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To download a Microsoft Powerpoint Document about Protein here |
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Primary Structure
This refers to the sequence order of amino acids in the polypeptide chain. |
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| The diagram below shows the basic structure of an amino acid. |
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Secondary Structure
In a protein molecule polypeptide chains are linked together in a number of different ways giving rise to molecules of definite shape. The links twist the polypeptide chain in to a specific coiled structure, such as a helix/spiral or pleated structure. This is known as the secondary structure |
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There are a number of ways that the links can occur between polypeptide chains.
The following are 2 such examples: |
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 | disulphide links |
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| hydrogen bonds |
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| Disulphide links occur when two sulphurs join, for example two cysteine amino acids (cysteine contains an SH group) link together. This can take place on the one polypeptide chain or bringing together two polypeptides chains. Insulin has disulphide links. |
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Polypeptide chains can also be linked by hydrogen bonds - the H in one chain and the oxygen in a neighbouring chain. Collagen is an example where polypeptide chains are linked by hydrogen bonds.
Tertiary Structure
Tertiary structure relates to the pattern of folding of the polypeptide chains. The helix/spiral of the secondary structure may be folded over and held firmly by links to form a globular structure. Tertiary Structure relates to the three-dimensional organisation of the polypeptide chain.
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Classification
Proteins can be classified in to simple and conjugated. Simple proteins can be further classified as those from animal sources and those from plant. Fibrous and globular proteins are the animal proteins. The plant proteins are less well understood but can be divided into 2 categories - glutelins and prolamines |
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Fibrous proteins are simpler than globular and made up of zigzag polypeptide chains held together by cross- links to form fibrous molecules with an elastic structure. They are insoluble. Collagen and myosin are fibrous proteins.
Globular proteins are more complex. The chain is folded in various ways to form molecules with an irregular shape. There is no general pattern of folding and so the exact nature of folding called the tertiary structure needs to be determined for each protein individually. Shape is approximately spherical. They are to be found in albumin and casein.
Glutelins are characterised by their insolubility in neutral solutions and solubility in acids and alkalis. They are to be found in wheat (glutenin), barley (hordenin) and rice (oryzenin).
Prolamines are insoluble in water but soluble in alcohol. They are to be found in wheat (gliadin) and maize (zein). Wheat has glutenin and gliadin. In combination these constitute gluten.
Sources
Actin 2.5% and myosin 7% are both found in the muscle fibres of meat. Myosin makes up the thicker filaments and actin the thinner ones. The cells of the muscle fibres contain ATP, which provides energy. After death ATP is broken down and in its absence myosin and actin combine to form rigid chains of actomysin. In this state, known as rigor mortis, meat is rigid and tough. It is therefore not consumed until after a period of storage known as conditioning occurs. During this time the stiffness diminishes and tenderness and flavour improve.
Gel Formation (Higher Level)
Proteins can be denatured by coagulation, foam formation and gel formation. A gel is a semi-solid viscous solution with a three dimensional network in which water molecules can become trapped. A gel is formed by gelatin. Gelatin is made from the protein collagen. When collagen is treated with hot water the cross- links are broken down and the resulting product is commercial gelatine. When cold water is added to gelatin it swells as it absorbs water. If this is heated it liquefies and forms a sol. On cooling the sol sets and becomes solid, a process which is known as gelation / gel formation. The gel formed is semi-rigid. The sol thus formed is not coagulated by heat. Gels are of considerable importance in food preparation. Soufflés and mousse illustrate this property showing the culinary uses gelatine.
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Functions
There are three main functions for protein - structural function, physiological function and nutritional function. |
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| Function | Examples
| | Structural | cellular membranes,
muscle,
skin | | Physiologically active | enzymes,
hormones,
blood proteins,
nucleoproteins | | Nutritional | | meat proteins - supply all essential amino acids |
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Structural proteins possess mechanical strength, because of the way their amino acids and polypeptide chains are arranged. This enables them to be involved in the formation of muscle tissue, skin, the keratin of hair, internal organs and cellular membranes. Some structural proteins are quite rigid (collagen in connective tissue) whereas others are elastic (elastin in arteries). All amino acids supplied by protein participate in growth and tissue maintenance. Some also perform other important physiologic and metabolic roles.
Physiologically active proteins are involved in the production of enzymes and hormones, nucleo-proteins and plasma proteins. Enzymes are the largest single group of proteins and are often referred to as organic catalysts. In fact without enzymes no biological process could take place and life would be impossible. Enzymes have many uses in food manufacture eg papain and bromelin used to tenderise meat.
Some hormones are also proteins and carry out many important metabolic processes.
Nucleoproteins are involved in the heredity process. They can be found in meat.
Plasma proteins are responsible for carrying essential substances around the body e.g. haemoglobin carries oxygen in the blood. They maintain physiological pH and osmotic balance and are necessary for the clotting mechanism and the immune response.
Nutrient proteins are those proteins that provide the essential amino acids (these must be taken in the diet as they cannot be made in the body). These are proteins that have a nutritional function e.g. caseins in milk that are passed from the mother to her offspring. The casein provides the essential amino acids for the child. Animal proteins are the main source of nutrient proteins.
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