzyme activity after heating. Heat denaturation is sometimes desirable-for example, the denaturation of whey proteins for the production of milk powder used in baking. Denaturation may sometimes result in flocculation of globu

27、lar proteins but may also lead to the formation of gels. Foods may be denatured, and their proteins destabilized, during freezing and frozen storage. Fish proteins are particularly susceptible to destabilization. Protein denaturation and coagulation are aspects of heat stability that can be related

28、to the amino acid composition and sequence of the protein. Denaturation can be defined as a major change in the native structure that does not involve alteration of the amino acid sequence. The effect of heat usually involves a change in the tertiary structure, leading to a less ordered arrangement

29、of the polypeptide chains. The temperature range in which denaturation and coagulation of most proteins take place is about 55 to 75. There are some notable exceptions to this general pattern. Casein and gelatin are examples of proteins that can be boiled without apparent change in stability. The ex

30、ceptional stability of casein makes it possible to boil, sterilize, and concentrate milk, without coagulation. Denaturing AgentsPhysical AgentsTemperature and DenaturationHeat is the most commonly used agent in food processing and preservation. Proteins undergo varying degrees of denaturation during

31、 processing. This can affect their functional properties in foods, and it is therefore important to understand the factors affecting protein denaturation. When a protein solution is gradually heated above a critical temperature, it undergoes a sharp transition from the native state to the denatured

32、state. The temperature at the transition midpoint, where the concentration ratio of native and denatured states is 1，is known either as the melting temperature Tm, or the denaturation temperature Td. The mechanism of temperature-induced denaturation is highly complex and involves primarily destabili

33、zadon of the major noncovalent interactions. Hydrogen bonding, electrostatic, and van der Waals interactions are exothermic (enthalpy driven) in nature. Therefore，they are destabilized at high temperatures and stabilized at low temperatures. However，since peptide hydrogen bonds in proteins are mostly buried in the interio