About the formation of protein
Photosynthesis results into formation of a carbon skeleton (a 3C or 6C skeleton). These newly formed carbon skeleton can be used to form a large number of bio-molecules which are required by a cell. For example the glucose can enter into aerobic respiration and the intermediates thus formed are used for synthesis of several molecules.


As you can specifically see from the above figure, alpha ketoglutarate is converted into glutamate which is then used for synthesis of proteins.

Alpha ketoglutarate is a keto acid can easily be aminated to give rise to amino acid.