Protein

Question 0

What bond joins amino acids together to form a polypeptide?

Answer 0

Peptide bonds

Question 1

Draw the structure of an amino acid and name the four main components.

Answer 1

Amino group (NH2), Central carbon bonded to hydrogen, carboxylic group (COOH), R group

Question 2

Describe how a peptide bond is formed.

Answer 2

The OH group of the carboxylic acid group on one amino acid combines with the H on the amino group of a second amino acid to form H2O whilst the carbon atom of the first amino acid covalently bonds to the nitrogen of the second amino acid.

Question 3

Describe the primary structure of a protein.

Answer 3

A linear sequence of amino acids where the R groups of adjacent amino acids are located on opposite sides of the chain and there are no interactions.

Question 3

Describe the secondary structure of a protein.

Answer 3

Two types of secondary structure: Beta pleated sheets and Alpha helix. It occurs when the sequence of amino acids are linked by hydrogen bonds.

Question 4

Describe the tertiary structure of protein.

Answer 4

This is a three dimensional arrangement where attractions are present between B-pleated sheets and Alpha helices. This can form globular like structures such as enzymes and transport proteins or fibrous structures such as hair, skin and nails.

Question 5

Describe the quarternary structure of a protein.

Answer 5

This is a protein containing more than one polypeptide chain which interact to form functional entities (Oligomer) such as insulin and haemoglobin.

Question 6

What are the requirements for protein?

Answer 6

There are no actual requirements but protein is required for the amino acids it contains which contribute towards the body’s amino acid pool.

Question 7

Define essential amino acids.

Answer 7

Amino acids that cannot be produced endogenously and must be provided in the diet.

Question 8

Define non-essential amino acids.

Answer 8

Amino acids that can be produced endogenously, by transamination of a carbon skeleton but relies on an availability of precursors.

Question 9

Define what is meant by conditionally essential amino acids.

Answer 9

Amino acids that can be produced endogenously but the rate of production is not sufficient under certain circumstances and so they become essential.

Question 10

Name the 9 essential amino acids.

Answer 10

Isoleucine, leucine, valine, lysine, methionine, threonine, phenylalanine, tryptophan & histidine.

Question 11

Name the 6 conditionally essential amino acids.

Answer 11

Arginine, cysteine, glycine, proline, serine, tyrosine.

Question 12

Name the 5 non-essential amino acids.

Answer 12

Alanine, aspartate, asparagine, glutamate & glutamine.

Question 13

Summarise protein digestion and absorption.

Answer 13

Dietary protein (complex macromolecule) is broken down by proteolytic enzymes in the stomach and small intestine into amino acids (small simple molecules). These are easily absorbed into the amino acid pool which are used to synthesise new proteins.

Question 14

Explain how protein is digested in the stomach.

Answer 14

Parietal cells in the stomach secrete fluid rich in HCL -> this denatures the protein causing it to straighten and uncoil allowing greater access to proteolytic enzymes. -> Zymogenic or peptic cells produce pepsin (a proteolytic enzyme) which is secreted as pepsinogen -> the acidity of he stomach activates the pepsin -> this cleaves proteins into smaller peptides.

Question 15

Describe how protein is digested in the small intestine.

Answer 15

Partially broken down protein is exposed to proteolytic enzymes -> these are secreted by the pancreas in the inactive Zymogen form -> requires a biochemical change to activate (e.g. Hydrolysis) -> proteins are then broken down by these enzymes via hydrolysis -> produces a mixture of amino acids and oligopeptides -> peptides then broken down further into tripeptides, dipeptides, and amino acids by membrane bound enzymes.

Question 16

Name three enzymes secreted by the pancreas which digest proteins (both there inactive Zymogen form and there activated form)

Answer 16

Trypsinogen -> trypsin
Chymotrypsinogen -> chymotrypsin
Procarboxypeptidase A & B -> carboxypeptidase A & B

Question 17

Name three ways amino acids are supplied to the amino acid pool

Answer 17

Dietary intake, de novo synthesis, protein degradation

Question 18

Name three demands for the amino acid pool

Answer 18

Oxidation, protein synthesis and other pathways

Question 19

During protein synthesis what is a limiting amino acid?

Answer 19

When a particular amino acid is in short supply protein synthesis will be limited. For instance, lysine is the first limiting amino acid.

Question 20

What nitrogen balance should adults and infants/children aim for?

Answer 20

Adults: nitrogen equilibrium

Infants/Children: positive nitrogen balance

Question 21

What are the major determinants of the nutritional quality of food proteins?

Answer 21

• The content of essential amino acids (complete/incomplete foods)
• The digestibility of the food source (I.e. The extent to which the amino acids are available to the host metabolism)

Question 22

Name three complete protein foods the contain all essential amino acids in sufficient quantities.

Answer 22

Poultry, fish, eggs, dairy

Question 23

Name two incomplete protein foods which lack one or more essential amino acids.

Answer 23

Corn, legumes (mainly plant foods)

Question 24

Are plant sources of protein more or less digestible than animal sources? Why?

Answer 24

Less digestible because of restriction to digestion by plant cell walls and the presence of anti-nutritional factors such as amylase and trypsin inhibitors.